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Properties of fusion welds
VIJAYAN M G
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Welding v casting
Weld must adhere to mould wall Heat is added continually to welds
High temperature gradient
Welds solidify much faster than castings In welds, the surface shape is constant There is strong mixing of weld pools
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Fusion weld structure
HAZWeld metal
HAZBasemetal
Fusion line
Weld preparation
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Thermal gradients in HAZ
Time
Temperature
Fusion lineFusion line + 2mmFusion line + 5 mm
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HAZ structure
High peak temperature High temperature gradient
Variable cooling rate
Superimposed HAZs in multipass welds Welding stresses affect transformation
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Multipass fusion weld
Last weld run
Previous weld run
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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
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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
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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)
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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
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Material properties
Melting & vaporisation temperatures Electrical & thermal properties
Conductivity, expansion coefficient, thermal capacity, latent heat
Affinity of weld metal for O, N & H Susceptibility to porosity, embrittlement or wetting
problems
Presence of a surface film on base metal Oxide, paint or metallic surface coating
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Service environment
Extreme environments Corrosive Low temperature (brittle failure) High temperature (oxidation, creep, embrittlement) Others (wear, fatigue, nuclear)
The more extreme the environment The more difficult it is to find suitable materials The more restricted the welding procedure becomes to
avoid service failure
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Welding Defects
Cracks In weld metal or HAZ
Porosity Gas bubbles entrapped in weld metal
Incomplete fusion (interrun or at preparation edge) Inclusions
Slag or other matter entrapped in weld
Defective profile Under-weld, over-weld, lack of penetration, overlap,
undercut
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Causes of weld defects
Poor design of weldment Lack of access to make weld
Mistakes by welder Lack of skill Poor placement of weld metal Inadequate cleaning
Poor welding procedure
Weld defects
Wormholeporosity
Incompletefusion
Slag inclusion
Overlap
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Solidification cracking
Contraction strains cause rupture of the weld at the point where the last material solidifies.
Solidification range Weld pool size & shape
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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
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XX XX
Residual stress in a butt weld
sx
sy
sx
0 TensionCompression
XX XX
y Tension
Compression
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Weld detailing
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Weld positions - plate
Flat(1G)
Horizontal(2G)
Vertical(3G)
Up or Down
Overhead(4G)
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Welding positions - pipe
Axis vertical2G
Axis horizontal5G
Axis inclined 45°6G
Weld details
Joints - Arrangements of members being joined
Butt, tee, lap, corner, flare
Welds - Geometry of weld detail selected to make the joint
Butt, fillet, plug & slot
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Joint types
Butt Tee
Lap Corner
Edge
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Weld types
Butt weld Between mating members Best quality High weld preparation cost
Fillet weld Easy preparation Asymmetric loads, lower design loads
Plug & slot welds Modified fillet welds in lap joints, using
holes through one member
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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
Fillet weld terms
Root
ToeWeld face
ToeThroatthickness
Apparent leg length
Gap
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Butt welds
Types: Double welded butt Permanent or temporary backing Single welded butt
Lower stress concentration Easier ultrasonic testing or radiography Expensive preparation
Butt weld types
Single veecan be single
or double welded
Single bevel Double vee
Backed butt (permanent or temporary)
Butt weld termsFusion face
Root face
Root gap
Included angle
Bevel angle
Root run Toe
Toe
Reinforcement
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Weld preparation dimensions
Compromise Weld cross sectional area
• Cost• Ease of welding (risk of defects)
Standard preparations AS1554, AWS D1.1, ANSI B31.3
Fabricator’s risk
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Single welded butt joints
Access to one side only Pipe & hollow section butt joints
Root run forms inside surface High fit-up tolerances required High skill required
Special welder qualification
Root may be contaminated Inside is difficult to inspect
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‘J’ & ‘U’ Preparations
Land
Root radius“U” preparation
Double “U” butt
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AS1101.2 drawing symbols
Tail
Arrow points to weld location
OTHER SIDE
ARROW SIDE
Weld type symbol
Reference line
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Typical AS1101.2 symbols
6mm6
CJP