Properties of fusion welds VIJAYAN M G
Qualmet Services
1
Welding v casting Weld must adhere to mould wall Heat is added continually to welds q
High temperature gradient
Welds solidify much faster than castings In welds, the surface shape is constant There is strong mixing of weld pools
Qualmet Services
2
Fusion weld structure Fusion line Weld preparation
Base metal
HAZ
Qualmet Services
Weld metal
HAZ
3
Thermal gradients in HAZ Temperature
Fusion line Fusion line + 2mm Fusion line + 5 mm
Time
Qualmet Services
4
HAZ structure High peak temperature High temperature gradient q
Variable cooling rate
Superimposed HAZs in multipass welds Welding stresses affect transformation
Qualmet Services
5
Multipass fusion weld
Last weld run
Previous weld run
Qualmet Services
6
Weld properties Weld metal has different composition & thermal history to base metal Welding heat modifies adjacent base metal (HAZ) Variation in strength, ductility & corrosion resistance across welds
Qualmet Services
7
Welding procedure Selection of process conditions & consumables for a weld Designed to match base material properties as closely as possible & avoid weld defects Needs to consider service conditions of the weld
Qualmet Services
8
Definition of weldability The capacity of a material to be welded under the imposed fabrication conditions into a specific, suitably designed structure & to perform satisfactorily in intended service. • (ANSI / AWS A3.0)
Qualmet Services
9
Factors affecting weldability Weldability is often considered to be a material property, however the effect of other variables should not be ignored. Design of weldment Its service conditions Choice of welding process
Qualmet Services
10
Material properties Melting & vaporisation temperatures Electrical & thermal properties q
Conductivity, expansion coefficient, thermal capacity, latent heat
Affinity of weld metal for O, N & H q
Susceptibility to porosity, embrittlement or wetting problems
Presence of a surface film on base metal q
Oxide, paint or metallic surface coating
Qualmet Services
11
Service environment Extreme environments q
q
Corrosive Low temperature (brittle failure)
q
High temperature (oxidation, creep, embrittlement)
q
Others (wear, fatigue, nuclear)
The more extreme the environment q
The more difficult it is to find suitable materials
q
The more restricted the welding procedure becomes to avoid service failure
Qualmet Services
12
Welding Defects Cracks q
In weld metal or HAZ
Porosity q
Gas bubbles entrapped in weld metal
Incomplete fusion (interrun or at preparation edge) Inclusions q
Slag or other matter entrapped in weld
Defective profile q
Under-weld, over-weld, lack of penetration, overlap, undercut
Qualmet Services
13
Causes of weld defects Poor design of weldment q
Lack of access to make weld
Mistakes by welder q
Lack of skill
q
Poor placement of weld metal
q
Inadequate cleaning
Poor welding procedure
Qualmet Services
14
Weld defects
Slag inclusion
Wormhole porosity Incomplete fusion
Overlap
Solidification cracking Contraction strains cause rupture of the weld at the point where the last material solidifies.
Qualmet Services
q
Solidification range
q
Weld pool size & shape
16
Residual stress & distortion Uneven heating leads to constrained thermal expansion At completion of the weld thermal cycle the weldment either distorts or if restrained will contain residual stress Residual stress fields are complex, but stresses as high as yield often occur Stresses may need to be removed by a stress relief heat treatment Qualmet Services
17
Residual stress in a butt weld s
σy
s
σx
X
X
s
σx
Compression
σy
X
0
Tension
Tension
X Compression
Qualmet Services
18
Weld detailing
Qualmet Services
19
Weld positions - plate
Flat (1G)
Horizontal (2G)
Qualmet Services
Vertical (3G) Up or Down
Overhead (4G) 20
Welding positions - pipe
Axis vertical 2G
Qualmet Services
Axis horizontal 5G
Axis inclined 45 6G
21
°
Weld details Joints - Arrangements of members being joined q
Butt, tee, lap, corner, flare
Welds - Geometry of weld detail selected to make the joint q
Butt, fillet, plug & slot
Joint types
Butt
Tee
Edge L ap
Qualmet Services
Corner
23
Weld types Butt weld q
q
q
Between mating members Best quality High weld preparation cost
Fillet weld q
q
Easy preparation Asymmetric loads, lower design loads
Plug & slot welds q
Qualmet Services
Modified fillet welds in lap joints, using holes through one member 24
Fillet welds Simple & cheap to assemble & weld Stress concentrations at toes & root Notch at root (fatigue, toughness) Critical dimension is throat thickness Root gap affects throat thickness Radiography & ultrasonic testing is of limited use Large fillets use a lot of weld metal & therefore are uneconomic
Qualmet Services
25
Fillet weld terms Toe Weld face
Toe Root
Throat thickness
Gap
Apparent leg length
Butt welds Types: q
Double welded butt
q
Permanent or temporary backing Single welded butt
q
Lower stress concentration Easier ultrasonic testing or radiography Expensive preparation
Qualmet Services
27
Butt weld types
Single vee can be single or double welded
Single bevel
Double vee
Backed butt (permanent or temporary)
Butt weld terms Fusion face
Included angle Bevel angle
Reinforcement Toe Root face Root gap
Root run
Toe
Weld preparation dimensions Compromise q
Weld cross sectional area • Cost • Ease of welding (risk of defects)
Standard preparations q
AS1554, AWS D1.1, ANSI B31.3
Fabricator’s risk
Qualmet Services
30
Single welded butt joints Access to one side only q
Pipe & hollow section butt joints
Root run forms inside surface High fit-up tolerances required High skill required q
Special welder qualification
Root may be contaminated Inside is difficult to inspect
Qualmet Services
31
‘J’ & ‘U’ Preparations
“U” preparation Root radius Land
Double “U” butt
Qualmet Services
32
AS1101.2 drawing symbols OTHER SIDE Tail
Reference line ARROW SIDE Arrow points to weld location Weld type symbol
Qualmet Services
33
Typical AS1101.2 symbols 6mm 6
Qualmet Services
CJP
34
