08MAT1.5 Nickel and Nickel Alloys

Topic 5 Nickel and nickel alloys

At the completion of this topic you should be able to:

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Apprecia Appreciate te the basic basic reason reason why nickel nickel is a major alloying alloying element element

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Understa Understand nd the effect effect of the various various alloying alloying element elements s used used in in conju conjunctio nction n with with nickel nickel

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Be aware aware of of the major major allo alloys ys based based on on nickel nickel,, their their stren strength gths s and wea weakne knesse sses s and their range range of applicati applications. ons. This will include include nickel-co nickel-copper pper,, nickel-chr nickel-chromiu omium, m, nickelnickelchromi chromiumum-iro iron, n, and the supera superallo lloys. ys. Some Some other other nickel nickel alloys alloys with with speci speciali alised sed applications will also be mentioned.

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Unde Unders rsta tand nd the gene genera rall char charac acte teri rist stic ics s that that make make nick nickel el simil similarar- but differ differen ent, t, to carbon steel when it is being welded Cleaning prior to welding Weld preparation preparation Effect of age hardening Welding processes MMAW; GTAW, GMAW Shielding gas characteristics Filler material characteristics

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Be aware aware of of the the requi requirem rements ents for solderin soldering g and and brazing brazing nickel nickel and and its its alloy alloys s

Module MAT1. Welding Mettallurgy Topic 5: Nickel and nickel alloys © University of Wollongong 2001, Cranfield University 2008. All rights reserved

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Readings Peritech Pty Ltd, 2001, Chapter 5.Nickel Alloys – Function of the Alloying Elements Chapter 6: Commercial High Nickel Alloys Chapter 6 The Joining of Nickel Alloys

References ASM Metals Handbook series with Volume 6 “Welding Brazing and Soldering” Welding Handbook, 1996, Eighth edition, Volume 3, Chapter 4, Nickel and cobalt alloys, American Welding Society

R. F. Decker and C. T. Sims, in The Superalloys, Eds. C. T. Sims and W. C. Hagel, Wiley, New York, 1972, p. 33. S. Kou Welding Metallurgy Second Edition , Wiley-Interscience, 2003.

Web sites (INCO Alloys International (www.incoalloys.com) Krupp VDM (/www.kruppvdm.de) Haynes Inc (www.haynesintl.com)


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Notes

The topic will cover four main areas: -

Nickel and its dilute alloys

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The function of the alloying elements

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Commercial nickel alloys

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The welding of high nickel alloys

Survey of Types of Nickel and its Alloys Nickel is one of the most useful alloying elements available to materials engineers. In smaller amounts (<10%) it gives increased toughness and better low temperature toughness to steel.

In larger amounts it imparts a degree of resistance to both high and low temperatures and to a wide range of corrosive atmospheres. Some of the alloys and the superalloys in particular have a high creep resistance and are therefore used for turbine blade components.

SA Q 1.

Nickel is used mainly as an alloying element but it can be used in its c o m m e r c ia ll y p u r e f o r m f o r s o m e a p p l i c at i o n s . T h e re a re t w o prob lem areas with the high er carbon grade nickel (N02200). State w h a t t h e s e ar e a n d w h y t h e y o c c u r .

Peritech 2001 Corrosion and heat resisting materials chapter 5 ‘Nickel alloys, function of the alloying elements’

Nickel Alloys - Function of the Alloying Elements Heat resistance: Heat resistance is also provided by chromium through the chromium

oxide film. The effect of this film can be enhanced by silicon and aluminium. Nickel also makes the scale formed at high temperatures more resistant to spalling and hence gives metals containing higher nickel superior heat resistance. There are also a range of other alloys added for the more advanced alloys used in gas turbine operations where service in excess of 1000°C is required Corrosion resistance: Chromium is the major element used to provide atmospheric

corrosion resistance, as well as heat resistance, to nickel. Nickel, in conjunction with chromium, allows the corrosion resistance provided by chromium oxide film formed on the surface to be utilised without the damaging metallurgical changes in the microstructure found in straight iron-chromium alloys. Module MAT1. Welding Mettallurgy Topic 5: Nickel and nickel alloys © University of Wollongong 2001, Cranfield University 2008. All rights reserved

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Pure nickel and alloys of nickel with copper have excellent corrosion resistance and are used extensively in the food industry. Molybdenum additions to these alloys give superior corrosion resistance, particularly in chloride environments. Nickel is also alloyed with molybdenum alone to provide the alloys most resistant to hydrochloric acid

In summary, the function of the principal alloying elements is as follows: Aluminium: Can give improved high temperature resistance because of effect on the

chromium oxide film. It is also used with titanium to give precipitation hardening by Ni3(Al,Ti) Carbon: Generally harmful but can provide dispersion hardening if carefully controlled Chromium: Solid solution strengthener but principally there for the chromium oxide film Copper: Copper is a solid solution strengthener, and inhibits the formation of graphite,

and enables the material to be more resistant to oxidising and reducing environments. Gives improvement in non-aerated sulphuric and hydrofluoric acids Iron: Mainly low cost, non-harmful replacement for nickel Molybdenum: Significant solid solution strengthener but also considerably improves

chloride pitting and crevice corrosion. Possibly harmful at high temperatures Niobium: Solid solution strengthener and carbide stabiliser. Age hardening alloy when

weld cracking possible. Tantalum, Titanium and Zirconium: Carbide stabilisers. Titanium is also a component of an age hardening Silicon: Can provide high temperature corrosion resistance but generally restricted. Can

also provide useful carburising resistance Tungsten: Significant solid solution strengthener

Now study the chapter ‘Nickel Alloys - Function of the Alloying Elements’ in the text supplied. In order to assist your understanding you will find it helpful to do the short answer test questions ‘Nickel Alloys - Function of the Alloying Elements’’ in conjunction with this .


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SAQ 2.

O f a ll o f t h e a ll o y i n g e le m e n t s a d d ed to h ig h n i c k el a ll o y s c h r o m i u m a n d i r o n a r e p r o b a b l y t h e m o s t c o m m o n . B r i e fl y d e s c r i b e t h e f u n c t i o n each of these.

SAQ 3.

W h a t p h as e f o r m s at c h r o m i u m c on t en t s a b ov e 2 5-3 5% ?

Commercial High Nickel Alloys The main nickel alloy groups are: 

Cupronickels - Usually low (<30% Ni) and used extensively in marine applications because of their excellent corrosion resistance to salt water. There are also a group of alloys based on 60-65% Ni generally referred to as Monel® after the name given

Corrosion and heat resisting materials. Chapter ‘Commercial high nickel alloys’

by International Nickel Inc.(INCO) to this alloy. This high nickel group are also used because of their resistance to atmospheric and marine corrosion. 

Nickel Chromium Alloys -These alloys are used in applications ranging from corrosion resistance in the chemical and petrochemical industries to high temperature service in furnaces and gas turbine engine components. Some alloys in this class have their resistance to chloride corrosion improved by molybdenum additions. One particular group used in hydrochloric acid service is based on only nickel and chromium

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Nickel Chromium Iron Alloys – Iron is a common addition in commercial alloys and can be present up to around 50%.

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Superalloys - The nickel superalloys are highly creep resistant due to precipitation hardening within the grains, and due to carbides which form along the grain boundaries and prevent grain boundary sliding.

The microstructure of a typical

superalloy is shown in Figure 1.

Figure 1 Microstructure observed in a Nickel base superalloy (Decker and Sims, 1972). There is also a wide range of other nickel based alloys for specialised electrical and magnetic applications. Most alloys are available in both cast and wrought form.


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Now study the chapter ‘Nickel Alloys - Commercial High Nickel Alloys’ in the text supplied

SAQ4

T h e U NS a ll o y N 06 60 0 i s s o m e t i m e s c a ll e d t h e 6 00 a ll o y . W o u l d t h i s a l lo y b e s u g g e s t ed f o r m a x im u m c o r r o s i o n r e s i s t a n c e in a n environment where chlorides were present. If not, what similar alloy w o u l d b e r e c o m m e n d e d .?

S A Q 5:

W h a t h a p p e n s t o t h e c o r r o s i o n r e s is t a n c e o f N i -F e- Cr a l lo y s i f t h e chromium content gets above about 25-35%?

Welding Processes Chapter 5, The Joining of Nickel Alloys

Nickel and its alloys can be welded by all of the major welding processes. Oxy-acetylene is not usually used because of the difficulty of satisfactorily controlling the air-gas ratio. Nickel alloys are susceptible to the following forms of cracking when welded: 

Solidification cracking. This occurs during the final stage of solidification. The tensile stresses caused by the contraction of the weld metal exceeds the strength of the last bit of weld metal to solidify. The solubility of phosphorus and sulphur both always present to some extent - is much lower in austenite than ferrite, resulting in strong rejection of these elements from primary austenite. They tend to form low strength or liquid films around the primary austenite particles, resulting in hot cracking under shrinkage stresses. Hence high levels of sulphur and phosphorus should be avoided.

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Liquation cracking. Similar to solidification cracking, but the low melting point region is on the grain boundaries of the HAZ of the base material. For the nickel alloys, the carbides which are present to enhance creep resistance melt before the surrounding grains. Hence the melted grain boundaries form cracks, which open when the weld cools and contracts.

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Post weld heat treatment cracking (also called strain-age cracking). Often the nickel alloys are post weld heat treated to relieve stress and obtain the maximum strength through precipitation hardening. The heat treatment involves solutionising the material first followed by aging. In the solutionising process some aging will occur as the material is heated up to temperature. This results in two effects: a) A reduction in the material’s ductility due to the precipitates that form as it is heated to the solutionising temperature. b) Strains in addition to those caused by welding due to the phase changes that occur during aging.


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Both of these effects can cause the nucleation of new cracks during aging as well as the growth of existing cracks.

Figure 2 Effect of Al and Ti contents on postweld heat treatment cracking. (Kou 2003)

The ease with which nickel alloys can be welded is dependent on the alloying. Alloys which have a large amount of either aluminium or titanium which produce γ’ precipitates are particularly difficult to weld as shown in Figure 2. They exhibit both liquation and post weld heat treatment cracking. Table 1 describes the typical problems that can be experienced when welding the nickel-base superalloys together with some solutions.

Table 1 Typical problems in welding nickel-base alloys, after (Kou, 2003).

Problem

Alloy Type

Solutions

Low strength in HAZ

Heat-treatable

Resolution and artificial aging after welding.

alloys Post weld heat treatment

Heat-treatable

cracking

alloys

Use less susceptible grade (IN 718) Heat treat in vacuum or inert atmosphere. Welding in overaged condition. Rapid heating through critical temperature range for cracking. Reduce heat input


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Problem

Alloy Type

Solutions

Liquation cracking

All types

Reduce heat input Reduce restraint Avoid coarse-grain structure and Laves phase.

More generally the principal points that must be considered when welding nickel alloys are: 

Cleanliness: This is probably the most significant. Oxides, sulphur compounds and contamination with zinc, lead and carbon can prove harmful.

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Metal viscosity: The higher viscosity of the metal compared to carbon steel can cause welders to increase the current. This does not solve the problem. Wider gaps and weaving patterns are better solutions.

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Penetration: The lower penetration compared to carbon steel again calls for a different type of joint preparation. It may also require specialised shielding gas compositions.

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Post weld heat treatment cracking: When welding precipita tion hardened nickel alloy grades care is needed to avoid post weld cracking. Use low heat inputs and minimise the time taken to reach the solutionising temperature.

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Temper: For those alloys that are precipitate hardened, they should always be welded in the solutionised or overaged condition. They should never be welded in the aged condition where peak hardness is obtained (and ductility is a minimum).

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Shielding: Shielding does not only prevent attack from oxygen, it can also control the arc characteristics and the distribution of heat energy across the weld pool. This is usually significant in nickel alloy welding.

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Filler Materials: The range of nickel alloys is considerable and the alloys have a wider range of component elements than most other systems. This means that the choice of filler is usually more significant than for conventional steel welding.

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Weld Corrosion: Highly alloyed weld pools are typical in nickel alloy weldments. These pools are subjected to alloy segregation and this can cause accelerated corrosion of the weld. For this reason over-alloyed welds are usually specified.

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Dissimilar Metal Welding: Ferrous welding normally involves welding one steel component to another steel component. In nickel alloy welding it is often the case that different metals are being welded. The procedures necessary when making


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dissimilar metal welds related to thermal expansion, thermal conductivity and alloy changes caused by intermixing of the dissimilar metals requires special consideration.

Overlaying Because of the cost of high nickel alloys, weld overlaying is a common alternative to using solid alloy sections in corrosion resistant applications. Overlaying can be done by a weld deposit laid down over the whole surface or by cladding the surface with sheet material, often referred to as ‘wall-papering’

The welding of material that is manufactured in a clad form, by either roll or explosive bonding, is also a significant component of high nickel alloy welding.

Quality control of welded joint In general the methods of quality control used for nickel alloys are the same as those applied to other welded joints. This involves welding procedure specifications (WPS), welder performance tests, inspection and testing of joints (mechanical and NDT). Because of the usually higher costs associated both with the preliminary equipment being welded and the actual welding operation, trial welds and testing of qualification welds are more common with this group of alloys. Each country typically has its own standards and codes.

Non-fusion Joining Brazing and soldering are also common non-fusion joining processes. These processes depend largely on obtaining a suitable low melting point alloy and developing a suitable flux to allow initial wetting of the surface are the major problems. One of the particular advantages of non-fusion joining is the ability to automate the joining of preassembled components that are self jigging.

SAQ 6

W h y i s c l ea n li n es s th e m o s t im p o r t a nt co n s i d er at i o n i n t h e w e ld i n g o f nickel and its alloys? Name as many cases as you ca n where contaminants that can influence the final weld, and state how these contaminants cause a deterioration of the weld properties.

SAQ 7

D es c r i b e t h e c au s e s o f so l id i f ic at i o n c r ac k i n g a n d l iq u a ti o n c r ac k i n g in nickel alloys and suggest some remedies.


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08MAT1.5 Nickel and Nickel Alloys

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