Welding Faults and Defects

Responses of Materials to Welding

·         Defects in Welds

·         Micro-structural changes

·         Stresses and Distortion

·         Heat treatment of parent metals and welds

Defects in Welds

·         Porosity

• restart porosity
• surface porosity
• crater pipes

·         Inclusions

·         Lack of fusion and penetration

·         Cracks

• hydrogen embrittlement
• Lamellar tearing
• Reheat cracking
• solidification cracking

Porosity

·         Uniform porosity from exsolution of dissolved gases

·         Restart porosity - from unstable arc at weld start (incomplete protection, poor welding technique)

·         Surface porosity - from excessive contamination (grease, dampness, atmosphere) or sometimes high sulphur in consumables

·         Crater pipes - from shrinkage crater at end of weld run

 

Reduction of Porosity

·         Proper selection of electrodes and filler materials

·         Improved welding technique

• preheating
• increasing heat input

·         Proper cleaning and prevention of contaminants entering weld zone

·         Slowing the welding speed to allow time for gas to escape

Crater pipes/Micro-porosity

·         Shrinkage of molten weld pool leads to porosity

• e.g. crater pipes in TIG, micro-porosity in submerged arc

·         prevent by

• improving welding technique
• Use welding set with current decay
• use a run-off tab

Inclusions

·         Two main types:-

• linear inclusions due to incomplete removal of slag in MMA
• isolated inclusions due to rust or mill scale on parent metal surfaces

·         Often associated with undercut or irregular surfaces in multi-pass welds

Lack of Fusion/Penetration

·         Caused by incorrect welding conditions

• current too low
• welding speed too high
• incorrect torch/gun angle
• incorrect edge preparation (e.g. too large root face)

·         Poor weld performance

Stresses and Distortion

·         Weld metal is deposited in molten state and cools to room temperature

·         Most of the parent metal is not heated and therefore is unchanged by welding

·         On cooling the weld pool contacts due to thermal expansion

·         This contraction leads to

• distortion if the parent metals are unrestrained
• stress if the parent metals are clamped

Distortion

Residual Stress

Transverse

Longitudinal

Magnitude of Stresses Generated

·         The thermal stress is simplistically given by (Eα∆T), where (E) is the Young’s Modulus, (∆T) is the temperature change and (α) the thermal expansion coefficient

·         For steel the melting point is around 1500^oC (thus ∆T=1475K), the Young’s Modulus is 200GPa and (α) the thermal expansion coefficient is 12x10^-6 K^-1

·         Thus a stress of 3.5GPa could be produced at room temperature - this will be limited by plastic deformation

Factors Promoting Hot Cracking

·         Welding current density (high levels promote cracking)

·         Heat distribution (joint design)

·         Restraint

·         Crack sensitivity of electrode material

·         Dilution of weld metal

·         Impurities (e.g. sulphur and phosphorus)

·         preheating (increases liability to cracking)

·         Welding procedure (high speeds, long arcs increase sensitivity)

Solidification Cracking

·         Caused by

• weld bead too deep or wide
• high current or welding speed
• large root gap
• C, P or S pick-up

·         Prevent by

• weld parameters chosen so that weld width is 0.5 to 0.8 weld depth
• Keep S and P in steel 0.6%
• Correct fit-up

Factors Promoting Cold Cracking

·         Joint restraint

·         Heat input

·         Weld of insufficient sectional area

·         Hydrogen in weld metal

·         Impurities

·         Embrittlement of the HAZ

·         High welding speeds and low welding currents

Lamellar Tearing

·         Caused by elongated non-metallic inclusion arrays in rolled plate

·         Occurs when weld metal is deposited on plate surface and where restraint is high

·         Prevent by design, low inclusion plate or use of castings/forgings

Reheat Cracking

·         Occurs in creep resisting and thick-section high strength low alloy steels during post weld heat treatment

·         Caused by poor creep ductility in HAZ

·         Accentuated by notches and defects

Reheat Cracking

·         Chromium, molybdenum and vanadium containing steels most susceptible

·         Prevented by

• Heat treat with a low temperature soak followed by rapid heating to high temperature
• Grinding or peening weld toes after welding
• Use two-layer welding technique to refine the coarse grained HAZ structure
• Use non-susceptible weld metal