Product Technology

TWI THE WELDING INSTITUTE

INDEX 1.0

Product Technology and Common Discontinuities

2.0

Steel Making 2.1 Blast Furnace 2.2 Basic Oxygen Steelmaking

3.0

Casting 3.1 Ingots 3.2 Killed Steel 3.3 Semi-killed Steel 3.4 Rimming Steel 3.5 Continuous Casting 3.6 Steel-casting Operations 3.7 Steel-casting Flow Control 3.8 Sand Casting 3.9 Shell Moulding 3.10 Die Casting 3.10.1. Hot Chamber Process 3.10.2 Cold Chamber Process 3.11 Investment Casting 3.11.1 Directional Solidification 3.11.2 Single Crystal Solidification 3.12 Casting Defects 3.12.1 Hot Tears 3.12.2 Restraint Cracks 3.12.3 Cold Cracks 3.12.4 Shrinkage 3.12.5 Sinks 3.12.6 Segregation 3.12.7 Entrapped Gas 3.12.7.1 Gas Porosity 3.12.7.2 Gas in Solution 3.12.7.3 Air Entrapment 3.12.7.3 Gas From Cores 3.12.8 Airlocks 3.12.9 Inclusions 3.12.10 Scabs 3.12.11 Cold Shut 3.12.12 Flash 3.12.13 Finning 3.12.14 Unfused Chaplets 3.12.15 Chills

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Wrought Products 4.1 Rolling 4.1.1 Slabs and Blooms 4.1.2 Plates, Strips and Sections 4.1.3 Planetary Rolling Mills 4.1.4 Plate Mills 4.1.5 Strip Mills 4.1.6 Seam Welded Pipe 4.1.7 Seamless Pipe 4.1.8 Long Product Mills 4.1.9 Cooling 4.1.10 Further Processing 4.1.11 Cold Rolling and Drawing 4.1.12 Hot Rolled Coil 4.1.13 Fabricating 4.2 Forging 4.2.1 Open Die Forging 4.2.2 Closed Die Forging 4.2.3 Upset Forging 4.2.4 Cold Forging 4.3 Extrusion 4.3.1 Direct Extrusion 4.3.2 Indirect Extrusion 4.4 Wrought Product Defects 4.4.1 Cracks 4.4.2 Seams 4.4.3 Rokes 4.4.4 Laps 4.4.5 Stringers 4.4.6 Slugs 4.4.7 Bursts 4.4.8 Laminations 4.4.9 Banding 4.4.10 Excessive Flash 4.4.11 Underfill 4.4.12 Internal Cracking 4.4.13 Mechanical Marks 4.5 Cold Working 4.5.1 Drawing 4.5.2 Cold Heading 4.5.3 Cold Rolling 4.5.4 Cold Extrusion

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TWI THE WELDING INSTITUTE 4.6

5.0

Defects Caused by Cold Rolling 4.6.1 Cracks 4.6.2 Splitting 4.6.3 Spalling Welding 5.1 Welding Process 5.1.1 Manual Metal Arc Welding 5.1.2 Metal Inert Gas Welding 5.1.3 Flux Cored Arc Welding 5.1.4 Tungsten Inert Gas Welding 5.1.5 Submerged Arc Welding 5.1.6 Electroslag Welding 5.2 Weld Terminology 5.2.1 Types of Joint 5.2.2 Types of Weld 5.2.3 Joint Preparation 5.2.4 Weld Zone Terms 5.2.5 Throat Thicknesses 5.3 Weld Defects 5.3.1 Cracks 5.3.2 Defects and their causes 5.3.3 Incomplete Root Penetration 5.3.4 Root Concavity 5.3.5 Burn Through 5.3.6 Excessive Penetration 5.3.7 Incompletely Filled Groove 5.3.8 Shrinkage Groove 5.3.9 Undercut 5.3.10 Overlap 5.3.11 Excessive Dressing 5.3.12 Mechanical Marks 5.3.13 Lack of Side Wall Fusion 5.3.14 Lack of Root Fusion 5.3.15 Slag Inclusion 5.3.16 Tungsten Inclusion 5.3.17 Copper Inclusion 5.3.18 Porosity 5.3.18.1 Distributed Porosity 5.3.18.2 Wormholes 5.3.18.3 Crater Pipe 5.3.19 Arc Strike 5.3.20 Spatter

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5.4 5.4.1 5.4.2 5.4.3 5.4.4 5.4.5 5.4.6 5.4.7 5.5 5.5.1 5.5.2 5.5.3 5.5.4 5.5.5 5.5.6 5.5.7 5.5.8 6.0

6

Post Heat Treatment Stress relieving Annealing Sub-critical Annealing Normalising Hardening Tempering Hydrogen Release Machining and Removal Turning Boring Drilling Milling Shaping and Planing Broaching Grinding Arc-Air

Service Defects 6.1 Fatigue Cracks 6.2 Stress Corrosion Cracking 6.3 Stress Corrosion Propagation Rate 6.4 Hydrogen Cracking 6.5 Grinding Cracks 6.6 Corrosion 6.6.1 Galvanic Corrosion 6.6.2 Exfoliation Corrosion 6.6.3 Pitting Corrosion 6.6.4 Crevice Corrosion 6.6.5 Fretting Corrosion 6.6.6 Filiform Corrosion 6.6.7 Microbiological Corrosion 6.6.8 Intercrystalline Corrosion 6.6.9 Weld Decay 6.7 6.7.1 6.7.2 6.7.3 6.7.4 6.8

Wear Abrasive Wear Adhesive Wear Fretting Wear Erosive Wear Creep

2


Basic Steel Metallurgy 7.1 Grain Structure 7.1.1 Body Centred Cubic Lattice 7.1.2 Face Centred Cubic Lattice 7.1.3 Hexagonal Close Packed Lattice

8.0

Grain Structure 8.1 Recrystallisation 8.2 Elastic/Plastic Deformation

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1.0

Product Technology and Common Discontinuities

Non-destructive testing is carried out on a large number of different materials, which will have undergone a variety of manufacturing processes. These may vary from the very simple to those which are exceedingly complex and costly. Each process and stage of manufacture has its own inherent discontinuities. Non-destructive testing is carried out to determine the suitability of these items to carry out the services they are intended under the operating conditions anticipated. What could well cause a catastrophic failure in one component will in another component under differing conditions and service live quite happily until the end of its useful life. It may be convenient to classify discontinuities into three groups:Inherent -

These may be described as arising from the initial manufacture of a material.

Processing -

Usually related to the various manufacturing processes, but are discontinuities which assume their characteristic shape while the part is being reformed.

Service -

Introduced in the operating cycle of the material or part.

Discontinuities are introduced at every stage of the manufacturing process and beyond; for example, in a steel fabrication these could be:1. During the initial casting, the solidification of the molten steel after conversion from the grey cast, or pig iron, or from casting into ingots, moulds or continuous casting. 2. During processing, produced by hot working such as forging, rolling and extruding, or cold working such as drawing, cold rolling or cold extruding 3. Welding, where material is fused together by bringing the atoms into such close proximity that atomic bonding occurs. 4. Finishing processes, such as machining, grinding etc. 5. In service degradation where defects may be generated due to deterioration of the component/structure as a result of one or more operating conditions which can result in deterioration of mechanical properties and possible catastrophic failures.

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2.0

STEEL MAKING

Steel production is a two stage process. The first stage is to convert the iron ore into pig iron or ‘hot metal’, which is iron with 3 to 5% carbon. The second stage is to reduce the carbon within the hot metal by passing oxygen, or oxygen rich gas through it to remove most of the carbon as carbon dioxide. Electric Arc Furnace (produces molten steel) Iron ore pellets

Steel Refining Facility

Oil Injection

Coal

Continuous casting

Coke ovens Limestone Blast Furnace (produces molten metal from iron ore)

Steel Making Flow Chart

2.1

Blooms and billets

Scrap

To rolling mills

Basic Oxygen Furnace (produces molten steel)

Pig iron casting

Blast Furnace

The blast furnace is the first step in producing steel from iron oxides. The first blast furnaces appeared in the 14th Century and produced one ton per day. Blast furnace equipment is in continuous evolution and modern, giant furnaces produce 13,000 tons per day. Even though equipment is improved and higher production rates can be achieved, the processes inside the blast furnace remain the same. Blast furnaces will survive into the next millennium because the larger, efficient furnaces can produce hot metal at costs competitive with other iron making technologies. The purpose of a blast furnace is to chemically reduce and physically convert iron oxides into liquid iron called "hot metal". The blast furnace is a huge, steel stack lined with refractory brick, where iron ore, coke and limestone are dumped into the top, and preheated air is blown into the bottom. The raw materials require 6 to 8 hours to descend to the bottom of the furnace where they become the final product of liquid slag and liquid iron. These liquid products are drained from the furnace at regular intervals. The hot air that was blown into the bottom of the furnace ascends to the top in 6 to 8 seconds after going through numerous chemical reactions. Once a blast furnace is started it will continuously run for four to ten years with only short stops to perform planned maintenance.

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TWI THE WELDING INSTITUTE Hot, dirty furnace gas

Waste gas stack Brick checkerwork columns

Gas cleaning equipment

Stock House

Brick lining

Hot blast Cold blast

Chutes – ore, coke, limestone

Molten metal

Liquid slag

Skip car

Cleaned gas

Blast Furnace Hot metal car

slag ladle

Iron oxides can come to the blast furnace plant in the form of raw ore, pellets or sinter. The raw ore is removed from the earth and sized into pieces that range from 0.5 to 1.5 inches. This ore is either Hematite (Fe2O3) or Magnetite (Fe3O4) and the iron content ranges from 50% to 70%. This iron rich ore can be charged directly into a blast furnace without any further processing. Iron ore that contains a lower iron content must be processed or beneficiated to increase its iron content. Pellets are produced from this lower iron content ore. This ore is crushed and ground into a powder so the waste material called gangue can be removed. The remaining iron-rich powder is rolled into balls and fired in a furnace to produce strong, marble-sized pellets that contain 60% to 65% iron. Sinter is produced from fine raw ore, small coke, sand-sized limestone and numerous other steel plant waste materials that contain some iron. These fine materials are proportioned to obtain desired product chemistry then mixed together. This raw material mix is then placed on a sintering strand, which is similar to a steel conveyor belt, where it is ignited by gas fired furnace and fused by the heat from the coke fines into larger size pieces that are from 0.5 to 2.0 inches. The iron ore, pellets and sinter then become the liquid iron produced in the blast furnace with any of their remaining impurities going to the liquid slag. The coke is produced from a mixture of coals. The coal is crushed and ground into a powder and then charged into an oven. As the oven is heated the coal is cooked so most of the volatile matter such as oil and tar are removed. The cooked coal, called coke, is removed from the oven after 18 to 24 hours of reaction time. The coke is cooled and screened into pieces ranging from one inch to four inches. The coke contains 90 to 93% carbon, some ash and sulphur but compared to raw coal is very strong. The strong pieces of coke with a high energy value provide permeability, heat and gases which are required to reduce and melt the iron ore, pellets and sinter.

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TWI THE WELDING INSTITUTE The final raw material in the iron making process in limestone. The limestone is removed from the earth by blasting with explosives. It is then crushed and screened to a size that ranges from 0.5 inch to 1.5 inch to become blast furnace flux. This flux can be pure high calcium limestone, dolomitic limestone containing magnesia or a blend of the two types of limestone. Since the limestone is melted to become the slag which removes sulphur and other impurities, the blast furnace operator may blend the different stones to produce the desired slag chemistry and create optimum slag properties such as a low melting point and a high fluidity. All of the raw materials are stored in an ore field and transferred to the stockhouse before charging. Once these materials are charged into the furnace top, they go through numerous chemical and physical reactions while descending to the bottom of the furnace. The iron ore, pellets and sinter are reduced which simply means the oxygen in the iron oxides is removed by a series of chemical reactions. These reactions occur as follows: 1) 3 Fe2O3 + CO = CO2 + 2 Fe3O4 2) Fe3O4 + CO = CO2 + 3 FeO 3) FeO + CO = CO2 + Fe or FeO + C = CO + Fe

Begins at 850° F Begins at 1100° F Begins at 1300° F

At the same time the iron oxides are going through these purifying reactions, they are also beginning to soften then melt and finally trickle as liquid iron through the coke to the bottom of the furnace. The coke descends to the bottom of the furnace to the level where the preheated air or hot blast enters the blast furnace. The coke is ignited by this hot blast and immediately reacts to generate heat as follows: C + O2 = CO2 + Heat Since the reaction takes place in the presence of excess carbon at a high temperature the carbon dioxide is reduced to carbon monoxide as follows: CO2+ C = 2CO The product of this reaction, carbon monoxide, is necessary to reduce the iron ore as seen in the previous iron oxide reactions. The limestone descends in the blast furnace and remains a solid while going through its first reaction as follows: CaCO3 = CaO + CO2 This reaction requires energy and starts at about 1600°F. The CaO formed from this reaction is used to remove sulphur from the iron which is necessary before the hot metal becomes steel. This sulphur removing reaction is: FeS + CaO + C = CaS + FeO + CO The CaS becomes part of the slag. The slag is also formed from any remaining Silica (SiO 2), Alumina (Al2O3), Magnesia (MgO) or Calcia (CaO) that entered with the iron ore, pellets, sinter or coke. The liquid slag then trickles through the coke bed to the bottom of the furnace where it floats on top of the liquid iron since it is less dense. Another product of the ironmaking process, in addition to molten iron and slag, is hot dirty gases. These gases exit the top of the blast furnace and proceed through gas cleaning equipment

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TWI THE WELDING INSTITUTE where particulate matter is removed from the gas and the gas is cooled. This gas has a considerable energy value so it is burned as a fuel in the "hot blast stoves" which are used to preheat the air entering the blast furnace to become "hot blast". Any of the gas not burned in the stoves is sent to the boiler house and is used to generate steam which turns a turbo blower that generates the compressed air known as "cold blast" that comes to the stoves. In summary, the blast furnace is a counter-current realtor where solids descend and gases ascend. In this reactor there are numerous chemical and physical reactions that produce the desired final product which is hot metal. Typical hot metal chemistry follows: Iron (Fe) Silicon (Si) Sulphur (S) Manganese (Mn) Phosphorus (P) Titanium (Ti) Carbon (C)

2.2

= 93.5 - 95.0% = 0.30 - 0.90% = 0.025 - 0.050% = 0.55 - 0.75% = 0.03 - 0.09% = 0.02 - 0.06% = 4.1 - 4.4%

Basic Oxygen Steelmaking

The second stage of the steel making process is to remove most of the excess carbon to change the pig iron to steel. There are two process routes for making steel in the UK today: the electric arc furnace and the basic oxygen converter. The latter requires a charge of molten iron, which is produced in blast furnaces. The BOS (Basic Oxygen Steelmaking) process is the major modern process for making bulk steels. In the UK, apart from special quality steels (such as stainless steel), all flat products, and long products over a certain size, are rolled from steel made by the BOS process. The BOS vessel is first tilted to allow materials to be tipped into it (charged). Scrap steel is first charged into the vessel, followed by hot metal (liquid iron) from the blast furnace. A watercooled lance is lowered into the vessel through which very pure oxygen is blown at high pressure. The oxygen, through a process known as oxidation, combines with the carbon, and with other unwanted elements, such as silicon, manganese and phosphorous ,separating them from the metal, leaving steel. Lime-based fluxes (materials that help the chemical process) are charged, and they combine with the "impurities" to form slag. The main gas formed as a by-product of the oxidation process is carbon-monoxide, and this is sometimes collected for use as a fuel elsewhere in the works. A careful balance between the amounts of hot metal and scrap charged into the converter is maintained as a means of controlling the temperature and to ensure that steel of the required specification is produced. After a sample has been taken to check that the chemical content of the steel is correct, the vessel is again tilted to allow the molten steel to flow out. This is known as tapping. The steel is tapped into a ladle, in which secondary steelmaking frequently takes place.

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TWI THE WELDING INSTITUTE Increasingly today, steels after they have been tapped (poured) from the furnace undergo a further stage of processing called secondary steelmaking before the steel is cast. This applies to both the basic oxygen process route and to the electric arc furnace route. The molten steel is tapped from the furnace into a ladle. A lid is placed over the ladle to conserve heat. A range of different processes is then available, such as stirring with argon, adding alloys, vacuum de-gassing or powder injection. The objective in all cases is to fine tune the chemical composition of the steel and/or to improve homogenisation of temperature (making sure that the steel is the same temperature throughout) and remove impurities. Ladle arc heating is a process used to ensure that the molten steel is at exactly the correct temperature for casting. During tapping small quantities of other metals and fluxes are often added to control the state of oxidation and to meet customer requirements for particular grades of steel. Finally the vessel is turned upside down and the slag tipped out into a container. Steelmaking slag is sometimes recycled to make road building materials. The modern BOS vessel makes up to 350 tonnes of steel at a time, and the whole process takes about 40 minutes.

Scrap charge

Oxygen blowing

Hot metal charge

Tap out and transfer to Ladle Metallurgy Facility

The steel is then teemed into ingots, or by means of the continuous casting process, directly into billets or blooms, which are used for further processing, such as rolling into slabs, plates, sections, or re-melted for pouring into moulds to make castings.

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3.0

CASTING

Castings are not only made from steel, but from a great variety of ferrous and non-ferrous materials, and range in size, from die-cast toy cars up to castings which could weigh hundreds of tons.

.

Ingot production from a 250 ton ladle 3.1 Ingots Ingots are produced after the steel making process as a general intermediate shape to be used for further processing. They are made in moulds made from cast iron, and are in two basic shapes: Wide end up (WEU) and Narrow end up (NEU). These can be poured from the top or the bottom and, dependant upon the addition of elements in the ladle can be killed, semi-killed or rimmed steel. Ingot casting continues to be the preferred method to produce steel for some uses, such as intermediate and large bar applications (e.g., power transmission) and high-performance bar and tubing applications (e.g., bearings and gears). Foundries and specialty producers also continue to use ingot casting and to produce large cross-sections or thick plates. Ingot casting is used for small batches of specialty steels or for end products with certain shape requirements (e.g., intermediate- and largebar applications or high-performance bar and tubing applications). Ingot casting also continues to be used by foundries and specialty steel makers to produce large cross sections or thick plates. During ingot casting, the molten steel is poured (teemed) into a series of moulds and allowed to solidify to form billets. After the moulds are stripped away, the ingots are heated to uniform temperature in soaking pits to prepare them for rolling. Continuous casting is much more energy efficient than ingot casting because of the need for soaking pits and increased scrap with the latter. While significantly lower than in other steelmaking processes, particulate emissions from casting occur when molten steel is poured into the moulds.

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TWI THE WELDING INSTITUTE 1

Hot Top (Refractory brick lining)

Killed Steel in Narrow end up top poured ingot

3.2

Semi-killed Steel in Wide end up bottom poured ingot

Rimmed Steel

Killed Steel

Killed steel ingots are produced by fully ‘killing’ or deoxidising the steel prior to the transfer of the steel from the ladle to the ingot mould. All the carbon/oxygen reactions are killed by the addition of silicon (Si) or aluminium (Al), either individually or combined. The aluminium also has the effect of refining the grains as they form, so producing a fine grained steel. This is the method used for the production of all engineering grade steel, including high alloy and tool steels.

3.3

Semi-killed Steels

These steels are produced as above, but instead of the aluminium or silicon being added in sufficient quantities to fully kill the carbon/oxygen reactions, a reduced amount of these elements is added therefore only partially killing the steel.

3.4

Rimming Steels

Rimming steels are produced by semi-killing the steel in the ladle with silicon just sufficient to allow oxygen to react with the carbon in the rim of the ingot producing blowholes of carbon dioxide and a rim of pure iron, free from carbon. The carbon dioxide blowholes usually weld shut upon rolling. Rimmed steels are mainly used for sheet, plate and rod in non-critical applications.

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3.5

Continuous Casting

In the continuous-casting process, molten steel is delivered in ladles and poured into a reservoir, or tundish, which is a refractory lined reservoir where many of the impurities are removed. From here it is released into the mould by gravity feed. The casting machine can have either one (single-strand caster) or multiple moulds (multistrand caster). The steel cools as it passes through the mould and forms a solid shell or “skin.” To stop the steel sticking to the water-cooled moulds the mould is vibrated, or oscillated, as the molten steel passes through. As the steel proceeds onto the runout table with a series of hot-handling rollers, the centre of the steel solidifies, yielding a semifinished shape at a specified width and thickness. Depending on the type of caster used, billets, blooms, rounds, thin slabs, or thick slabs are produced. A cutting torch is used at the end of the roll line to cut the steel to the desired length.

3.6

Steel-casting Operations

The functions of the caster pouring system are to transfer metal from the ladle to the caster, control flow to the caster, minimize slag entrainment, minimize oxygen pickup from the pouring system, cause flotation of inclusions, and minimize heat loss. Flow is controlled both at the ladle and tundish. The ladle flow control system includes a hydraulic slide gate and a reusable shroud to control oxygen contamination. The tundish flow control system includes a hydraulic slide gate or stopper rod system, a tundish block for positioning the nozzle over the mould and weirs and dams or baffles to control flow, temperature, and composition uniformity. Oxide-based refractories are used to form carbon monoxide and thus reduce oxygen pick-up. Silica-based refractories are not used because of the potential for steel contamination. In some cases, defectforming inclusions are removed with ceramic filters prior to casting.

Ladle Flow control device Water Spray

Tundish Flow control device

Continuous casting

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3.7

Steel-casting Flow Control

Slide gates can be of solid construction or contain integral gas passages to aid opening, reduce oxygen ingress, and prevent clogging. Stopper rods are a source of high maintenance because of erosion. During ladle refining, aluminium is added to remove excess oxygen. If the molten metal is not adequately protected from exposure to air during the casting operation, oxygen contamination can recur. Nitrogen pick-up also occurs and can be detrimental to the steel chemistry. Heating of the tundish is commonly provided by gas- or oil-fired burners in open containers. To limit oxygen pick-up during pouring from the ladle, a thin-wall ladle shroud is attached to the bottom of the ladle; the shroud extends into the molten metal contained in the tundish. A similar shroud is used to protect the molten metal stream as it leaves the tundish when not protected by the nozzle itself. The high-alumina refractories commonly used for the shrouds have a life expectancy of 1-10 heats before they need to be replaced. Conventional continuous casting occurs at speeds of 1-6 m/min with mould temperatures approaching 1600C. These operating conditions result in mould friction, surface defects, and gas bubbles when uncooled refractory moulds are used. Improvements in as-cast surface finish and dimensional control have been achieved by using water-cooled copper moulds. Clogging of the caster pouring system is the single largest operational problem resulting in reduced quality and production delays. Sources of clogging include dirty steel, air infiltration, high iron oxide content, ladle slag and misalignment of the tundish suppressor pad.

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3.8

Sand Casting

This is the traditional method of casting, using compressed sand as a mould which is formed round a pattern, usually made of wood. The pattern is removed from the sand mould and when the two parts of the mould are re-assembled molten metal is poured into the void through the pouring cup, sprue and runners. Matchplate patterns feature impressions on both the cope (top) and drag (bottom) sides, and typically are used to produce moulds for small parts, such as this elbow casting. The sand used for casting may be:Green sand – silica sand mixed with clay and a little water Synthetic sand – silica sand with clay mixed into it Core sand – green sand mixed with water-soluble core sand binder Cement bonded sand Alkaline Phenolic Bonded Sand Cold setting sand Urea formaldehyde modified resins Loam sand A coarse, strongly bonded moulding sand used for loam and drysand moulding. Sodium silicate sand Moulding sand mixed with sodium silicate and the mould is gassed with carbon dioxide gas to produce a hard mould or core. Zirconia sand A composition for coating the moulding surfaces of moulds for casting metal parts, an aqueous dispersion contains finely divided zirconia, finely divided mica, finely divided zircon, finely divided bentonite and, optionally, an inert colouring agent. This protects the moulding surfaces against high temperature molten metal and produces castings having substantially defect-free surfaces Mould Cavity Pouring Cup Breather Blind Riser Sprue Open Riser Cope Side

Parting Line

Drag Side Core

Moulding Sand

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Chaplet

Runner

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3.9

Shell Moulding

Rather than using a wooden pattern, a ferrous or aluminium pattern is coated with a silicone release agent, so forming a mould from resin-bonded sand mixtures brought in contact with preheated (300°F – 500°F) metal patterns, resulting in a firm shell with a cavity corresponding to the outline of the pattern.

3.10 Die Casting Primarily used in the making of aluminium, magnesium and low melting point alloys. The mould is generally made of metal, with the molten metal forced into the die at pressures between 0.7 and 700MN/mm2. There are two main methods:3.10.1 Hot Chamber Process Hot chamber machines are used primarily for zinc, copper, magnesium, lead and other low melting point alloys that do not readily attack and erode metal pots, cylinders and plungers. The injection mechanism of a hot chamber machine is immersed in the molten metal bath of a metal holding furnace. The furnace is attached to the machine by a metal feed system called a gooseneck. As the injection cylinder plunger rises, a port in the injection cylinder opens, allowing molten metal to fill the cylinder. As the plunger moves downward it seals the port and forces molten metal through the gooseneck and nozzle into the die cavity. After the metal has solidified in the die cavity, the plunger is withdrawn, the die opens and the casting is ejected.

Nozzle

Gooseneck

Stationary Ejector Platen Platen (moves) Cavity

Plunger Rod Plunger Ejector Pot Die Half Molten Metal

Ejector Die

Die Cavity

Hydraulic Shot Cylinder

Furnace Cover Die

Hot Chamber Die Casting

Molten Metal

Ladle

ShotHydraulic Ram Sleeve Cold Chamber Die Casting

3.10.2 Cold Chamber Process Cold chamber machines are used for alloys such as aluminium and other alloys with high melting points. The molten metal is poured into a “cold chamber,” or cylindrical sleeve, manually by a hand ladle or by an automatic ladle. A hydraulically operated plunger seals the cold chamber port and forces metal into the locked die at high pressures.

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3.11 Investment Casting The investment casting or “lost wax” process is ancient in its origin, dating back to 4000 BC. Although the process is ancient in origin we can attribute the success of some of our modern technical advances to the process. Originally used to cast sculptures and other “works of art”, it is now used to cast some of the most complex castings for some of the most critical applications. From moulds for small items of jewellery to jet engine turbine blades, from a few grams to several thousand pounds, the process provides millions of castings that meet everyday practical needs.

Die Construction

De-waxing

Wax Injection

Casting

Wax Assembly

Shell Removal

Slurry and stucco coating

Cut Off

Finishing

Inspection

Every casting produced by the process starts as a disposable wax duplicate (thus the term “lost wax”) that is injection moulded in a permanent die built with allowances for the process. Wax shrinkage, refractory shrinkage and alloy shrinkage are all factors considered when cutting the die cavity. Very complex shapes can be achieved by incorporating the use of soluble wax cores and/or preformed ceramic cores or a combination of both into the wax part. Each wax part is then assembled with the runner or sprue system, also called gating, to form what is commonly called a “tree”, “case” or “setup”. A tree can consist of one wax part, or in some cases, hundreds of wax parts. With very few exceptions, the ceramic shell has replaced the solid mould method of investing the wax assembly. This is accomplished by dipping the wax assembly or tree into a ceramic slurry followed immediately by a coating (stucco) of dry refractory grain. (The composition of the slurry and refractory grain is selected primarily based upon the alloys cast). The coated assembly is then allowed to dry in a controlled environment. The dip, stucco and dry steps are repeated until a shell of sufficient thickness has been formed. Typically a minimum 3/8 inch thick shell is required to achieve a green strength capable of withstanding the pressures of wax expansion during de-wax. Since investment shells are cast unsupported, it is also necessary

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TWI THE WELDING INSTITUTE to have a fired strength capable of holding the weight of the metal cast, another consideration for determining the shell thickness. When the shell is complete, it is necessary to remove the wax invested within. This is accomplished by either placing the shell into a steam autoclave or directly into a preheated furnace. To minimize shell cracks from wax expansion, it absolutely necessary to reach de-wax temperature in a very few seconds. As the wax melts it exits the shell through the runner or sprue system of the assembly. After de-wax, the shells can be stored in an uncontrolled environment until scheduled to cast. Prior to casting the shell is fired primarily to develop the fired strength of the ceramic (green or unfired shells have insufficient strength to contain the metal), and secondarily to remove any traces of the wax. After proper firing, the shells are removed from the furnace and immediately cast unsupported. The metal enters the shell through the runner or sprue system, which must be of proper design to prevent metallurgical defects due to improper gating. Because of metal shrinkage during solidification much of the ceramic shell literally falls off the castings; however, additional cleaning is required to remove all the shell material. This is normally accomplished mechanically and/or chemically. There are other casting techniques such as centrifugal and semi-centrifugal casting, gravity die casting and spray die casting, as well as variations on the above mentioned processes. These include controlled solidification techniques, which produce castings with high mechanical properties, i.e. ceramic mould casting, which produces a casting with a very fine grain structure. This can be further modified to give grains produced in one direction only, or as a single crystal. 3.11.1 Directional Solidification The ceramic mould is pre-heated, but a water chilled base plate encourages grain nucleation and resultant grain growth in a preferential columnar direction. This results in little or no transverse grain boundaries. This would, for example, produce turbine blades with superior mechanical properties in the direction of centrifugal forces acting in it when in service.

Heated Baffles

Molten Metal Columnar Crystals Water Cooled Chill Plate

3.11.2 Single Crystal Solidification Heated Baffles Here the mould is constructed with a chill base plate to encourage grain nucleation, but above the chill plate is a Molten corkscrew feeder which will only allow a single crystal to Metal pass through into the mould. The ceramic mould, a turbine blade for example, is lowered into the liquid metal and the single crystal grows through the cooling liquid metal as solidification takes place. The resultant blade has no grainCorkscrew constriction boundaries for stress cracking to propagate along, thus giving improved resistance to creep and thermal shock.

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Water Cooled Chill Plate

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Casting Defects

Due to the process and resultant problems encountered, various defects are inherent with the casting of metals. These are present before any further working, such as rolling, forging, extruding or welding is carried out:3.12.1 Hot tears – generally occur in the region of changes in section or contour, which does not necessarily need to be abrupt. They occur when there is a conflict of solidification between large columnar growths. Hot tears have an oxide skin at its crack surface, and are usually found either on the surface, or slightly sub-surface, they are often found in groups which may be in line or multidirectional. Hot tears are usually caused by rough handling after solidification, but before cooling to ambient temperature, restraint to contraction by the mould/core system or the temperature being too high when the casting is stripped from the mould.

3.12.2 Restraint cracks – these are caused by nonuniform cooling which results in the raising of localised stresses. They occur with their major axis along the direction of the applied stress and are usually ragged lines with a multitude of branches radiating away from the major axis.

3.12.3 Cold cracks – These cracks occur after the casting has been removed from the mould and has cooled to ambient temperature. The crack does not exhibit an oxide skin. Rough handling is one of the causes of cold cracks. Such cracks are usually hairline and visually difficult to detect. When high levels of residual casting stress are present, such cracks may occur. The high stresses may be caused by uneven cooling in the mould or restriction to metal contraction and the stresses exceed material strength.

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TWI THE WELDING INSTITUTE 3.12.4 Shrinkage – When metals are heated they expand, when they cool they contract. Most steels have a coefficient of about 0.000011 per degree Celsius. In metric terms, that means that an unrestrained steel bar, one meter long, will increase in length 11 millionths of a meter, or 11 thousandths of a millimetre, for each 1 degree C rise in temperature. A temperature rise of 1000 C will cause a one-meter bar to increase 11 mm in length. When steel solidifies and cools it will contract, in the case of an ingot it will reduce in cross sectional area. Due to the heat dissipation patterns the last part to solidify will be in the centre, towards the top of the ingot. Initially the liquid metal level will fall as the rest of the ingot solidifies and contracts. Gradually the metal will solidify on the surface. Solidification will start from the edges inwards resulting in a V shaped depression on the surface. When this takes place, and the solidified void is open to the surface, primary pipe is formed. Where the piping has not been open to the atmosphere then the secondary pipe is formed. As the metal in the secondary piping has not oxidised, small cavities may close up and weld shut on rolling or forging. Large cavities of secondary piping may be manually welded up prior to further working. The ingot usually has a lining of refractory bricks at the top to retard the cooling/solidification process, this is called the hot top. When the ingot has solidified and cooled, the hot top containing the primary piping and gross secondary piping is removed and is returned with the scrap to form part of the next melt. 0

Primary Pipe (open to the surface)

Shrinkage in the dendritic arms

Secondary Pipe (enclosed) interdendritic ‘A’ segregation ‘V’ segregation– (also known as ‘coring’)

Cooling gradient – starts from bottom and outside towards top centre

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Location of piping, dendritic segregation and shrinkage

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TWI THE WELDING INSTITUTE Castings other than ingots may have internal solidification voids, the term used for these voids is “shrinkage cavities”, these usually occur in heavy sections and under risers and heads where the metal will remain molten longer. Cavities will be of varying size, from large holes revealed by machining, NDT or pressure testing, to porosity only visible under a microscope. Inter-dendritic shrinkage is shrinkage associated with dendrite solidification, occurs sub-surface and exists on both microscopic and macroscopic scales.

Typical Dendritic microstructure

Interdendritic shrinkage

Shrinkage, which occurs in the last metal to solidify, is often associated with changes of crosssection. Centre line shrinkage consists of cavities or porous areas along the central axis of a casting, again due to this being the last metal to solidify. Like nearly all materials, metal is less dense as a liquid than a solid, and so a casting shrinks as it cools – mostly as it solidifies, but also as the temperature of the solid material drops. The shrinkage caused by solidification can leave cavities in a casting, weakening it. Risers provide additional material to the casting as it solidifies. Risers add cost because some of their material must be discarded as waste, but they are often necessary to produce usable parts. Shrinkage after solidification can be dealt with by using an oversized pattern designed for the relevant alloy. Causes of shrinkage include:Premature solidification of the ingates due to poor design of the runner system; Pouring with too slow, or with too low a temperature; Too high metal pouring temperature; Lack of mould rigidity; Metal composition out of specification.

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3.12.5 Sinks – These are surface depressions which occur where there is a localised change of section thickness. They are caused by the reduction in volume due to the liquid changing to solid with the resultant contraction lowering the surface level of the casting (similar to the primary pipe on an ingot). There is often an associated area of porosity underneath the sink.

3.12.6 Segregation – When an ingot solidifies the alloying agents and impurities within the steel are not always uniformly distributed within the steel. Differing alloys and impurities have differing melting points. During solidification of an ingot, the steel production process involves unavoidable segregation of the alloying elements. The solidification process begins with the formation of crystals within the melt. These crystals have a tree-like, branching appearance and are referred to as dendrites. The first dendrites that form in the molten steel have a relatively low carbon content and will be richer in composition of some alloying agents than the average, depending on the melting point of these constituents. As the freezing continues, these dendrites will become surrounded with the remaining liquid steel that is comprised of higher carbon levels. The melt that then freezes around the original dendrites will therefore have a different chemical composition. Pure metals do not exhibit segregation. Carbon steels forms segregates of carbon, sulphur, silicon, manganese and phosphorous. The segregation forms below the secondary piping, again caused to congregate in this area by the solidification process. Segregation can only be seen either microscopically or macroscopically after sectioning, polishing and etching. When continuously cast steel cools there is a band of segregation that can be seen upon polishing and etching, this is referred to as ‘banding’.

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Banding in concast steel

Dendrite Growth

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TWI THE WELDING INSTITUTE 3.12.7 Entrapped gas 3.12.7.1 Gas porosity. This can be sub-divided into three causes: • Firstly, gas held in solution in the molten metal can be precipitated as the metal solidifies, simply as a result of the reduced solubility on freezing. • Secondly, if the mould is filled under very poor conditions, air can be entrained in the metal stream and then trapped as the metal solidifies. • Finally, the sand binders used to make the moulds and cores often break down when in contact with the molten metal and the gaseous decomposition products can force their way into the solidifying metal, leading to defects which are normally known as ‘blows’. These different types of gas porosity defect vary in their size, distribution and distance below the casting surface. 3.12.7.2 Gas in Solution. Whilst the metal is in liquid form many of the alloying agents have gas forming elements, which are either insoluble in the molten metal, or during solidification, if conditions are favourable, may be rejected in the form of gas pores, These can as the metal solidifies, diffuse towards the surface, combining to form gas molecules. If the metal solidifies before they reach the surface they will be trapped within the metal, otherwise they will evaporate into the environment. The environment of the furnace is complex: the top surface of the liquid may be in contact with the air and so able to equilibrate directly with the atmosphere. However, in many cases, a surface oxide film, slag or flux layer may be present. These additional layers will present a further barrier to the passage of gas atoms emerging from the metal, slowing equilibration in the furnace even further. Conditions above the liquid may also be changing rapidly as waste combustion products, high in water vapour, are directed onto the surface, or blow across from time to time. The environment of the liquid metal in the mould is perhaps a little clearer. If the mould is a metal die, then the environment is likely to be dry and thus relatively free from water vapour and its decomposition product, hydrogen. The liquid metal may lose hydrogen to this environment, since the equilibrium pressure of hydrogen in the melt will be less than that of the partial pressure of the environment. In contrast, if the mould is made from sand, either chemically-bonded or especially if bonded with a clay-water mixture as in a greensand mould, then the environment all around the metal will contain nearly pure steam at close to one atmosphere pressure. The water will decompose in contact with the steel as follows: 3 H2O + 2 Fe = 3 H2 + Fe2O3 Thus steam will yield equal volumes of hydrogen gas, still at one atmosphere pressure, which will be available for solution in the liquid steel. It is likely that the melt will gain hydrogen in this environment.

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TWI THE WELDING INSTITUTE Gas precipitation from solution in the metal leads to small bubbles, normally in the size range 0.05 – 0.5 mm, as a result of the high internal pressure of gas due to the micro-segregation between the dendrite arms. The bubbles are distributed uniformly throughout the casting, with the exception of a bubble-free surface layer about 1 – 2 mm deep 3.12.7.3 Air Entrapment Moving on to the entrapment of air, taking as an example a sump casting that has been poorly designed (or ‘methoded’) in relation to runners, sprues and headers and will generate surface turbulence in the metal stream as it fills the mould, leading to a chaotic, scrambled mess of metal and air. The air cannot escape easily because it is held in place by the oxide film. Furthermore, as the air bubbles move through the molten metal, they leave behind a collapsed sac of oxide, forming a ‘bubble trail’ which is another form of defect in the casting. Bubbles tend to get trapped on horizontal surfaces, such as above ingates, on the cope surfaces or under any window-type features in the vertical sides of a casting. These bubbles are intermediate in size between those precipitated from solution and those blown from cores. They are also irregular in size, reflecting the randomness, or chaos, inherent with turbulence. They normally fall into the size range 0.5 – 5 mm and are often only found when ingates are cut off or the casting is shot blasted or machined. Since they arrive with the incoming metal, they are always close to the casting surface, and usually only the thickness of the oxide skin separates them from the casting surface. This partly explains the size range of the bubbles: they are only the remnants which were too small to generate sufficient buoyancy force to break through the oxide on the surface of the liquid, whereas their bigger neighbours escaped. When viewed on a polished cross section under the optical microscope, the bubbles are always seen to be associated with considerable quantities of oxide films – the remnants of bubble trails. These are often thought – incorrectly – to be ‘gas’ but the problem will certainly not be solved by degassing the metal. The solution will almost certainly lie in the design of the running system (i.e. the methoding) of the casting. 3.12.7.3 Gas From Cores The final type of gas defect is blown from cores. When a metal is poured into a sand mould containing cores, any water within the sand will instantly turn to steam, and the gas present in the core expands and attempts to escape. The steam and gas can usually escape from the core via the core prints, but if the core prints are too small or if the mould and core have a low permeability, the pressure will build up inside the core. If the pressure reaches the level where it exceeds the opposing pressure of the molten metal, bubbles can be formed in the metal and float up towards the top of the casting resulting in blowholes in or near the surface of the casting.

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3.12.8 Airlocks – Also known as misrun. A part of the casting is missing, usually on the cope or remote from the ingates. Air within the mould is unable to escape and so the molten metal is unable to completely fill the cavity. The edges adjacent to the missing parts are rounded and generally shiny.

3.12.9 Inclusions- These are formed during melting, solidification, and moulding. Generally non-metallic, they are regarded as harmful because they act like stress raisers and reduce the strength of the casting. They can often be filtered out during processing of the molten metal, for example, by use of ceramic filters at the end of the runners. Inclusions may form during melting because of reaction of the molten metal with the environment (usually oxygen) or the crucible material. Chemical reactions among components in the molten metal may produce inclusions; slags and other foreign material entrapped in the molten metal also become inclusions. Reactions between the metal and the mould material may produce inclusions. Spalling of the mould and core surfaces also produces Sand inclusions, indicating the importance of the quality and Inclusions maintenance of moulds. Sand from the mould material, often but not exclusively in the cope surface of the casting, may become detached from the mould and form irregular shaped cavities. These may be caused by erosion or washing of the sand from the mould by excessive turbulence, pieces of sand broken from the core or mould due to insufficient ramming or poor mould strength. Sand inclusions are usually accompanied by lumps on the surface where the sand has broken away .

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3.12.10 Scabs – A thin, irregular metal layer on the surface of the casting, attached to the casting either by a thin fin or at points, but separated from it by a thin layer of sand. The scab usually rests in a depression or groove on the casting surface and appears on the cope cast surface. It is usually caused before molten metal evenly covers the mould surface and uneven heating causes the surface layer of sand to buckle or distort and so become detached allowing the metal to flow behind the sand.

3.12.11 Cold Shut- A defect produced as a result of lack of fusion between metal streams where, due to the geometry of the mould, runners or cores, two or more streams of molten metal meet. If the temperature of the molten metal is too low, there is slag on the surface or there is an interruption to the pouring, the metal does not fuse and if the defect appears on the surface it will show as a shallow groove with rounded edges.

3.12.12 Flash – Projections in the form of metal wafers of varying thickness occurring at the mould joints, around core prints or between core and mould joints. This is caused by inaccuracies between mould or core faces which leaves a gap open to metal ingress. 3.12.13 Finning – Thin, rough and irregular fins of metal projecting from the casting surface, usually occurring in corners or recesses formed by the cores or mould. Heat from the pouring metal causes the core or mould face to expand and crack from the resultant stresses. The liquid metal penetrates the cracks and solidifies as a fin. .

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TWI THE WELDING INSTITUTE 3.12.14 Unfused Chaplets – If a hollow section is required a core is placed within the mould cavity. Chaplets are used to support and locate the correct position of the core. The chaplet should fuse into the casting wall to become a homogeneous part of the casting. If there is contamination, such as oil, rust, paint, dampness, etc., or where the teeming temperature is too low, then the chaplet will not fuse within the wall.

3.12.15 Chills - Where localised cooling is required to aid uniform solidification, chills, which may become an integral part of the casting, or are placed on the surface, to be removed later by fettling, may be used. Blowholes in the casting section are often associated with the use of chills.

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4.0

WROUGHT PRODUCTS

A wrought product is an item which is primarily produced by hot working, for example, by rolling, forging, extruding or drawing, where grain recrystalisation takes place as the item is being worked. For steels this would be in the range of 950 0C to 12500C. Cold working takes place below 9500C, where grain recrystalisation will not take place, and so grain deformation will be present. The main processes used in wrought working are:1) Rolling 2) Forging 3) Extrusion 4) Drawing

4.1

Rolling

Rolling is the most important metal working process and can be performed on either hot or cold metal. Material is passed between cast of forged steel rolls which compress it and move it forward. Rolling is an economical method of deformation if metal is required in long lengths of uniform cross section. Normal rolling achieves thickness reduction of about 2:1. The ingots or blooms from the primary production source, i.e. BOS or Concast works, are rolled into slabs or billets. These are in turn rolled into long, uniform cross sections such as squares, rounds, H or I sections, rails, plate or strip. The two main reasons for hot rolling are: 1) More material can be reduced in thickness in any one pass in the hot state due to the increased ductility at elevated temperatures. 2) Hot working refines the grain structure from coarse, elongated grains to fine, uniform, equiaxial grains which enhance ductility and gives improved strength. Rolling may also close up and weld, or bond, some of the internal defects such as porosity, piping and shrinkage cavities. Gross defects or defects which have oxidised surfaces do not bond but are elongated in the direction of rolling. Coarse grains

Deformed, elongated grains Recrystalisation complete Grains in liquation Hot Rolling

Formation of Laminations

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TWI THE WELDING INSTITUTE Pickling and oiling

Hot Strip

Cold Strip

Skelp

Slab:- semi-finished product with sides equal or greater than 50mm and a width to thickness ratio equal or greater than 2:1. Flat slab: - slab with width to thickness ratio greater than 4:1.

Welded Pipe

Plate

Sectional Shapes Rails

Square bloom:- semi-finished product with sides greater than 120mm. Rectangular Bloom:- cross-section greater than 14,400mm2, ratio width to thickness between 1:1 and 2:1.

Hot Rolled Bar

Cold Drawn Bars

Rod Wire and Wire Products Square billet:- semi-finished product with sides generally equal to or greater than 50mm and less than or equal to 120mm. Rectangular Billet:- billet with cross-sectional area of 2500mm2 or greater, but equal or less than 14400mm2 with a width to thickness ratio between 1:1 and 2:1 Round Billet:- diameter equal to or greater 75mm. (below 75mm it is termed as round bar).

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Tube Rounds Seamless Pipe and Tubes

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TWI THE WELDING INSTITUTE 4.1.1

Slabs and Blooms

Ingots are first rolled into either rectangular slabs or square blooms which are produced as intermediate stages. In this rolling process the ingots are passed through the plain rolls repeatedly in one direction and then in the reverse direction at each stage the rolls are brought closer together. If square blooms are required the material is rotated through 90 o between rolling operations. 4.1.2

Plates, Strips and Sections.

The rolling process can be used to produce plates, strips and rolled sections including channels, Universal Columns angles sections etc. The plates and strips are generally formed using plain rolls. The rolls can bow which results in the plate being thicker at the middle. The rolls can be backed up in four high roll arrangements with additional rolls to reduce this tendency.. 4.1.3

Planetary Rolling Mills

Adjustable Rollers

Small diameter rollers are more effective than large ones in conveying rolling forces to deforming metal. Planetary mills take advantage of this principle. This process can achieve thickness reductions of up to 25:1

Back-up Roll

Small Diameter Rolls

Semi-finished products called blooms, billets and slabs are transported from the steelmaking plant to the rolling mills. In many plants steelmaking and rolling are both Planetary Rolling Millcarried out on the same Fourthere Highare Roll Mill site. However also many stand-alone rolling mills in the UK (some are independently owned while others are part of a larger group but located away from the steelmaking works). Steel products can be classified into two basic types according to their shape: flat products and long products. Slabs are used to roll flat products, while blooms and billets are mostly used to roll long products. Uniquely, at Corus' Teesside works, slabs are used to roll large long products (such as beams).

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TWI THE WELDING INSTITUTE Semi-finished products are first heated in a re-heat furnace until they are red hot (around 1200 0 C). On all types of mill the semi-finished products go first to a roughing stand. A stand is a collection of steel rolls (or drums) on which pressure can be applied to squeeze the hot steel passing through them, and arranged so as to form the steel into the required shape. The roughing stand is the first part of the rolling mill, another name for these are cogging rollers. The large semi-finished product is often passed backwards and forwards through it several times. Each pass gradually changes the shape and dimension of the steel closer to that of the required finished product. 4.1.4 Plate Mills Slabs are used to make plate. Typically, after leaving the plate mill's roughing stand, they are passed through a finishing stand. This is a reversing mill: like on the roughing stand, the steel is passed backwards and forwards through the mill. It is also turned 90 0 and rolled sideways at one stage during the process. Plate is a large, flat piece of steel perhaps 10mm or 20mm thick (although it can be up to 50mm thick) and up to 5 metres wide. It is used for example to make the hulls and decks of ships or to make large tanks and boilers. It can also be rolled up and welded to form a large steel tube, used for oil and gas pipelines.

Stainless Steel Plate

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TWI THE WELDING INSTITUTE 4.1.5 Strip mills Slabs are also used to make steel strip, normally called hot rolled coil. After leaving the roughing stand, the slab passes continuously through a series of finishing stands which progressively squeeze the steel to make it thinner. As the steel becomes thinner, it also of course becomes longer, and starts moving faster. And because the single piece of steel will be a whole range of different thicknesses along its length as each section of it passes through a different stand, different parts of the same piece of steel are travelling at different speeds. This requires very close control of the speeds at which each individual stand rolls; and the entire process is controlled by computer. By the time it reaches the end of the mill, the steel is traveling at about 40 miles per hour. Finally the long strip of steel is coiled and allowed to cool. Hot rolled strip is a flat product which has been coiled to make storage and handling easier. It is a lot thinner than plate, typically a few millimetres thick, although it can be as thin as 1mm. Its width can vary from 150mm to nearly 2 metres. It frequently goes through further stages of processing such as cold rolling and is also used to make tubes (smaller tubes than those made from plate ).

Hot rolled wide coil being produced on the strip mill at Corus' Port Talbot works. © Corus

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TWI THE WELDING INSTITUTE 4.1.6 Seam Welded Pipe Seamed pipe is made from a roll of strip steel called ‘skelp’. The skelp is heated to welding temperature and then grasped by tongs attached to a moving chain, which pulls it through a die called a welding bell. Because the skelp is heated to weld temperature the edges fuse when they are pulled together. If the correct temperature is not maintained, there may be intermittent seams ( lack of fusion or cracks) either inside or outside the pipe. If there are any discontinuities in the skelp they will be transferred to the pipe. Pipe

Skelp 4.1.7 Seamless Pipe The making of seamless tube or pipe leads on from extrusion except the billet is pulled through rollers and over a piercing mandrel. It would appear that seamless tubes can only be made to a certain maximum length related to the mandrel length. However, modern processes use a floating mandrel held in place by another set of rollers through which the pierced tube is squeezed and drawn.

Top View

Sizing Mandrel Slag inclusion

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4.1.8 Long Product Mills Blooms and billets are used to make long products. After leaving the roughing stand, the piece of steel passes through a succession of stands which do not just reduce the size of the steel, but also change its shape. In a universal mill, all faces of the piece of steel are rolled at the same time. In other mills, only two sides of the steel are rolled at any one time, the piece of steel being turned over to allow the other two sides to be rolled. Long products are so called because they come off the mill as long bars of steel. They are however produced in a vast range of different shapes and sizes. They can have cross-sections shaped like an H or I (called joists, beams and columns), a U (channels) or a T. These types of steel "section" are used for construction. Bars can have cross-sections the shape of squares, rectangles, circles, hexagons, angles. These bars can also be used for construction, but many types of bar are also used for engineering purposes. Rod is coiled up after use and is used for drawing into wire or for fabricating into products used to reinforce concrete buildings, as are some types of bar.

Steel bars come in many varieties of cross-section. These 'U' shaped sections are known as channels.

Steel rails

© Caparo plc Merchant Bar

Other types of long product include railway rails and piling. Some long product mills make unique shapes of steel to a customer's individual specification. These are known as special sections. 4.1.9 Cooling In all rolling processes, cooling the steel is a critical factor. The speed at which the rolled product is cooled will affect the mechanical properties of the steel. Cooling speed is controlled normally by spraying water on the steel as it passes through and/or leaves the mill, although occasionally the rolled steel is air-cooled using large fans.

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TWI THE WELDING INSTITUTE 4.1.10 Further Processing Hot rolled products can undergo many forms of further processing before they are finally used to make an end-product (such as a steel-framed building or a consumer product). Such processing includes: 4.1.11 Cold Rolling and Drawing. After hot rolling, many steel products undergo a further processing in the cold state. This stage of processing does not alter the shape of the steel product, but it does reduce it in thickness and significantly improve its performance characteristics 4.1.12 Rolled Coil is initially hot rolled to strip and then cold rolled (also known as cold reduced) for the final thickness reductions. The strip is first de-coiled (uncoiled) and then passes through a series of rolling mill stands which apply pressure to the strip and progressively reduce its thickness - down to as low as 0.15 mm. The strip is then recoiled. Cold rolling processes are also used to improve the surface quality of the steel. Cold rolling also has the effect of hardening steel, so cold reduced strip is subsequently annealed: a process of very carefully controlled heating and cooling to soften it. Cold reduced strip and sheet is able to withstand subsequent forming and pressing operations without the steel cracking. The elaborate shapes used to make car bodies are pressed out of cold reduced sheets. Very thin cold reduced sheet (known also as blackplate), after coating with a thin layer of tin, is used to make food and drink cans. So sophisticated have modern steelmaking and rolling techniques become that it is now possible to press the shape of the complete can (bottom and sides) from sheet steel, leaving only the lid to be sealed on after filling. Many drinks cans are formed in this way.

Precision cold rolled coil being slit to narrow strips . © Outokumpu Stainless Cold Rolled Coil

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TWI THE WELDING INSTITUTE Another form of cold processing is cold drawing. Steel rod is dragged at pressure (drawn) through a series of dies which progressively reduce the rod's circumference to produce wire. The drawing process substantially increases the steel's tensile strength - steel wires can be spun into huge ropes strong enough to support the world's largest suspension bridges.

Steel wire can be drawn into a wide range of different shapes. Profiled colour coated steel sheets. © Corus

4.1.13 Fabricating. Steel sections are cut, welded and otherwise prepared to form the steel frame of a building. Rods and bars are similarly cut and shaped to form the steel reinforcement for concrete buildings.

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4.2.

Forging

Forging dates back to ancient times and was associated with the village blacksmith. Because materials are more malleable at high temperatures, and grain restructuring takes place, hot forging is easier to perform, both for the amount of deformation possible with a given force, and the extent to which a material will deform without suffering from ruptures and bursts. The work piece can be a billet, a wrought bar, a cast or sintered ingot etc. Hammering, or pressing, the work piece to the desired shape, can then complete the forging process. Hand hammering is not greatly utilised in industry; power assisted hammers as detailed below are the norm. There are two main types of power assisted hammers used in industry: gravity drop and power drop. Gravity drop hammers are where a forging ram is raised by chain, belt, air, steam, hydraulic etc. and then allowed to fall freely under gravity, using the mass and velocity of the hammer to deform the metal. Power drop hammers are similar, but the down stroke is assisted by a pressurised ram , operated by air, steam or hydraulics to intensify the impact. Press forging imparts a pressing action, either by mechanical presses, or screw presses. Mechanical presses have a crank or eccentric actuator to impart very high forces to the part, but these are limited by the inherent length of the stroke. Screw presses use the stored energy of a flywheel or centrifugal mass to impart the pressure. The mass of the flywheel or centrifugal mass is the limiting factor, and is only used for relatively light work.

Boar d Forging Stock

Board

Belt

Chai n

Ram

Air, steam or oil

Downstroke

Upstroke – gravity only on downstroke

Upstroke

Friction Drive Flywheel Screw Ram

Ram

Dies

Belt

Chain Gravity

Pneumatic

Pneumatic Power

Knucklejoint

Flywheel

Mechanical

Forging has a marked beneficial effect on the metals being shaped. Their toughness and strength are improved because the process results in a beneficial orientation of the metal grain structure. The repeated hot working causes the metal to become denser and the grain "flow lines" to follow the contour of the final component. The flow lines may be revealed by sectioning and etching.

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Forging flow lines increase the strength of the item much as wood is stronger in the direction of the grain growth.

4.2.1 Open Die Forging Open die (smith forgings) are made by using hammers or presses in conjunction with blacksmith tools or flat type dies. There is little lateral confinement of the work piece. The desired shape is obtained by manipulating the workpiece between blows. It is performed between flat dies with no precut profiles in the dies. The lower of the dies is usually called the “anvil”, whilst the hammer, or upper die is called the “Tup”. Movement of the work piece is the key to this method. Larger parts over 150 tons and 80 feet in length can be hammered or pressed into shape this way. This process employs low cost tooling, is relatively simple, but has less control in determining grain flow, mechanical properties and dimensions than other forging methods and so the process has to be carried out by skilled operators. Called open-die because the metal is not confined laterally by impression dies during forging, this process progressively works the starting stock into the desired shape, most commonly between flat-faced dies. In practice, open-die forging comprises many process variations, permitting an extremely broad range of shapes and sizes to be produced. In fact, when design criteria dictate optimum structural integrity for a huge metal component, the sheer size capability of open-die forging makes it the clear process choice over non-forging alternatives. At the high end of the size range, open-die forgings are limited only by the size of the starting stock, namely, the largest ingot that can be cast.

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TWI THE WELDING INSTITUTE Practically all forgeable ferrous and non-ferrous alloys can be open-die forged, including some exotic materials like age-hardening superalloys and corrosion-resistant refractory alloys. Open-die shape capability is indeed wide in latitude. In addition to round, square, rectangular, hexagonal bars and other basic shapes, open-die processes can produce: • • • •

Step shafts solid shafts (spindles or rotors) whose diameter increases or decreases (steps down) at multiple locations along the longitudinal axis. Hollows cylindrical in shape, usually with length much greater than the diameter of the part. Length, wall thickness, ID and OD can be varied as needed. Ring-like parts can resemble washers or approach hollow cylinders in shape, depending on the height/wall thickness ratio. Contour-formed metal shells like pressure vessels, which may incorporate extruded nozzles and other design features.

Not unlike successive forging operations in a sequence of dies, multiple open-die forging operations can be combined to produce the required shape. At the same time, these forging methods can be tailored to attain the proper amount of total deformation and optimum grain-flow structure, thereby maximizing property enhancement and ultimate performance for a particular application. Forging an integral gear blank and hub, for example, may entail multiple drawing or solid forging operations, then upsetting. Similarly, blanks for rings may be prepared by upsetting an ingot, then piercing the centre, prior to forging the ring.

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SHAFTS

1. Starting stock, held by manipulator.

2. Open-die forging.

3. Progressive forging. 4. Lathe turning to near net-shape.

2. Preliminary upsetting.

3. Progressive upsetting/ forging to disc dimensions.

DISCS

1. Starting stock.

4. Pierced for saddle/mandrel ring hollow "sleeve type" preform.

SADDLE/MANDREL RINGS

1. Preform 2. Metal displacementmounted on reduce preform wall saddle/mandrel. thickness to increase diameter.

3. Progressive 4. Matching to reduction of wall near net shape. thickness to produce ring dimensions.

HOLLOW "SLEEVE TYPE" FORGING

1. Punched or trepanned disc on tapered draw bar.

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2. Progressive reduction of outside diameter (inside diameter remains constant) increases overall length of sleeve.

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TWI THE WELDING INSTITUTE 4.2.2 Closed Die Forging This process is based on hammering the work pieces into the desired shape by means of closing dies. The hammering or pressing is performed, respectively, by a mechanical or hydraulic press. Small and medium sized forgings are generally made in presses ranging in capacity from 500 to 10000 Te. Closed die forgings have good dimensional accuracy, with improved mechanical properties compared to open die forgings. The process has good reproducibility and rapid production rates are possible. The initial cost of tooling is very high. In the simplest example of impression die forging, two dies are brought together and the workpiece undergoes plastic deformation until its enlarged sides touch the side walls of the die. Then, a small amount of material begins to flow outside the die impression forming flash that is gradually thinned. The flash cools rapidly and presents increased resistance to deformation and helps build up pressure increased resistance to deformation and helps build up pressure inside the bulk of the workpiece that aids material flow into unfilled impressions.

Flattener or Preform

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Blocker

Finisher

Trimmed Flashing (Scrap)

Finished Forged Part

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TWI THE WELDING INSTITUTE 4.2.3 Upset Forging This process uses barstock which is heated at the end which is being forged. The bar is gripped in the fixed half of a die so that the length of material being forged projects. The forging blow is delivered by a moving die. Simple shapes are produced in a single stage but more complicated shapes require multiple stages. Fundamentally, impression die forgings produced on horizontal forging machines (upsetters) are similar to those produced by hammers or presses. Each is the result of forcing metal into cavities in dies which separate at parting lines.

Header Die

Fixed Die

Moving Die

The impression in the ram-operated "heading tool" is the equivalent of a hammer or press top die. The "grip dies" contain the impressions corresponding to the hammer or press bottom die. Grip dies consist of a stationary die and a moving die which, when closed, act to grip the stock and hold it in position for forging. After each workstroke of the machine, these dies permit the transfer of stock from one cavity to another in the multipleimpression dies.

Drawing and squeezing are not major considerations in forging, however, upsetting is. This is because the metal is being forced back on itself and the metal really doesn’t want to do this. There are several guidelines that have to be considered when upsetting metal. The method being used - hand forging, drop forging, etc. - does not effect the upsetting. The following rules have to be observed. The length of unsupported material that can be uspet without buckling the stock is three times the material diameter. Lengths of stock greater than three times the diameter may be upset if the diameter of the die is not more than one 1½ times the diameter of the stock. When the required upset is more than three times the diameter of the stock and the diameter of the upset is less than 1½ the diameter of the stock, the length of unsupported stock beyond the die must not exceed the diameter of the stock.

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TWI THE WELDING INSTITUTE 4.2.4 Cold Forging Most forging is done as hot work, at temperatures up to 2300 degrees F, however, a variation of impression die forging is cold forging. Cold forging encompasses many processes -- bending, cold drawing, cold heading, coining, extrusions and more, to yield a diverse range of part shapes. The temperature of metals being cold forged may range from room temperature to several hundred degrees. Cold forging encompasses many processes:- bending, cold drawing, cold heading, coining, extrusion, punching, thread rolling and more to yield a diverse range of part shapes. These include various shaft-like components, cup-shaped geometry's, hollow parts with stems and shafts, all kinds of upset (headed) and bent configurations, as well as combinations. With cold forging of steel rod, wire, or bar, shaft-like parts with 3-plane bends and headed design features are not uncommon. Typical parts are most cost-effective in the range of 10 lbs. or less; symmetrical parts up to 7 lbs. readily lend themselves to automated processing. Material options range form lower-alloy and carbon steels to 300 and 400 series stainless, selected aluminium alloys, brass and bronze. There are times when warm forging practices are selected over cold forging especially for higher carbon grades of steel or where in-process anneals can be eliminated. Often chosen for integral design features such as built-in flanges and bosses, cold forgings are frequently used in automotive steering and suspension parts, antilock-braking systems, hardware, defence components, and other applications where high strength, close tolerances and volume production make them an economical choice. In the process, a chemically lubricated bar slug is forced into a closed die under extreme pressure. The unheated metal thus flows into the desired shape. As shown, forward extrusion involves steel flow in the direction of the ram force. It is used when the diameter of the bar is to be decreased and the length increased. Backward extrusion, where the metal flows opposite to the ram force, generates hollow parts. In upsetting, the metal flows at right angles to the ram force, increasing diameter and reducing length. Process Operations

1. Forward extrusion reduces slug diameter and increases its length to produce parts such as stepped shafts and cylinders.

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2. In backward extrusion, the steel flows back and around the descending punch to form cupshaped pieces.

3. Upsetting, or heading, a common technique for making fasteners, gathers steel in the head and other sections along the length of the part.

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4.3.

Extrusion

This is the term used to cover several industrial processes which include the impact method, used to produce very thin walled tubes, such as toothpaste containers, and the hydraulic powered extrusion machines which are used to make thicker walled products, where the cross section may be of a very convoluted shape but will be the same through the whole length. 4.3.1 Direct Extrusion This is where the hot billet is placed in the chamber and then forced out forward under pressure through the die opening. Die Die

Billet

Billet

Ram

Ram

4.3.2 Indirect Extrusion Also known as reverse extrusion, where the billet is held in the chamber and the die holder is forced into the billet extruding the section out. Die Ram Billet

Extrusion is usually carried out at elevated temperatures thus increasing the range of plastic deformation within the material making it easier to extrude – hot extrusion, although it can be carried out at ambient temperatures – cold extrusion. For both types of extrusion, but particularly cold extrusion, the material must be ductile, aluminium, copper, magnesium and their alloys possess ductility, as do some steels, but to a lesser extent. The extrusion process can be used to produce a range of components from clad or coated sections to tubed and stepped items. A thin residue is sometimes left in the extrusion chamber which is referred to as “skull”, which confirms that the extruded item is free from oxides. Steel Slug Die Punch Impact Extrusion Process

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4.4

WROUGHT PRODUCT DEFECTS

4.4.1 Cracks There are many types of cracks and tears which may be present in wrought products which can appear in many configurations, most, but not all, are surface breaking. These may be due to defects which were in the primary material, e.g. in the ingot; they may be caused during the various processes involved in the producing of the wrought product, such as thermal shock cracks formed whilst re-heating, during forging or rolling; stress cracks formed during the rolling or forging process – if the rate of deformation is too high; if the temperature drops too low, or due to uneven cooling or heating; attempting to reduce the cross section too much in one operation; the presence of impurities in the material, especially copper or sulphur.

4.4.2 Seams Shallow grooves, or striations, most obvious when the piece is upended, cross-cut or pickled. They are generally formed by the elongation during rolling of oxidized surface, subsurface blowholes or the result of splashes of molten metal on the surface of the ingot mould present in the ingot. They may also arise from a badly rippled surface or from recurrent teeming laps. 4.4.3 Rokes Defects on the exterior of bars. They consist of fissures which have become elongated in the direction of working, but have been only partially closed up during rolling, their surfaces being separated by a thin film of scale. They originate from blow-holes, formed immediately below the surface of the ingot, which have been broken down during forging or rolling and become oxidized or decarburized. The term should be restricted to isolated deep seams. 4.4.4 Laps During the rolling or forging process, particularly when faulty or oversize rolls or dies are used, there is overfill on the forming process and the material folds over and is flattened but not fused onto the surface of the component on subsequent passes.

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TWI THE WELDING INSTITUTE 4.4.5 Stringers (in bar stock) As the billet is rolled out into bar stock, if there is a concentration of non-metallic inclusions, such as sulphides, which normally congregate round the centre of the bar, these may be rolled out into longer, thinner discontinuities.

4.4.6 Slugs A piece of foreign material, such as a splash of molten metal within the ingot mould during teeming, is pressed or rolled and subsequently elongated into the surface of the component. The slug not fused but may become detached upon further working, blasting or pickling.

Slug Attached

detached

4.4.7 Burst (in forging) These are surface or internal ruptures caused by excessive working, working at too low a temperature or metal movement during working.

4.4.8 Lamination (in plate) Laminations are planar defects which are aligned parallel to the surface of the material and run in the direction of rolling. They are usually the result of original casting defects such as piping or gas pores which are flattened and elongated by the rolling process.

Inclusions Lamination

4.4.9 Banding If segregation is present in the ingot, or more commonly, down the centre of concast steels, then forging or rolling will elongate the segregation into bands of thin, dark lines that are revealed when the component is cross-sectioned, polished and etched. Banding is not normally considered as significant. The majority of NDT methods cannot pick it up, only by eddy current where it may be detected by the change of permeability within the material.

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TWI THE WELDING INSTITUTE 4.4.10 Excessive Flash (forgings) Caused by too large a blank being used in closed dies. This results in too much metal being squeezed outside the required form.

4.4.11 Underfill (forgings) Undersize blank is placed in the die and the forging is incomplete.

4.4.12 Internal Cracking (Extrusions) These appear as arrowhead shaped fractures, also known as ‘chevron cracking’ or ‘cupping’. It is caused by impurities in the material or incorrect die angles.

4.4.13 Mechanical Marks Surface marks, often repetitive in nature, caused by damaged or worn equipment, such as damaged rollers, guides, manipulators, worn dies, rolls and box hole roll collars (used in primary rolling).

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4.5

Cold Working

Cold working, compared to hot working, produces very good surface finish with accurate dimensions. Some of the properties of the cold worked components cannot be achieved by hot working and due to the difficulties in maintaining correct working temperatures on hot worked products cold working is preferable for items with small cross-sections such as wire, rods and small diameters tubes. 4.5.1 Drawing The material is reduced and/or changed in profile by pulling through a die. It is possible to produce rod in solid, uniform cross-sections which can vary from round, square or star to irregular shapes depending on the die used. Tubes can also be drawn, starting with a larger, thicker hollow tube. A variety of diameters and wall thicknesses can be produced from the same initial tube stock, depending on the size of the die and the mandrel used.

α F

Af

A0

F

The force required (F) to draw the wire or rod is affected by the die angle (α) reduction in cross-sectional area per pass (A0 – Af) speed of drawing, temperature and lubrication used.

Die

Examples of tube drawing processes, with and without an internal mandrel.

No Mandrel

Fixed Mandrel

Die

Die

Moving Mandrel Floating Mandrel

Die

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Die

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TWI THE WELDING INSTITUTE The rod or tube to be drawn is pulled through a die which is shaped to provide an ‘entrance angle’, where it is progressively reduced down to the ‘bearing land’ where the final shape is produced. Lubrication is applied before the material enters the die. ‘Wet drawing’ is where the die is immersed in oil, whilst ‘dry drawing’ is where the drawn material is coated with lubricant such as soap before entering the die. There is a limit to the amount of drawing possible before fracturing occurs due to hardening of the material. Intermediate or ‘patenting’ can be carried out which results in regrowth of the grain structure, with resultant softening of the material. ‘Sizing passes’, where only small reductions of cross-section are employed to improve tolerances and surface finish. 4.5.2 Cold Heading This is a cold forging method which is used to manufacture nuts, bolts, screws etc. in a closed die forge. 4.5.3 Cold Rolling Plate sheet and strip is manufactured by cold rolling where fine tolerances are required, to improve mechanical properties or when it is impractical to carry out hot rolling on thin sections. 4.5.4 Cold Extrusion Cold Extrusion may be used in preference to hot extrusion where improved mechanical properties or fine tolerances are required.

4.6

Defects Caused by Cold Rolling

Defects which may already be in the supplied material have been detailed previously, and so the following defects are only those which are caused by the cold working process. 4.6.1 Cracks These are caused mainly when the rate of deformation is too grewat, or when attempting to reduce the section too much in one operation. Cracks can either be surface breaking or subsurface and occur in a variety of configurations. 4.6.2 Splitting This tern is usually used in connection with flat products to describe a rupture caused by excessive cold working. 4.6.3 Spalling This is the fallout of surface material due to excessive local working.

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5.0

WELDING

Welding is the term used to describe the joining of two or more materials by fusing them together with heat, pressure or both so that the atoms are brought into such close contact that atomic bonding takes place. Although welding is normally associated with the joining of metals it can be carried out on other materials, for example, glass and plastics. The first welding to be carried out was ‘forge welding’ (as carried out by blacksmiths in the smithy, or forge.) Pieces of iron were heated red hot and hammered to fuse the separate parts together. As there is no melting of the materials in this method it comes under the heading of welding with pressure or hot solid phase welding. To achieve the atomic bonding the pressure applied must cause plastic deformation of the surfaces in order to break up and remove the oxides from the surfaces. The weld is then obtained by atomic diffusion followed by grain growth across the surfaces to be joined. The application of heat, or the generation of heat through friction, has the effect of reducing the amount of plastic deformation required to achieve atomic bonding. Welding with pressure has a low heat imput compared to other welding processes, this can be advantageous in many welding applications. Dissimilar metals, which may be difficult to join by fusion methods can be joined by the welding with pressure methods. Fusion welding relies on the properties of molten materials to easily form atomic bonds. When a solid melts the lattice structures which form the material are destroyed allowing the atoms to mix together freely. When the molten material solidifies the lattice structures re-form, although not necessarily in the same structure as before. These differences may take place due to the rate of heating and cooling, the temperatures reached, and any additions or changes to the chemical composition of the molten material. This may well give the weld material properties which are very different to the parent materials. As the fusion method requires the materials to liquify, the heat imput is far higherthan required for the welding with pressure processes. For steels the temperature needed to achieve this is in the region of 14000C to 15000C. The temperature in the molten weld-pool may be around the 25000C to 30000C mark, while the arc temperature will be around 6000 0C. The heat dissipated into the parent material adjacent to the weld and into the surrounding air. The molten material in the weld-pool readily combines with the surrounding air, (primarily nitrogen and oxygen), forming undesirable nitrides and oxides. To guard against atmospheric contamination the molten weld-pool is shielded from the atmosphere either by a shielding gas directly surrounding the arc and molten metal, or by flux which when heated and melted by the arc or other heating methods will release shielding gasses around the molten material.

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5.1

Welding Processes Welding with Pressure

Resistance Welding

Forge Welding

Blacksmith Welding

Pressure Welding

Hammer Roll Welding Welding

Resistance Butt Flash Welding Welding

Stitch Multiple Spot Welding Welding

Spot Welding

Series Spot Welding

Ultrasonic Welding

Diffusion Welding

HF Pressure Oxy-acetylene Cold Friction Explosive Welding Welding Welding Welding Welding

Projection Welding

Seam Welding

Roller Spot Welding

Percussion Welding

Butt Seam Welding

HF Resistance Welding

Foil Butt Seam Welding

Fusion Welding Arc Gas Welding Welding

Aluminothermic Welding

Electron Beam Welding

Electroslag Welding

Light Radiation Welding Laser Welding

Metal-Arc Welding

Carbon-Arc Welding

Manual Metal Arc Welding

Tungsten Inert Gas Welding

Submerged Arc Welding

Flux Cored Arc Welding

Arc-Spot Welding

Atomic Hydrogen Welding

Metal Inert Gas Welding

Arc Image Welding Arc Plasma Welding

Metal Active Gas Welding

Electro Gas Welding

CO2 Welding

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TWI THE WELDING INSTITUTE 5.1.1 Manual Metal Arc Welding Manual Metal Arc Welding, also known as MMA, SMAW (Shielded Metal Arc Welding) or Stick Welding, is the most versatile of the welding processes, as it is suitable for both ferrous and most non-ferrous metals. It can be used in all positional welding and for all thickness ranges from 18 gauge to the maximum encountered. For thicknesses over ¼” a bevelled edge preparation is used and the multi-pass welding technique is employed. The arc is under the control of and is visible to the welder. Slag removal is required. The electrodes are normally between 1/16” to ¼” diameter, (but can be as large as ½”) with a normal length of 9” to 18”, though 14” is the most common. The covering of the electrode dictates the useability of the elecrode and provides:1. Gas from the decomposition of of certain ingredients, usually wood pulp, in the flux coating to shield the arc from contamination from the atmosphere 2. Deoxidisers for purifying the deposited weld metal. 3. Slag formers to protect the deposited weld metal from oxidisation. 4. Ionising elements to make the arc operate more smoothly. 5. Alloying agents to provide a higher strength deposited weld. 6. (Optional) Iron powder to improve the productivity of the electrode. There are three main types of electrode coverings commonly used:1. Rutile – containing large quantities of components derived from titaniun oxide, they are a general purpose electrode, easy to use and the slag is easy to remove after welding. 2. Cellulosic – made primarily from finely divided cellulose, rutile (TiO 2) and deoxidants, deep penetration is obtainable, can be used in all positions, commonly used for vertically down welding (stovepipe), which is a fast technique. 3. Basic – these rods have a thick covering, containing considerable amounts of calcium and other basic carbonates and fluorspar, and are therefore basic in character. Gives welds with good metallurgical and mechanical properties with low hydrogen weld deposits, (although the weld is suceptible to porosity in the stop/starts and general weld body porosity if long arcs are used). The electrode requires pre-heating or drying before use to remove moisture as this can give rise to high hydrogen concentrations in the weld which can give rise to hydrogen induced cracking (HIC).

Weld Metal

Metal Droplets Covered in Electrode Molten Slag Flux Slag

Electrode Holder

Filler Wire Weld Preparation Arc Parent Material Shielding Gas Weld Pool

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Current Selector Switches (Coarse and Fine) Power Supply Unit Welding Lead

Return Lead

Electrode Holder

Work Piece

Separate Earth Connection

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TWI THE WELDING INSTITUTE The welding machine power supply unit is the most important part of the welding equipment. Its primary purpose is to provide electrical power of the correct current and voltage sufficient to maintain the welding arc.A rule of thumb of 35 to 40 amps per millimetre of diameter of electrode is used to provide a suitable amperage for welding. MMA can be accomplished by either AC or DC current. Straight (electrode negative) or reverse (electrode positive) polarity can be used with direct current. The electric arc melts the parent plate and the electrode to form a weld pool which is protected by decompositionof the flux covering. The operator adjusts electrode travel speed, weave, arc length, angle of welding, amprage and voltage. Care must be taken as the electric arc emits brilliant visible light and ultraviolet light which can be hazardous to personnel in the vicinity of the arc. A protective helmet is required with a dark coloured filter in the helmet allowing the welder to watch the arc while his eyes are protected from the harmful effects of the arc. Ventilation must be used in confined spaces due to the toxic fumes given off from the burning of the electrode.

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5.1.2 Metal Inert Gas Welding (MIG) The consumable electrode wire is carried on a spool, typically 0.8 to 1.6mm diameter and is fed automatically from a 15 Kg coil to a manually operated or automatic gun and through a nozzle into the weld arc. In addition to the electrode wire, a shielding gas is fed to the gun together with the welding current supply. Argon, which is one of the inert gasses, is used when welding non-ferrous metals, but for ferrous metals carbon dioxide is used to give a more stable arc and better fusion. However carbon dioxide is an active gas, it reacts chemically with the weld pool, and so deoxidizers are added to the wire to produce an acceptable weld. This process is called CO2 or MAG (metal active gas) welding. When other active gasses are used, such as argon/nitrogen, for the welding of copper, or argon/oxygen, for the welding of high strength corrosion resistant ferrous alloys, then the term MAG welding is also used. The micro-wire technique will weld most steels in thinner gauges than previously possible with an arc process. This uses a short circuit, or dip transfer method, where the filler wire connects directly with the weld pool and the resultant short circuit melts the wire so producing the welding process. This method allows for all positional welding, but the lower amperage used means there is an increased risk of lack of fusion. The spray transfer method uses an argon/oxygen shielding gas mixture, here higher amperes are used and the plasma arc vaporises the filler wire and the droplets are sprayed across to the weld pool. This produces high speed welding with minimal clean up times. The advantages of MIG/MAG welding are:• Good quality welds in almost all metals and alloys used in industry • Minimum post weld cleaning required, with no slag produced to be trapped in the weld • The arc and pool are clearly visible to the welder • Welding in all positions is possible, depending on wire diameter and process • Relatively high speeds of welding are possible with larger weld beads • It is a low hydrogen process, preheat may not be required Disadvantages:• Increased risk of porosity due to displaced shielding gas • More maintenance of plant required • High risk of lack of fusion, particularly with dip transfer due to lower amperage Workpiece

Manually held gun

Voltage and current control

Wire reel

Shielding gas with regulator

Gas Nozzle Contact Tube

Consumable Electrode Wire, gas and current in feed cable

Return lead

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Parent Weld Metal Pool

Gas Shield

Arc

Weld Metal

Welding machine

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TWI THE WELDING INSTITUTE 5.1.3 Flux Cored Arc Welding (FCAW) The FCAW process is similar to the MIG welding process, except that the shielding gas is completely or partially replaced with flux inside the filler wire. When additional shielding is obtained from an external gas then it is usually CO 2 for steels, however, for stainless steels and certain alloys a mixture of argon/CO 2or argon/oxygen is used. The flux is inside the tubular electrode wire which provides the required gas shielding when the arc is struck between the filler wire and the work. The process may be semiautomatic or automatic, although semiautomatic is the more widely used method. The two variations of the process provide different welding features. With the external gas shielding the properties are for smooth, sound welds with deep penetration and good characteristics for volumetric NDT (ultrasonics and radiography). The self shielding variation offers an elimination of the external gas shield, along with the gas nozzle and controls, the weld gives moderate penetration and the ability to weld in draughts and in breezy conditions. Both processes give an arc which is visible to the welder, all positional welding is possible (depending on wire diameter used) and any weld joint configuration can be made.

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TWI THE WELDING INSTITUTE 5.1.4 Tungsten Inert Gas Welding (TIG) In the TIG process the arc is formed between a pointed tungsten electrode and the work-piece in an inert atmosphere of argon or helium. The small intense arc provided by the pointed electrode is ideal for high quality and precision welding. Because the electrode is not consumed during welding, the welder does not have to balance the heat input from the arc as the metal is deposited from the melting electrode. When filler metal is required, it must be added separately to the weld-pool. TIG must be operated with a drooping, constant current power source - either DC or AC. A constant current power source is essential to avoid excessively high currents being drawn when the electrode is short-circuited on to the work-piece surface. This could happen either deliberately during arc starting or inadvertently during welding. If, as in MIG welding, a flat characteristic power source is used, any contact with the work-piece surface would damage the electrode tip or fuse the electrode to the work-piece surface. In DC, because arc heat is distributed approximately one-third at the cathode (negative) and two-thirds at the anode (positive), the electrode is always negative polarity to prevent overheating and melting. However, the alternative power source connection of DC electrode positive polarity has the advantage in that when the cathode is on the work-piece, the surface is cleaned of oxide contamination. For this reason, AC is used when welding materials with a tenacious surface oxide film, such as aluminium. The welding arc can be started by scratching the surface, forming a short-circuit. It is only when the short-circuit is broken that the main welding current will flow. However, there is a risk that the electrode may stick to the surface and cause a tungsten inclusion in the weld. This risk can be minimised using the 'lift arc' technique where the short-circuit is formed at a very low current level. The most common way of starting the TIG arc is to use HF (High Frequency). HF consists of high voltage sparks of several thousand volts which last for a few microseconds. The HF sparks will cause the electrode/work-piece gap to break down or ionise. Once an electron/ion cloud is formed, current can flow from the power source. HF is also important in stabilising the AC arc; in AC, electrode polarity is reversed at a frequency of about 50 times per second, causing the arc to be extinguished at each polarity change. To ensure that the arc is reignited at each reversal of polarity, HF sparks are generated across the electrode/work-piece gap to coincide with the beginning of each half-cycle. Electrodes for DC welding are normally pure tungsten with 1 to 4% thoria to improve arc ignition. Alternative additives are lanthanum oxide and cerium oxide which are claimed to give superior performance (arc starting and lower electrode consumption). It is important to select the correct electrode diameter and tip angle for the level of welding current. As a rule, the lower the current the smaller the electrode diameter and tip angle. In AC welding, as the electrode will be operating at a much higher temperature, tungsten with a zirconia addition is used to reduce electrode erosion. It should be noted that because of the large amount of heat generated at the electrode, it is difficult to maintain a pointed tip and the end of the electrode assumes a spherical or 'ball' profile.

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TWI THE WELDING INSTITUTE Shielding gas is selected according to the material being welded. Argon - the most commonly-used shielding gas which can be used for welding a wide range of materials including steels, stainless steel, aluminium and titanium. Argon + 2 to 5% H2 - the addition of hydrogen to argon will make the gas slightly reducing, assisting the production of cleaner-looking welds without surface oxidation. As the arc is hotter and more constricted, it permits higher welding speeds. Disadvantages include risk of hydrogen cracking in carbon steels and weld metal porosity in aluminium alloys. Helium and helium/argon mixtures - adding helium to argon will raise the temperature of the arc. This promotes higher welding speeds and deeper weld penetration. Disadvantages of using helium or a helium/argon mixture is the high cost of gas and difficulty in starting the arc. TIG is applied in all industrial sectors but is especially suitable for high quality welding. In manual welding, the relatively small arc is ideal for thin sheet material or controlled penetration (in the root run of pipe welds). Because deposition rate can be quite low (using a separate filler rod) MMA or MIG may be preferable for thicker material and for fill passes in thick-wall pipe welds.

Externally applied filler (optional)

Gas Supply

Cup Tungsten Electrode

Gas Orifice Shielding Gas

Power Connection

Work

TIG is also widely applied in mechanised systems either autogenously or with filler wire. However, several 'off the shelf' systems are available for orbital welding of pipes, used in the manufacture of chemical plant or boilers. The systems require no manipulative skill, but the operator must be well trained. Because the welder has less control over arc and weld-pool behaviour, careful attention must be paid to edge preparation (machined rather than handprepared), joint fit-up and control of welding parameters.

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TWI THE WELDING INSTITUTE 5.1.5 Submerged Arc Welding Submerged arc welding (SAW) is similar to MIG, that it has an arc between the continuously fed bare-wire electrode and the work-piece. A granular flux is poured in front of the wire that covers the arc so that no shield gas is required when welding.. A hopper above the work-piece stores the granular flux and a tube to the work-piece feeds a constant stream of flux over the weld-pool.. The excess flux is then recycled and is used again. The slag remaining on the weld, which helps to exclude oxygen as the molten pool is cooling, is easily removed after completion. Submerged arc welding is ideally used for butt or fillet, multiple pass welds. Longitudinal and circumferential butt welds are the most common types of welding. Pipe is usually rotated with the weld in the 1G position. There are usually either one, two or three wires being welded at one time. The more wire the faster rate of travel. The main features of SAW are , high speed of welding, easy slag removal, wide range of thicknesses weldable and good quality welds suitable for welds requiring x-ray and ultrasonic inspection. Welding is done in the flat and horizontal positions, with the arc not visible to the welding operator. Wire Feed Reel

Flux Hopper Flux Feed Electrode

Wire Feed Control

Power Cable

Flux Feed

Return Cable Flux Retrieval

Flux Parent Material

Arc and Shield Gas Flux

Electrode

Flux Retrieval

Molten Metal Slag Flux

Weld Metal

Slag Earth Clamp

Parent Material

The welding machines used for submerged arc welding is especially designed for the process, with both AC and DC power used. In either case the power source should be rated at 100% duty cycle, as the submerged arc welding operations are continuous and the length of time the welding set will be in operation will usually exceed the 10 minutes base period used for figuring duty cycle. Welding machines range in size from 200 to 5000 amperes, although it is normal to have the current somewhere between 1000 and 2000 amperes and deposit multi run welds because of the improved metallurgical properties. Alternating current is primarily with automatic machines, and also used in conjunction with DC for multi-wire SAW. Welding is done under a blanket of granular, fusible flux. This flux operates in much the same way as the covered coating on an MMA electrode. It protects the weld metal from contamination by atmospheric oxygen and nitrogen and also acts as a scavenger to clean and purify the weld deposit. Additionally, it may also be used to add alloying elements to the deposited weld metal. A portion of the flux is melted

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TWI THE WELDING INSTITUTE by the intense heat from the welding arc and becomes molten. The molten flux cools and solidifies forming a slag on the surface of the completed weld. The upper , or non-melted part of the flux can be recovered and re-used. The electrode wire for SAW is solid and bare, except for a very thin protective coating on the surface, usually copper, to prevent rusting. The electrode wire contains special de-oxidisers which help to clean and scavenge the weld metal to produce sound, quality welds. Alloying elements may also be included in the wire to provide additional strength to the weld. The electrode wire is available in sizes between 1/16” and ¼” diameter and is supplied in coils ranging from 50lbs to 1000lbs in weight. Submerged arc welding is extensively used in shipbuilding, structural steel work, general engineering and the fabrication of pipes and pipelines for the longitudinal and circumferential butt welds (at double jointing stations). Because of the high deposition rates and the liquid slag it is only possible to weld in the flat or horizontal positions, although circumferential butt welds in pipes and vessels are welded with SAW, the welding heads remain stationary and the work-piece rotates underneath it.

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TWI THE WELDING INSTITUTE 5.1.6 Electroslag Welding (ESW) Electroslag is a single-pass process-once a weld begins, it continues without interruption. It requires a welding arc only at the beginning of the weld cycle. The arc heats a supply of granular flux and forms a molten slag bath that extinguishes the arc. Water Cooled Copper Shoe Parent Material

Wire Feed Wheel and Drive Control Panel

To Welding Set

Water Supply and Return

Sump

Guide Tube Weld Wire

Unsubmerged Wire Extension

Flux Slag Bath Droplet Transfer

Completed Weld

Submerged Wire Extension

Electroslag Weld in Progress

Cross-Section of Weld, Enlarged View The electrically-charged slag bath (hence the name electroslag) is resistant to the welding current passing through it. This resistance heats the slag and keeps it molten at a temperature of approximately 3500° F. The slag is hot enough at this temperature to sustain continuous melting of the welding wire without an arc. The melted wire falls through the floating slag bath and joins the weld puddle. The fluidity of the slag bath and molten weld puddle requires a containment method and forces a vertical weld. Welding wire is pulled from its source, and fed down a steel tube to the molten puddle. The tube serves to guide the wire between the two plates so that the neither the wire nor the tube touches the plates being joined. This guide tube needs to be protected from touching the parent material to avoid a short that stops the welding operation. To avoid this, the guide tube must be insulated. As the weld progresses and the flux bath and weld puddle rise, the guide tube melts. It is important to understand how welding wire transfers into weld metal through the slag pool. As wire passes through the flux bath to the molten weld pool, the melted wire transfers in droplet form, down through the flux bath. Wire passes, in this manner, from a solid to a liquid to form weld metal. During this process, the welding wire melts completely inside the slag pool before it reaches the surface of the weld metal pool. As wire plunges into the molten flux bath, a certain amount of wire extends below the end of the guide tube and into the top of the molten flux bath. This length of wire is the "unsubmerged wire extension". Below this extension, an additional length of the wire plunges into the molten slag bath. This length of wire is the "submerged wire extension". This submerged portion of the wire is significant to the welding operation. Heat generated in the wire extension occurs due to resistance of the metal wire. The longer the length of wire submerged in the molten flux, the thinner the wire becomes. The thinner the wire

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TWI THE WELDING INSTITUTE becomes, the greater resistance to current flow. The greater the resistance to current flow, the greater the amount of heat generated by the current and the hotter the flux bath becomes. Two things that increase submerged extensions are higher wire-feed speeds, and solid wire. Solid wires have more mass than hollow-cored wires, and therefore take longer to transfer from a solid to a liquid. This creates a longer submerged extension for solid wires, and more resistant heating of the flux bath. The increased heating of the flux bath creates a hotter and deeper weld metal pool. The depth of the weld metal pool directly affects the grain formation "form factor" which determines the physical characteristics of an electroslag weld. For thicker plates the guide may be oscillated and multiple electrodes can also be used, for example, a single electrode would be used from 17 to 75mm, from 50 to 125mm the electrode would be oscillated, while from 125mm up to 300mm two oscillating electrodes would be used and up to six electrodes could be utilised for plate thicknesses up to 900mm. Since an electroslag flux bath and resultant weld metal pool are so fluid, the process requires a vertical position with devices to contain the fluids on the bottom and sides of the weld. To ensure that complete fusion occurs at the start of each weld, a sump is attached to the bottom of the material being welded. The depth of the sump allows the flux to melt and the molten weld pool to liquify enough to tie in all sides of the parent material. The sump is usually cut into a "U" shape, and tack welded to either side of the weld gap, on the bottom of the two plates being welded. Since one inch of flux generally covers the molten weld puddle, run-off tabs are added to the top of the plates being welded. When the flux puddle reaches the top, the weld metal is still one inch below. Therefore, if two-inch run-off tabs are tack-welded to either side of the weld gap, the flux puddle can rise two inches above the parent material. When the weld stops, and the flux is cleaned off, weld metal should be one inch above the parent material. Water-cooled copper shoes contain the molten weld puddle on either side of the gap between the two pieces of parent material. The cavity formed between the two pieces of parent material and the two copper shoes contains the molten slag bath and weld pool. The shape of the contoured face of each copper shoe also determines the finished surface appearance of the weld bead. As the weld is being produced, the slag bath comes in contact with the copper shoe and cools which causes a thin layer of flux to deposit on the exposed portion of the shoe. If the contact point of the copper shoe does not fit firmly against the parent material, an air gap forms between the plate and the copper shoe. As the weld progresses, hot flux flows between the shoe and the parent material, causing the plate to undercut. Parent Material

Water Cooled Copper Shoes

Plan View of Weld

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TWI THE WELDING INSTITUTE A certain amount of flux is lost as it plates against the copper shoes. This lost flux is replaced periodically to maintain a depth of approximately one inch. If the flux level falls lower than one inch, the weld becomes unstable. The sound of the puddle is louder as the puddle becomes more agitated. If the puddle depth shrinks too much (to less than one-half inch), arcing may occur between the guide tube and the sidewalls of the parent material. To maintain a proper flux height, flux is continually added to the puddle during the welding operation. Sometimes the welding operator adds too much flux during the weld, and the puddle becomes too deep. When the flux is too deep, it chills the weld and incomplete fusion, or cold-roll on the wet lines occurs. Flux should be added only as needed. As operators gain more experience, the sound of the weld is a guide for adding or withholding flux. Electroslag welding differs from the arc welding processes because there is no arc except at the beginning of the weld. The resistance of the molten flux to the electrical current produces the heat needed for welding. Conventional constant-voltage power sources allow easier control of the amperage and voltage. Constant-current power sources are not recommended because control has proven difficult. In constant-voltage systems, the power supply maintains the voltage at a constant level, which gives a flat, or nearly flat, volt-ampere characteristic. Processes that require continuously fed electrode wire frequently use this type of power source. Setting the voltage level on the power source controls the welding voltage. The electrical load on the power supply controls the current. The load varies by the speed of the welding wire feed and by the diameter of the wire. That is why amperage varies so much during an electroslag weld. As the slag puddle rises, it eventually reaches the bottom of the guide tube, and submerses the guide tube in the molten puddle. The power supply sees this large cross-sectional load, and increases current to supply the load. As the slag puddle melts the bottom of the guide tube, a gap forms between the bottom of the guide tube and the top of the slag puddle and power supply amperage decreases. In a like manner, as feed speed increases, power supply amperage increases. As wire feed speed decreases, power supply amperage decreases. The self-regulating characteristic of the constantvoltage power source comes about by the ability of this type of power source to adjust its welding current in order to maintain a fixed voltage level.

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5.2

WELD TERMINOLOGY

5.2.1

Types of Joint

Butt

Corner

Lap

Tee

Edge Cruciform

5.2.2

Types of Weld

Fillet

Single Vee Butt

Double Vee Butt

Part Penetration

Plug

Single Bevel Butt

Fillet

Spot (fusion) Edge

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Joint Preparations Angle of Bevel

Included Angle

Angle of Bevel

Included Angle

Root Face

Root Face Root Radius

Root Gap

Single Vee Butt

Root Gap

Single J Butt

Included Angle

Included Angle

Root Face

Root Face

Root Gap

Single Bevel Vee Butt

Included Angle

Angle of Bevel

Root Gap

Single Bevel J Butt

Included Angle

Angle of Bevel

Root Face

Root Face

Root Gap

Double Vee Butt

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Root Radius

Root Gap

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Weld Zone Terms Fusion Penetration

Weld Zone Fusion Face

Cap

Fusion Penetration

Weld Zone

Weld Junction

Fusion Face

Weld Root Hot Pass Weld Root

Weld Junction

HAZ Heat Affected Zone

HAZ Heat Affected Zone

Root, fusion penetration, weld junction and zones of typical weld

Throat Weld width

Weld width Weld Toes

Weld Toes

Weld Toes Leg (Length)

Weld Toes

Weld Toes

Weld Toes

Leg (Length)

Examples of toes, legs and weld widths of typical welds

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Throat Thicknesses

Actual Throat Thickness


Satisfactory Throat Thickness

Design Throat Thickness

Throat Thickness Under Design Thickness




Throat Thickness in Excess of Design Thickness


Satisfactory Throat Thickness Design Throat Thickness



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5.3

WELD DEFECTS

5.3.1

Cracks

“A linear discontinuity caused by fracture”

A crack is the most serious type of defect. If the localised stresses within the weld or base metal exceed the tensile strength of the material a crack will occur. Welding related cracks are generally brittle in nature, exhibiting little plastic deformation at the crack boundaries. When a crack exists there is a very real possibility that it will propagate due to the forces exerted at the crack boundaries caused by the stresses within the material. If a crack exists in the weld zone, the application specification may require the complete weld be cut out, rather than a localised repair be attempted. Some specifications will allow local repairs of a cracked area, but very few will allow the acceptance of a detected crack, no matter how small. Cracks may be defined by their location and/or direction and shape:_ Toe, underbead, edge, centreline, fusion zone, HAZ, crater, parent metal, longitudinal, transverse, multi-directional, chevron or branched. The crack may be smooth, jagged, exist as a single line, branched, intermittently or be multi-directional. Essentially there are four crack types caused by welding:A. Solidification cracks B. Hydrogen induced cracks (HIC) C. Lamellar Tearing D. Reheat cracks A. Solidification Cracks Cracking which occurs during the weld solidification process is termed as solidification or hot cracking, and usually takes place in steels which have a high sulphur content. The sulphur, which has a high melting point, causes lowered ductility at elevated temperatures. In order for a crack to develop the solidifying metal must be subject to high tensile stresses, often caused by the contraction of the solidifying weld metal which is under constraint. Solidification cracking usually occurs longitudinally down the centre of the weld, hence it’s other common name of centreline cracking. Due to the segregation of impurities congregating in this area upon solidification of the weld metal and if there are high longitudinal stresses then transverse cracks may develop, for example, on large submerged arc welds. If the stresses are in the transverse direction then there is a strong possibility of centreline cracking occurring. A crater crack, one which forms in the stop/start craters of a weld run (due to incorrect welding technique), is a form of solidification crack. As the weld pool in the crater starts to solidify, contraction occurs which may give rise to the formation of a pore in the middle of the crater. The stresses induced by the contraction may also allow the weld metal to crack. The characteristic shape of these cracks gives the crater crack it’s other name of star crack, while it’s position gives it the name of stop/start crack. Liquation cracks, which are very small and can initiate hydrogen induced cracking, are another form of solidification crack and may be caused in part by the presence of materials within the metal which have a lower defined melting point than that of the metal itself., These low melting point materials usually congregate near the grain boundaries and can cause problems in the HAZ,

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TWI THE WELDING INSTITUTE where the temperature is not sufficient to melt the parent material, but is enough to melt the grain boundaries. Where this melting takes place in the presence of high tensile stresses (where the weld material is in contraction), then the grain boundaries are pulled apart and a liquation crack will occur. Sulphur is a major liquation material. If the weld has a high heat input, then the sulphur in the HAZ is taken into solution by the surrounding steel, and upon cooling precipitates out as sulphides which cause embrittlement in the grain boundaries thereby significantly weakening the steel. When this happens the steel is said to be burned.. Another cause of liquation cracking may be from incorrectly fitted earth clamps or poorly insulated welding cables which can result in copper pick-up. B. Hydrogen Induced Cracking The causes of hydrogen induced cracking are involved and not fully understood, but the formation of HICC (hydrogen induced cold cracking) in steel is thought to be caused by hydrogen entering the weld through the welding arc, either from moisture in the atmosphere, contamination on the weld preparation, moisture in the electrode flux or even from the incorrect selection of flux in MMA or SAW welding. The intense heat in the arc is sufficient to break down the molecular hydrogen (H2) from the moisture (H2O) into atomic hydrogen (H). When the weld is molten the iron atoms are in a mobile state and there are relatively large gaps between the atoms. As the hydrogen atoms are so small, the smallest atom known, they are able to infiltrate these gaps between the iron atoms. As the weld cools down the majority of the hydrogen atoms diffuse outwards into the parent material and to atmosphere, but some of the atoms are trapped within the iron matrix as it cools down and contracts. Below 2000C the hydrogen prefers to be in the molecular form as opposed to the atomic form and so the atoms are attracted to each other, and as the cooling continues they congregate as microscopic bubbles of hydrogen. As the metal continues to cool and contract the hydrogen these bubbles exert a great deal of pressure on the structure of the metal, - between 60,000 and 200,000 psi. When the hydrogen molecules exist in large numbers this pressure will make the surrounding metal react in one of two ways:1. Where pearlite or other ductile grain structures exist, resulting in a body centred cubic lattice, it will deform slightly to reduce the pressure. 2. Where the grain structure is more brittle, such as martensite, (where the crystal structure is stretched/strained, but with the same characteristics as a BCC lattice), then the metal is liable to separate under the pressure and it will crack. These fractures are more likely to occur in the HAZ, as this area is more likely to contain brittle grain structures. The presence of external stress is normally required to initiate and propagate a crack. Lower temperatures will also decrease the fracture toughness of the steel as well as increasing the H2 pressure. Therefore it can be seen that before HIC can occur the following three criteria must exist. Susceptible grain structure

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TWI THE WELDING INSTITUTE Removing one or more of the above will reduce the possibility of HIC. This is usually achieved by controlling the hydrogen content by using low hydrogen electrodes which have been stored in an atmosphere with a low moisture content, or if they have already absorbed moisture, be dried before use according to the manufacturer’s recommendations. The part to be welded must also be dried to remove moisture from the surface, by applying a minimum preheat with either gas burners or electric heating pads. Correct fabrication sequencing to reduce stress levels should also be applied. C. Lamellar Tearing Lamellar tearing may occur in the parent plate or HAZ of steels with poor through thickness ductility where the fusion boundary is parallel to the plate or pipe surface – always in the direction of rolling. It is usually associated with restrained joints subject to through thickness stresses such as corners, tees or cruciform joints – where a large volume of weld is deposited with high restraint. Plates with high sulphur content, although other non-metallic inclusions may play their part, and the presence of hydrogen increases a steel’s susceptibility quite significantly. Lamellar tearing has a characteristic step like appearance.

Direction of rolling

D. Reheat Cracking Most alloy steels are subject to an increase of embrittlement of the coarse grained region of the HAZ when heated above 6000C, for example, during heat treatment or when working at elevated temperatures. Re-heat cracking, also known as stress relaxation cracking, mainly occurs in the HAZ of welds, particularly low alloy steels when heated above these temperatures. The problem is made worse in thicker sections containing chromium, copper, molybdenum, vanadium, niobium, titanium and those containing sulphur and phosphorus. Typical steels susceptible to reheat cracking would be creep resisting steels, such as 2Cr-Mo-V. During the post weld heat treatment process, or when a steel is operating at elevated temperatures, the residual stresses will be relieved by creep deformation - where the grains either deform or slide over each other at the boundaries. If, due to high creep resistance this cannot happen, then the grain boundaries may be forced apart, resulting in cracks appearing.

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TWI THE WELDING INSTITUTE Re-heat cracks most frequently occur in areas of high stress concentration, such as from existing defects e.g. incomplete root penetration, toes of poorly formed fillets, LOF, undercut, etc. To try and overcome this problem the dressing of weld profiles, rejection of poor profiles, use of full penetration welds in favour of partial penetration welds, selection of steels resistant to liquation cracking, employing steels with the lowest strength acceptable for the job and carrying out controlled heat treatment may be carried out. It should be noted that even though a crack may only be detected after post weld heat treatment has been carried out it is not necessarily a re-heat crack.

a

b

c

d

e

f

g

a) Crater Crack b) Toe Crack c) Root Crack d) Centreline Crack e) Sidewall Crack f) Chevron Crack g) Transverse Crack 5.3.2 Defects and their causes Not all defects associated with welding are cracks, the following are a brief description of the defects which may be found and their probable causes. 5.3.3 Incomplete Root Penetration The failure of the weld metal to extend through the full depth of the root faces of the weld preparation. Other terms for this defect are lack of penetration (LOP) or lack of root penetration (LORP). In the case of double V preparations this could be called lack of interpenetration or lack of cross penetration.

Incomplete Penetration Single V

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TWI THE WELDING INSTITUTE The causes of incomplete penetration are:• root face too large • root gap too tight • welding arc too long • wrong polarity • electrode too large for the joint preparation • incorrect electrode angle • travel speed too fast for current selected 5.3.4 Root Concavity A depression on the underside of the root pass, although the weld has caught on both sides of the weld preparation (no sharp edge of root preparation evident). This may also be called underwashing, or on short lengths a suckback. Often, specifications will allow root concavity providing that the through thickness of the nominal thickness is maintained, (enough cap is present to compensate for the lack of root material). Causes include:• Root face too large • Low arc energy • Excessive backpurge with TIG welding

5.3.5 Burn Through A severe depression or crater type hole in the root area of single sided welds where the molten pool has collapsed due to excessive penetration or excessive weld current. The burn through, or suckback is usually a ‘keyhole’ shape with the diameter larger than the root run. Causes include:• Excessive amperage during the welding of the root run and hot pass • Excessive grinding of root run, resulting in insufficient material remaining to support hot pass allowing it to blow through the root • Improper welding technique

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TWI THE WELDING INSTITUTE 5.3.6 Excessive Pentration Excess weld metal protruding beyond the normal depth of root penetration bead in a single V butt, or through weld metal previously deposited from either side of a multi-run joint. Causes include:• Root gap too large • High arc energy • Speed of travel too slow

5.3.7 Incompletely Filled Groove A continuous , or intermittent, channelling of the surface of the weld, running along the length, due to insufficient weld metal. The channel may be along the centre or to one or both sides of the weld groove. Causes include:• Root gap too large • High arc energy • Speed of travel too slow

5.3.8 Shrinkage Groove A shallow groove in the root caused by the contraction in the weld metal along each side of the penetration bead. Causes include:• Electrode too large • High arc energy • Speed of travel too fast • Incorrect welding angle

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TWI THE WELDING INSTITUTE 5.3.9 Undercut An irregular groove at the toe of a run running along the edge of the parent material, or previously deposited weld metal, due to welding. Causes include:• Excessive welding current • Welding speed too high • Incorrect welding angle • Excessive weaving • Electrode too large

5.3.10 Overlap Excess of weld metal at the toe of a weld covering the parent metal, but not fused to it Causes include:• Excessive welding current • Welding speed too low • Incorrect welding angle • Excessive weaving • Electrode too large

5.3.11 Excessive dressing (underflushing) A reduction of metal thickness caused by the removal of the surface of the weld and adjacent areas to below the surface of the parent metal.

Causes include:• Excessive grinding

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TWI THE WELDING INSTITUTE 5.3.12 Mechanical Marks Grooves or indentations in the surface of the parent material or weld caused by grinding wheels, hammer blows, chipping hammers or torn surface where temporary attachments have been broken off. a) b) c) d) e) a

b

c

Grinding marks Hammer Blows Chipping Hammer Marks Torn Surface Corrosion

e d

5.3.13 Lack of Side Wall Fusion and Lack of Inter-run Fusion Lack of union between the weld metal and the parent material on the fusion face, or between adjacent runs of the weld metal in a multi-run weld. Causes include:• Contaminated weld preparation, preventing the melting of the material underneath • Amperage too low • Amperage too high, causing the welder to increase speed of travel • Excessive inductance in MIG or MAG dip transfer welding • See also causes for LOP

5.3.14 Lack of Root Fusion Lack of union at the root of the joint. Similar to LOP, but the weld has fused at one side of the preparation, but not the other. Causes include:• As above for LOSWF

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TWI THE WELDING INSTITUTE 5.3.15 Slag Inclusion Weld slag or other foreign matter trapped between runs of weld metal. Usually formed by slag from the previous run that has not been re-melted. The defect is normally more irregular in shape than a gas pore. If the slag inclusion is against the side wall then one side of the defect will exhibit all the characteristics of LOSWF (as shown below). Causes include:• Lack of Inter-run cleaning • Amperage too low, preventing the slag from melting from the previous run • Speed of travel too fast.

5.3.16 Tungsten Inclusion An inclusion of tungsten from the electrode in tungsten inert gas welding, where the electrode is brushed against the weld pool or the preparation whilst the tip is semi-molten. Causes include:• Poor welding technique

5.3.17 Copper Inclusion An inclusion of copper due to the accidental melting of the contact tube or nozzle in selfadjusting and controlled-arc welding techniques, or to pick-up by the contact between the copper nozzle and the molten pool in MIG/MAG welding. The image on a radiograph will not be as white or well defined as a tungsten inclusion as the tungsten is a denser material. Causes include:• Poor welding technique

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TWI THE WELDING INSTITUTE 5.3.18 Porosity Porosity is the presence of cavities in the weld metal caused by the freezing in of gas released from the weld pool as it solidifies. The porosity can take several forms: • distributed • surface breaking pores • wormhole • crater pipes 5.3.18.1 Distributed porosity and surface pores Distributed porosity is normally found as fine pores throughout the weld bead. Surface breaking pores usually indicate a large amount of distributed porosity

Uniformly distributed porosity

Surface breaking pores (T fillet weld in primed plate)

Porosity is caused by the absorption of nitrogen, oxygen and hydrogen in the molten weld pool which is then released on solidification to become trapped in the weld metal. Nitrogen and oxygen absorption in the weld pool usually originates from poor gas shielding. As little as 1% air entrainment in the shielding gas will cause distributed porosity and greater than 1.5% results in gross surface breaking pores. Leaks in the gas line, too high a gas flow rate, draughts and excessive turbulence in the weld pool are frequent causes of porosity. Hydrogen can originate from a number of sources including moisture from inadequately dried electrodes, fluxes or the workpiece surface. Grease and oil on the surface of the workpiece or filler wire are also common sources of hydrogen. Surface coatings like primer paints and surface treatments such as zinc coatings, may generate copious amounts of fume during welding. The risk of trapping the evolved gas will be greater in T joints than butt joints especially when fillet welding on both sides.

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TWI THE WELDING INSTITUTE Unwanted gas sources should be identified and removed as follows: • Air entrainment - seal any air leak - avoid weld pool turbulence - use filler with adequate level of deoxidants - reduce excessively high gas flow - avoid draughts • Hydrogen - dry the electrode and flux - clean and degrease the workpiece surface • Surface coatings - clean the joint edges immediately before welding - check that the weldable primer is below the recommended maximum thickness 5.3.18.2 Wormholes Characteristically, wormholes are elongated pores which produce a herring bone appearance on the radiograph. Wormholes are indicative of a large amount of gas being formed which is then trapped in the solidifying weld metal. Excessive gas will be formed from gross surface contamination or very thick paint or primer coatings. Entrapment is more likely in crevices such as the gap beneath the vertical member of a horizontal-vertical, T joint which is fillet welded on both sides. When welding T joints in primed plates it is essential that the coating thickness on the edge of the vertical member is not Elongated pores or wormholes above the manufacturer's recommended maximum, typically 20µm, through over-spraying. Eliminating the gas and cavities prevents wormholes. • Gas generation - clean the workpiece surfaces - remove any coatings from the joint area - check the primer thickness is below the manufacturer's maximum • Joint geometry - avoid a joint geometry which creates a cavity 5.3.18.3 Crater pipe A crater pipe forms during the final solidified weld pool and is often associated with some gas porosity. This imperfection results from shrinkage on weld pool solidification. Consequently, conditions which exaggerate the liquid to solid volume change will promote its formation. Switching off the welding current will result in the rapid solidification of a large weld pool. In TIG welding, autogenous techniques, or stopping the wire before switching off the welding current, will cause crater formation and the pipe imperfection.

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TWI THE WELDING INSTITUTE Crater pipe imperfection can be prevented by removing the stop or by welder technique. • Removal of stop - use run-off tag in butt joints - grind out the stop before continuing with the next electrode or depositing the subsequent weld run • Welder technique - progressively reduce the welding current to reduce the weld pool size - add filler (TIG) to compensate for the weld pool shrinkage If the imperfections are surface breaking, they can be detected using a penetrant or magnetic particle inspection technique. For sub surface imperfections, detection is by radiography or ultrasonic inspection. Radiography is normally more effective in detecting and characterising porosity imperfections. However, detection of small pores is difficult especially in thick sections. Remedial action normally needs removal by localised gouging or grinding but if the porosity is widespread, the entire weld should be removed. The joint should be re-prepared and re-welded as specified in the agreed procedure. 5.3.19 Arc Strike Accidental striking an arc from the electrode onto the parent material outside the weld preparation area primarily causes this type of defect. Due to the very fast heating and cooling which takes place the arc strike will have a very brittle structure, especially on high carbon equivalent steels. This gives rise to stress raisers and a very real danger of cracks propagating from them. Other possible causes of arc strikes are poor insulation on the electrode holder or welding cables, poor earth contacts and magnetic particle inspection using current flow techniques.

5.3.20 Spatter During the welding process small droplets of weld material may be ejected from the weld due to high arc energy. Some of these droplets may solidify on the surface, which may or may not fuse with the parent material or weld surface. Apart from looking unsightly, these globules of metal may mask the presence of other defects, limit the ability of NDT inspections such as ultrasonics to be carried out and, if they are detached after painting or protective coatings are applied then localised corrosion will occur. Causes include:• Excessive arc energy • Excessive arc length • Damp electrodes • Arc-blow, where localised magnetic fields cause the arc to wander

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TWI THE WELDING INSTITUTE 5.4. Post Heat Treatment Post heat treatments are processes carried out whilst the metal is in the solid state designed to improve or change the metallurgical or mechanical properties of the metal, or to remove residual stresses, by controlled heating, soaking and cooling after casting, forging or welding. The metals can be softened, toughened, hardened or stresses relaxed dependant upon the temperatures attained, the length of time the temperatures are maintained and the rate of cooling. 5.4.1 Stress relieving The metal is heated to a temperature where residual stresses from previous working, such as welding or cold working, are relaxed. The temperatures attained are not sufficient to produce austenitic grain structure and so there will be no change in the metallurgical structure. Different metals and alloys will have different eutectoid temperatures, for example, the eutectoid temperature of steel is 7270C. Stress relieving is achieved by heating to below the eutectoid temperature, i.e. between 550 – 6500C, holding for the required time (often stated as 1 hour for each 25mm wall thickness), and then cooled in still air. Heating is carried out by electric heating elements, gas burners or by placing the component in a furnace. 5.4.2 Annealing Annealing is the term given to a class of heat treatments used to soften metals, to allow metalworking operations to be carried out economically and without damage to the work-piece or tooling. Annealing improves hot and cold working characteristics, increases machinability, reduces internal stresses arising from machining, forging, pressing and welding, and also conditions material for subsequent hardening operations. The temperature required for annealing should be slightly above the critical point, which varies for different steels. Low-carbon steel should be annealed at about 9000C, and high-carbon steel at between 760 and 815 0C. This temperature should be maintained just long enough to heat the entire piece evenly throughout. Care should be taken not to heat the steel much above the decalescence or hardening point. When steel is heated above this temperature, the grain assumes a definite size for that particular temperature, the coarseness increasing with an increase in temperature. Moreover, if steel that has been heated above the critical point is cooled slowly, the coarseness of the grain corresponds to the coarseness at the maximum temperature; hence, the grain of annealed steel is coarser, the higher the temperature to which it is heated above the critical point. Full annealing consists of heating the work-piece to above the upper critical temperature and slow cooling, usually in the furnace. It is generally only needed for the higher alloy steels, cast irons and complex alloys. Long treatment times are necessary to produce optimum softening in the case of the higher alloy steels. 5.4.3 Sub-critical Annealing Minimum hardness and maximum ductility of steel can be produced by a sub-critical annealing which is, as the name implies, carried out at temperatures below the lower critical temperature. It is mainly carried out in the temperature range 630 to 700 deg.C. It reduces hardness by transforming the iron carbide into small spheres or nodules in the ferrite matrix. In order to start with small grains the process is usually performed on normalised steel. It is possible to produce spheroidisation of the cementite phase instead of forming the normal lammellar pearlite and

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TWI THE WELDING INSTITUTE ferrite structure. Spheroidising is a useful technique for softening high carbon steels to improve machinablity, however because of the lengthy soak times and therefore high costs spherodising is not carried out nearly as much as annealing or normalising. 5.4.4

Normalising

The purpose of normalising is somewhat similar to that of annealing with the exceptions that the steel is not reduced to its softest condition and the pearlite is left rather finer instead of the larger, coarser grains in annealed steels. Normalising involves heating the steel to about 40 oC above its upper critical limit. (The same temperature as that used for annealing). The steel is then held at this temperature for a period of time and is then cooled in air. It is desirable that the temperature of the steel shall be maintained for a time period more than 2 minutes per mm of section thickness and shall not exceed the upper critical temperature by more than 50oC. The structure produced by this process is pearlite (eutectoid) or pearlite in a ferrite matrix (hypoeutectoid) or pearlite in a cementite matrix (hypereutectoid). Because the steel is cooled in air the process results in a fine pearlite formation with improved mechanical properties compared to the full annealing process. Refinement of grain size, relief of internal stress and improvement of structural uniformity together with recovery of some ductility provide high toughness qualities in normalised steels Normalising is used to •

To refine the grain structure and to create a more homogeneous austenite when a steel is to be reheated for quench hardening or full annealing

•

To encourage reduced grain segregation in castings and forgings and provide a more uniform structure

•

To provide moderate hardening

5.4.5 Hardening Hardening involves heating a steel to its normalising temperature and cooling (Quenching ) rapidly in a suitable fluid e.g oil, water or air. Steel is basically an alloy iron and carbon some steels alloys have have various other elements in solution. When steel is heated above the upper critical temperature (about 760oC), the iron crystal structure will change to face centered cubic (FCC), and the carbon atoms will migrate into the central position formerly occupied by an iron atom. This form of red-hot steel is called austentite (γ iron). If this steel form cools slowly, the iron atoms move back into the cube forcing the carbon atoms back out, resulting in soft steel called pearlite. If the sample was formerly hard, this softening process is called annealing. If the steel is cooled quickly (quench) by immersing it in oil or water, the carbon atoms are trapped, and the result is a very hard, brittle steel. This steel crystal structure is now a body centered tetragonal(BCT) form called martensite.

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TWI THE WELDING INSTITUTE 5.4.6 Tempering Hardened steel can be tempered or made softer and less brittle by re-heating it to a certain temperature (depending on the nature of the steel and its intended use), and then cooling. When steel is tempered by the colour method, the temper is gauged by the colours formed on the surface as the heat increases. First the surface is brightened to reveal the colour changes, and then the steel is heated either by placing it upon a piece of red-hot metal, a gas-heated plate or in any other available way. As the temper increases, various colours appear on the brightened surface. First there is a faint yellow which blends into straw, then light brown, dark brown, purple, blue and dark blue, with various intermediate shades. The temperatures corresponding to the different colours and shades. Turning and planing tools, chisels, etc., are commonly tempered by first heating the cutting end to a cherry-red, and then quenching the part to be hardened. When the tool is removed from the bath, the heat remaining in the unquenched part raises the temperature of the cooled cutting end until the desired colour (which will show on a brightened surface) is obtained, after which the entire tool is quenched. The foregoing methods are convenient, especially when only a few tools are to be treated, but the colour method of gauging temperatures is not dependable, as the colour is affected, to some extent, by the composition of the metal. The modern method of tempering, especially in quantity, is to heat the hardened parts to the required temperature in a bath of molten lead, heated oil, or other liquids; the parts are then removed from the bath and quenched. The bath method makes it possible to heat the work uniformly, and to a given temperature with close limits. 5.4.7 Hydrogen Release Where trapped hydrogen within a weld may give rise to hydrogen induced cracking (HIC) it is often advantageous to assist its removal to atmosphere by means of controlled heat treatment. Stress relieving, annealing or normalising will help the release of hydrogen from the weld, but there are times when either it is not required to carry out the full heat treatment cycle, or it is not financially viable. The weld is heated to 150 to 2000C and soaked at that temperature for between 2 and 24 hours. This will allow the molecules within the material to expand sufficiently for the hydrogen to migrate to atmosphere.

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TWI THE WELDING INSTITUTE 5.5 Machining and Material Removal The removal of material from a component depends on a large number of factors, so it is difficult to define the ideal method to use in terms of material properties. Hardness, grain structure, direction of working or rolling and heat treatment condition all play an important part in deciding the machinability of a material. Removal of excess material may be achieved in a number of ways all of which use cutting tools ranging from diamonds, (the hardest material known to man), to water (Ultra-high pressure water jets can cut through steels up to several inches thick). 5.5.1 Turning The material is rotated whilst clamped in a lathe with the cutting tool, usually high speed tool steel, removing metal into a circular configuration. 5.5.2 Boring Similar to turning, but the material will be removed from the internal diameters. The component may be held stationary and the tool revolved round to produce the cutting action, or vice versa. The main problems encountered with turning and boring are:incorrect location of holes, incorrect diameters, rough surface finish, surface tears, chatter marks. 5.5.3 Drilling The twist drill is the most common tool for cutting holes of specified diameters, which is usually held in a chuck or spindle and rotated to produce a hole the same diameter as the drill bit. A reamer is sometimes used if a greater accuracy or surface finish is required. Faults include:poor surface, metal spread, oversize holes, tapered holes and broken drill bits in the hole. 5.5.4 Milling Milling machines can perform a range of cutting operations for shaping the surface using a rotating multi-toothed cutting tool transversing the work table. Faults include:- poor surface finish, tearing and burnishing the surface . 5.5.5 Shaping and Planing Both operations use a tool which travels in a straight line with a reciprocating action. The tool either cuts in a vertical or horizontal line with the tool cutting on the forward stroke. Problems are the same as for milling. 5.5.6 Broaching Similar to shaping, but using multiple teeth cutters with the teeth giving a progressively deeper cut. Broaching is used to cut both external and internal shapes, commonly slots. Problems are the same as for milling. 5.5.7 Grinding Because any shape surface made by any other process or machine require grinding as a finishing operation, or material may be removed by grinding alone, there are a great number of grinding machine types from hand grinders to large, high speed grinding finishers. There are various

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TWI THE WELDING INSTITUTE materials, including: caborundum (silicon carbide), zirconia alumina, ceramics , diamond and tool steels which are used with a variety of belts, discs, wheels, cones, segments, pads and rolls. 5.5.8 Arc-Air Gouging An electric arc is generated between the tip of a carbon electrode and the work piece. The metal becomes molten and a high velocity air jet streams down the electrode to blow it away, thus leaving a clean groove. The process is simple to apply (using the same equipment as MMA welding), has a high metal removal rate, and gouge profile can be closely controlled. Disadvantages are that the air jet causes the molten metal to be ejected over quite a large distance and, because of high currents (up to 2000A) and high air pressures (80 to 100 psi), there will be changes to the microstructure and mechanical properties of the parent material.

Turning

Shaping

Boring

Broaching

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6.0 SERVICE DEFECTS Not all defects are caused during manufacture, whilst the component is in service degradation occurs due to movement, heating, corrosion or erosion. 6.1 Fatigue Cracks A component may fail under sufficient cycles of repeated loading, even though the maximum load applied is considerably less than the actual tensile strength of the material. Failure will occur at an even lower stress level if the cyclic loading is reversed, alternating tension and compression rather than when the load is repeated in the same direction time after time. Fatigue failure normally starts at some spot where the stress concentration is high because of the shape of the member or some imperfection. Holes through the material, notches on the surface, internal flaws, such as voids, cracks, inclusions or even surface scratches and faults caused by corrosive attack on the grain boundaries, may be the source of fatigue cracking. With repeated stressing, a crack starts at one of these fatigue nuclei and grows until insufficient solid material remains to carry the load. Complete failure results in a sudden, brittle manner. The exposed surface of the fatigue failure shows it to be a smooth and polished, while the rest exhibits a welldefined grain structure. The crystalline portion was separated in a sudden, final break. Fatigue failure accounts for up to 90% of failures in moving or parts subject to vibration. 6.2 Stress Corrosion Cracking A serious type of corrosion which occurs when the material is in a state of tensile stress and in contact with a corrosive medium. The corrosion will follow the grain boundaries from the metal surface. The impact of stress corrosion cracking (SCC) on a material usually falls between dry cracking and the fatigue threshold of that material. The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses, such as in an aircraft component (see below) Stress on aircraft parts may be residual within the part as a result of the production process or externally applied cyclic loading. Press-fit bushings, tapered bolts and severe metal forming are examples of high residual tensile stresses which can lead to stress cracking.

Cold deformation and forming, welding, heat treatment, machining and grinding can introduce residual stresses. The magnitude and importance of such stresses is often underestimated. The residual stresses set up as a result of welding operations tend to approach the yield strength. The build-up of corrosion products in confined spaces can also generate significant stresses and should not be overlooked. SCC usually occurs in certain specific alloy-environment-stress combinations.

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Usually, most of the surface remains unattacked, but with fine cracks penetrating into the material. Note that the crack follows the line of the grain boundaries. (Inconel X500)

The micrograph above (X300) illustrates SCC in 316 stainless steel. Chloride stress corrosion cracking in austenitic stainless steel is characterised by the multi-branched "lightning bolt" transgranular crack pattern.

Chloride SCC One of the most important forms of stress corrosion that concerns the nuclear industry is chloride stress corrosion. Chloride stress corrosion is a type of intergranular corrosion and occurs in austenitic stainless steel under tensile stress in the presence of oxygen, chloride ions, and high temperature. It is thought to start with chromium carbide deposits along grain boundaries that leave the metal open to corrosion. This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and use of low carbon steels. Caustic SCC Despite the extensive qualification of Inconel for specific applications, a number of corrosion problems have arisen with Inconel tubing. Improved resistance to caustic stress corrosion cracking can be given to Inconel by heat treating it at 620 oC to 705oC, depending upon prior solution treating temperature. Other problems that have been observed with Inconel include wastage, tube denting, pitting, and intergranular attack. Prevention or Controlling SCC The most effective means of controlling SCC is:1) design properly with the right materials; 2) reduce stresses; 3) remove critical environmental species such as hydroxides, chlorides, and oxygen; 4) and avoid stagnant areas and crevices in heat exchangers where chloride and hydroxide might become concentrated. Low alloy steels are less susceptible than high alloy steels, but they are subject to SCC in water containing chloride ions.

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6.3 Stress Corrosion Propagation Rate Stress corrosion cracks propagate over a range of velocities from about 0.0001to 10 mm/h, depending upon the combination of alloy and environment involved. Their geometry is such that if they grow to appropriate lengths they may reach a critical size that results in a transition from the relatively slow crack growth rates associated with stress corrosion to the fast crack propagation rates associated with purely mechanical failure. 6.4 Hydrogen Cracking Hydrogen induced cracking is normally thought of as a welding defect, but if enough hydrogen enters the material whilst in service; - from chemicals being transported or stored, or from hydrogen sulphide in stagnant water or rotting plants - or secondary processing; – such as during etching, pickling or plating – then the defect can be categorised as an in-service defect. As mentioned in section 5.3.1 the structure must not only have sufficient hydrogen present, but also have a susceptible grain structure and suffer from sufficient stress. Hydrogen cracking may be in the form of a fine network of multidirectional cracks, as a linear or wandering line or it may propagate as blistering on the surface. 6.5 Grinding Cracks Grinding cracks only occur in materials which can be hardened and usually occur in groups at right angles to the direction of grinding, or, if rotary grinding is carried out, as a network of cracks. They are commonly caused by use of the incorrect grade of abrasive, resulting in a too aggressive surface removal, by applying too much force or by loss of grinding fluid. The surface will be in each of these cases be heated up to too high a temperature, and due to the localised heating, will cool down very quickly, resulting in a form of quenching. 6.6 Corrosion In general, corrosion is the deterioration of metals by the chemical action of the steel with either liquid, gas, or both. The term corrosion is used to describe action which is detrimental., but the principal is actually used to benefit in some cases. For example, acids and alkalies are used to corrode metal away in the manufacturing process of chemical milling. Aluminium alloys are frequently anodised to produce an oxide coating that resists further oxidation. Corrosion attacks metals by direct chemical action or by electrolysis, or commonly, a combination of the two. Iron ore is an oxide of iron in chemical balance with the environment Fe2O3 or Fe3O4. When the iron ore is converted to iron, this balance is changed, the iron becomes active and it corrodes on contact with a suitable environment, such as salt water which provides an electrolyte for a corrosion cell to form. Oxidation is the reaction of a metal with oxygen to form an oxide, for example, millscale is an oxide of iron produced when the steel is hot worked. When the white hot metal comes in contact with the air it forms an oxide comprising of three layers: FeO nearest the steel, Fe3O4. And then finally Fe2O3 on the outside. The total thickness of the millscale is usually about 25 to 100µm. Due to the differences in composition of the steel and the millscale, when the steel is placed in a bath of dilute sulphuric acid the millscale will be preferentially attacked and dissolved, with no residue, with only very minor attack of the steel plate. The more reactive metal will always be attacked in preference to the more passive of the two.

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The galvanic series lists materials in order of reactivity, starting with the more reactive down to the most passive:

More Reactive

More Passive

Magnesium Aluminium Zinc Iron Steel Tin Lead Nickel Brass Bronze Copper Stainless Steel Silver Gold Platinum

It is often assumed that aluminium is less reactive than carbon steel, this is not true as can be seen by the above chart – aluminium is highly reactive, but the oxide of aluminium forms a hard, impervious layer very quickly. This hard oxide layer prevents further oxides forming and so gives the appearance of being a passive material. 6.6.1 Galvanic Corrosion When two dissimilar metals are in contact, in the presence of an electrolyte, e.g. salty water which will conduct electricity, the difference in reactivity determines the extent of reaction between them. If there is a large difference between, them in terms of reactivity, then there will be a strong reaction and a great deal of corrosion taking place. On ships, oil platforms, etc. zinc is employed to serve as cathodic protection. In the presence of an electrolyte, the zinc becomes the anode (+) and the steel the cathode (-). An electric circuit is formed, with electrons leaving and ions discharged at the anode, and electrons entering and negative ions formed at the cathode, and so as the zinc will corrode in preference to the steel. The corrosion reactions will be accelerated if there are variations in oxygen concentrations on the material surface; the presence of residual stresses, in the presence of acids, alkali’s, chlorides or sulphates or where other metals of higher nobility are in contact with the metal. Except where sacrificial corrosion protection is planned, it is not normally good practice to design products with contacting surfaces of radically different galvanic reaction if there is a likelihood of exposure to any corrosive medium.

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6.6.2 Exfoliation Corrosion This is caused by the selective corrosion on the exposed end grain structures of fibrous banded steels resulting in a laminated appearance with the ends expanding due to corrosion products. This appears as flaking and loss of metal. 6.6.3 Pitting corrosion Pitting is a localised corrosion by which pits that extend deep into the metal develop. This is a more serious corrosion than the slower general type because the pits may decrease the wall thickness, sometimes to the extent of leakage paths forming or to form the nuclei for fatigue failure. With some materials pitting rate may increase with time 6.6.4 Crevice Corrosion If two different metals, or even where two similar metals with a non-uniform electrolyte, are joined a similar reaction (current flow) to the galvanic cell, called a concentration cell, is formed. This is particularly detrimental when the chemical variation of the electrolyte is in its oxygen content. Moisture, or liquid, penetrates into the narrow gap between metal to metal contacts and, particularly if it stagnates, an electric cell is set up which will locally corrode. As it will corrode on the interface between the metals it will be very difficult to detect unless the component is dismantled. Variation of oxygen content

Variation of ion content

6.6.5 Fretting Corrosion Fretting is corrosion assisted wear resulting from small oscillatory movements between mating surfaces under load, this gives in effect two forms of breakdown; wear and corrosion. Characteristic deposits are formed .eg. ferrous based materials show a red coloured deposit, while light alloys, such as aluminium and magnesium show black deposits. 6.6.6 Filiform Corrosion When water or corrosive materials is able to infiltrate under paint or other protective surfaces, it sets up corrosive cells which results in blisters or bulges, as are often seen on car bodies. 6.6.7 Microbiological Corrosion Microbiological corrosion (MIC) refers to corrosion and ensuing loss of metal caused by biological organisms. MIC can occur in any aqueous environments, and because of the omni present nature of microbes in fluid systems, MIC is a commonly occurring phenomenon. MIC is a common problem in industrial processes due to the presence of microbes, adequate nutrients and corrosive byproducts.

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TWI THE WELDING INSTITUTE A number of metals, such as structural steels, copper alloys etc., tend to corrode generally over the entire surface in the absence of crevices or galvanic effects. In such cases, corrosion is determinated by the rate at which dissolved oxygen can be delivered to the metal surface. Biological organisms present in the aqueous medium often have the potential to increase or decrease oxygen transport to the surface; consequently, these organisms have a role in increasing or decreasing general corrosion. Most MIC, however, manifests as localized corrosion because most organisms do not form in a continuous film on the metal surface. Microscopic organisms also tend to settle on metal surfaces in the form of discrete colonies or at least spotty, rather than continuous films. 6.6.8 Intercrystalline Corrosion Created when the attack is against the grain boundaries. Following the grain boundaries from the metal surface, a crack-like discontinuity develops. Such cracks can cause material failure under static loading by reduction of load supporting cross-section. In the case of dynamic loading, they are likely to be the initiator for fatigue failure. 6.6.9 Weld Decay Due to the elevated temperatures of welding or during improper heat treatment, chromium carbides can form in the grain boundaries of stainless steel. This chemical reaction robs the alloy of chromium in the zone near the grain boundary, making those areas much less resistant to corrosion. This creates a galvanic couple with the well-protected alloy nearby, which leads to weld decay (corrosion of the grain boundaries near welds) in highly corrosive environments. Special alloys, either with low carbon content or with added carbon "getters" such as titanium and niobium (in types 321 and 347, respectively), can prevent this effect, but the latter require special heat treatment after welding to prevent the similar phenomenon of knifeline attack. As its name implies, this is limited to a small zone, often only a few micrometres across, which causes it to proceed more rapidly. This zone is very near the weld, making it even less noticeable 6.7 Wear As two materials move over each other, both materials will have their surfaces abraded due to friction. Wear occurs through a variety of mechanisms depending on the application and environment 6.7.1 Abrasive Wear Abrasion is the most common form of wear in industry. It is defined as: 'The action of a hard, sharp material cutting through the surface of a softer material' The abrasive may be products like coal, cement, rock, glass, ceramics or ore. It may be being processed; that is mined, crushed, extruded or conveyed. The consequent wear of machinery and parts is called: 'High Stress, TwoBody Abrasion' and requires hard, tough and thick surface coatings to combat it. The abrasive may be present as a contaminant in a product, for instance sand in oil being pumped ashore. Facilities like pumps and valves will have abrasive trapped between rubbing surfaces (for instance bearings), with crushing of the particles to cause sharp abrasive edges. This is called: 'Three-Body Abrasion' and, since high stresses are involved, requires hard, tough surfaces

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TWI THE WELDING INSTITUTE Finally, the abrasive may be present within the product, either as a filler or a pigment. For instance, glass fibres in plastics, pigments like TiO2 in textiles or Cr2O3 in ink (printing, paper handling), cellulose in wood or grain, abrasive elements in medical, tobacco or food products, etc. Components handling such products suffer what is called: 'Low Stress Abrasive Wear' and can usually be combated with hard, relatively thin coatings. 6.7.2 Adhesive Wear Adhesive wear is the second most common form of wear in industry. It is defined as: 'The action of one material sliding over another with surface interaction and welding (adhesion) at localised contact areas'. Adhesive wear may be between metallic materials, ceramics or polymers, or combinations. It is dependent on adhesion between the materials and that in turn depends on surface films like oxides or lubricants, as well as the mutual affinity of one material for another If loads are light and the natural spontaneous oxidation of a metal can keep up with the rate of its removal by wear, then that wear rate will be relatively low (the oxide acting as a lubricant). It is called: ‘Mild Wear’. If loads are high and the protective oxide is continually disrupted to allow intimate metallic contact and adhesion, then the wear rate will be high. It is called: ‘Severe Wear’. With materials which have thin, brittle oxides, notably stainless steel, aluminium alloys and titanium, the protective oxide is easily disrupted and the consequent massive adhesion and wear is called: ‘Galling’. The terms Mild wear, Severe wear and Galling are used with specific meanings. They are in relation to unlubricated sliding. Mild wear is characteristic of dry sliding metals where the conditions are such that the naturally protecting oxide can continuously reform at the slidng contact, so acting with a degree of dry lubrication and reducing the wear rate. It also occurs with hardened alloys (usually steels) when, even under high contact loads and speeds, the underlying substrate can support the oxide and prevent its disruption by deformation below it. Severe wear occurs (generally in soft metals or alloys) when the conditions are such that the oxide is disrupted at a greater rate than which it can reform, so that clean metal is exposed below and massive adhesion occurs between the mating surfaces. It is not uncommon for soft materials to show sudden transitions between these two wear regimes. With mild steel at low load, mild wear results. As the load is increased, a point is reached when the oxide cannot keep pace and there is a sudden 100 fold increase in wear rate. At even higher loads, the frictional heating is such that the oxidation rate rapidly increases and can again form a protective layer; and mild wear is re-established. The objective of Surface Engineering a component is often to eliminate this possibility of severe wear by hardening the surface (nitriding, carburising, etc) and supporting the natural oxide. Galling is a particular form of very severe adhesive wear reserved for materials that have thin, brittle oxides that are easily disrupted under load. It leads to seizure of fasteners and couplings in particular. It is also referred to as 'pick-up' or 'transfer'. 6.7.3 Fretting Wear Fretting occurs in situation where parts are vibrating or impacting against each other over a very limited contact, so that the relative motion is in the order of 50 to 500µ. This is particularly the case in splines and couplings where motion is transmitted from one part to another via a loaded contact. It can also occur in parts fastened or fitted together where there is a source of external vibration (e.g. heat-exchangers), as well as on bearing housings. The sequence of events is characterised by:

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TWI THE WELDING INSTITUTE 1. Vibration and sliding 2. Adhesive wear and generation of debris 3. Oxidation of that debris, which remains trapped in the small contact area 4. Abrasion by that debris, with an increased wear rate and even more debris production 5. It results in significant localised damage and sometimes a 'runaway situation. Fretting Corrosion is the term applied to situations where significant amounts of oxide dust is generated in and around the contact; a characteristic red colour in the case of steel components. Fretting Fatigue applies to situations where the load and cycles are sufficient to initiate and propagate fatigue cracks, with the onset of failure accelerated by the corrosive element of the wear process. Fretting is best combated by oxidation resistant coatings (e.g. Electroless Nickel) or softer, ductile self-lubricating coatings like silver or indium 6.7.4 Erosive Wear Erosion occurs when particles or fluids impinge upon a surface. There are two main types: • High Angle Erosion, where much of the energy is expended in deformation of the surface. It requires a resilient coating, often elastomeric. • Low Angle Erosion, where the action is more akin to abrasion and cutting. It requires a hard surface to reduce the wear rate.

High Angle Erosion

Low Angle Erosion

In general, the rate of wear is proportional to the cube of the impact velocity, as well as being related to the number of particles, their hardness, shape (sharpness) and size.

Strain

6.8 Creep The term creep is used to describe the continuous deformation of a material under constant load, producing unit stress below those of the elastic limit. At normal temperatures, the effect of creep is very small and can be disregarded. At higher operating temperatures this deformation by slow plastic flow can become very important, particularly on higher strength materials to be used at elevated temperatures.

Creep Curve Stage III

Stage I

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TWI THE WELDING INSTITUTE In a creep test a constant load is applied to a tensile specimen maintained at a constant temperature. Strain is then measured over a period of time. The slope of the curve, identified in the above figure, is the strain rate of the test during stage II or the creep rate of the material. Primary creep, Stage I, is a period of decreasing creep rate. Primary creep is a period of primarily transient creep. During this period deformation takes place and the resistance to creep increases until stage II. Secondary creep, Stage II, is a period of roughly constant creep rate. Stage II is referred to as steady state creep. Tertiary creep, Stage III, occurs when there is a reduction in cross sectional area due to necking or effective reduction in area due to internal void formation. Reheat cracking may occur in low alloy steels containing alloying additions of chromium, vanadium and molybdenum when the welded component is being subjected to post weld heat treatment, such as stress relief heat treatment, or has been subjected to high temperature service (typically 350 to 550°C). Cracking is almost exclusively found in the coarse grained regions of the heat affected zone (HAZ) beneath the weld, or cladding, and in the coarse grained regions within the weld metal. The cracks can often be seen visually, usually associated with areas of stress concentration such as the weld toe. Cracking may be in the form of coarse macro-cracks or colonies of micro-cracks. A macro-crack will appear as a 'rough' crack, often with branching, following the coarse grain region. Cracking is always intergranular along the prior austenite grain boundaries. Macro-cracks in the weld metal can be oriented either longitudinal or transverse to the direction of welding. Cracks in the HAZ, however, are always parallel to the direction of welding. The principal cause is that when heat treating susceptible steels, the grain interior becomes strengthened by carbide precipitation, forcing the relaxation of residual stresses by creep deformation at the grain boundaries. The presence of impurities which segregate to the grain boundaries and promote temper embrittlement, e.g. sulphur, arsenic, tin and phosphorus, will increase the susceptibility to reheat cracking. The joint design can increase the risk of cracking. For example, joints likely to contain stress concentration, such as partial penetration welds, are more liable to initiate cracks. The welding procedure also has an influence. Large weld beads are undesirable, as they produce a coarse grained HAZ which is less likely to be refined by the subsequent pass, and therefore will be more susceptible to reheat cracking. The risk of reheat cracking can be reduced through the choice of steel, specifying the maximum impurity level and by adopting a more tolerant welding procedure / technique.

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BASIC STEEL METALLURGY

Although plain carbon steels (less than 0.05% carbon) work well for numerous uses, and are the cheapest and most used of the alloy steels, they cannot fulfil the requirements for some work. Individual or groups of properties can be improved by the addition of various elements in the form of alloys. Even plain carbon steels are alloys of at least iron, carbon and manganese, (used to control sulphur), but the term ‘alloy steels’ is reserved for those steels which have elements added in controlled quantities greater than impurity concentrations, or in the case of manganese, greater than 1.5% In general the addition of alloying elements have marked effects on the properties of the steel by means of their interaction with the carbon. For example, the hardness and strength of any steel, alloy or otherwise, depends primarily on the amount and form of the iron carbide or other metal carbides present. Certain low alloy steels, sold under various trade names, have been developed to provide a low cost structural material with higher yield strength than plain carbon steel. The addition of small amounts of some alloying agents can raise the yield strength of hot-rolled sections without heat treatment to 30% or 40%greater than plain carbon steels. Designing to higher working stresses can reduce the required section size by 25% to 30% with a resultant cut in weight and cost. There are many elements which may be present in steels, some of which are added or reduced to certain levels to produce specific properties, for example: • Carbon (C) The key element in steels. Has a major influence on strength, toughness and hardness. Even in unhardened steel, carbon produces an increase in hardness and strength with a consequent loss of ductility. The improvements in machinability and loss of weldability are based on this loss of ductility. The term machinability is used to describe the relative ease with which any material may be machined. • Manganese (Mn) Primary desulphuriser and secondary deoxidiser. Often added in order to enable the carbon content to be reduced. Affects strength and hardness. • Silicon (S) Primary deoxidiser. Reduces toughness if too much is added. • Aluminium (Al) Grain refiner and tertiary deoxidiser. • Molybdenum (Mo) Improves creep resistance and reduces temper embrittlement. Used to improve hardenability and wear resistance, reduces weldability. • Chromium (Cr) Improves hardness and resistance to wear. In austenitic stainless steels it improves resistance to corrosion resistance. • Nickel (Ni) Improved ductility, strength and toughness. In austenitic stainless steels it improves resistance to corrosion from acids. • Sulphur (S) Considered an impurity, usually kept below 0.05% because it may promote ‘hot shortness’ which is the susceptibility of the material to crack during hot working. • Phosphorous Again considered an impurity, may promote ‘cold shortness’, the susceptibility of the material to crack during cold working.

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7.1 Grain Structure As the temperature of liquid metal is reduced and the atoms become less active, they are attracted to each other and take definite positions in three dimensional geometric patterns that are characteristic of the metal. These structures are called ‘space lattices’ and consist of network groupings of identical unit cells that are aligned in parallel planes. There fourteen types of crystal lattices, but the three main structures, which are most common and commercially important, are: 1. Body centred cubic 2. Face centred cubic 3. Hexagonal close packed 7.1.1 Body Centred Cubic Lattice The body centred cubic lattice (BCC) is made up of nine atoms. Eight are located on the corners of the cube with the ninth positioned centrally between them. The BCC is a strong structure, and in general, the metals that are hard and strong are in this form at normal temperatures. These metals include chromium, iron, molybdenum, tantalum tungsten and vanadium.

7.1.2 Face Centred Cubic Lattice Face centred cubic lattices consist of fourteen atoms with eight at the corners and the other six centred in the cube faces. This structure is characteristic of ductile materials, such as aluminium, copper, gold, lead, nickel, platinum and silver. Iron, which is BCC at room temperature, is also FCC from about 9100C to 1,4000C

7.1.3 Hexagonal Close Packed Lattice Seventeen atoms combine to make this lattice structure. Seven atoms are located in each hexagonal face, with one at each corner and the seventh in the centre. The remaining three atoms take up a triangular position in the centre of the cell equidistant from each face. The metals with this structure are quite susceptible to work hardening. Such metals include cadmium, cobalt, magnesium titanium and zinc.

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8.0

GRAIN STRUCTURE

When metals solidify and the atoms take their positions certain atoms will assume their positions ahead of the others and will form a nucleus for crystal formation. Orderly growth continues in all directions until the crystal, or as usually referred to for metals, the grain, runs into interference from other grains that are forming from other nuclei. If two grains that have the same orientation meet, they will join to form a larger grain, but if they are forming about different axes, the last atoms to solidify between the growing grains will have to assume compromise positions. These misplaced atoms are in layers about the grains and are known as grain boundaries. They are interruptions in the orderly arrangement of the space lattice and offer resistance to deformation of the metal.

a) Nucleation of crystals

b) grain growth

c) irregular grains form as grain grows

d) grain boundaries as seen under a microscope

A fine grain metal with large numbers of interruptions will therefore be harder and stronger than a coarse grained metal of the same composition and condition. The faster the metal is cooled, the more numerous will be the nucleation points and so the grain structure will be more refined. A pure metal that possesses single crystals or single grains is called anisotropic, while polycrystalline structures, consisting of many grains, are isotropic and, unlike anisotropic materials, have similar properties in all directions. The grain structure of a material will effect its mechanical properties, weldability and in-service performance. Single or multiple grain structures may be present in a material; the type and number of these grain structures will be primarily influenced by the elements in the material, the temperature reached during processing and the subsequent cooling rates. If the material is being worked, the degree of working will also influence grain structures.

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In heavy sections of castings and ingots, the first Fine equiaxial grains grains to form, round the outside, would be fine equiaxed due to the heat escaping quickly to the outside wall of the ingot or mould. Columnar and dendritic structure would then appear moving towards the centre. Finally the centre section would cool down slowly, as the heat would have greater difficulty in dissipating from the centre. This centre section will be made up from large equiaxial grains and so be the weakest structure. Changes in this grain-growth pattern can be caused by numerous Course equiaxial factors affecting the cooling rate. Thin sections that cool very quickly will not exhibit columnar or grains Typical grain structure from coarse equiaxial grains, while variable section sizes solidification of a heavy and changes in cross section will cause interference section and variations in grain-structure pattern.

Columnar grains

8.1 Recrystallisation Although some of the major distortions are eliminated by stress relieving, most of the distorted crystalline lattice remains as it was produced by cold working. The elastic limit will have been raised to close to the ultimate strength, and further deformation will cause fracture failure. Recovery of ductility can only be regained by elimination of the deformed grains, and this can be accomplished by recrystallisation. By heat treatment, new grains will be formed with fully recovered capacity for plastic flow can be formed by solid state change in the metal. It is important to note that no grain-size changes can be accomplished at any temperature below melting point unless the strained condition of cold worked metal is present. 8.2 Elastic/Plastic Deformation When a sufficient load is applied to a metal or other structural material, it will cause the material to change shape. This change in shape is called deformation. A temporary shape change that is self-reversing after the force is removed, so that the object returns to its original shape, is called elastic deformation. In other words, elastic deformation is a change in shape of a material at low stress that is recoverable after the stress is removed. This type of deformation involves stretching of the bonds, but the atoms do not slip past each other. When the stress is sufficient to permanently deform the metal, it is called plastic deformation. Plastic deformation involves the breaking of a limited number of atomic bonds by the movement of dislocations. Millions of dislocations result for plastic forming operations such as rolling and extruding. It should be noted that the force needed to break the bonds of all the atoms in a crystal plane all at once is very great. However, the movement of dislocations allows atoms in crystal planes to slip past one another at much lower stress levels. Since the energy required to move is lowest along the densest planes of atoms, dislocations have a preferred direction of travel within a grain of the material. This results in slips that occur along parallel planes within the grain. These parallel slip planes group together to form slip bands, which can be seen with an optical

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TWI THE WELDING INSTITUTE microscope. A slip band appears as a single line under the microscope, but it is in fact made up of closely spaced parallel slip planes as shown in the image below. 100 Atomic Diameters

Grain Boundaries

Individual Slip Planes

Grains

Slip Bands

Unstrained Grain

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Product Technology

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