AWS AWS Weld Metallurgy & Heat Treatment 5 — Questions and Answers
Question 1: Which weld zone is defined as the region that reached temperatures above the liquidus but below the solidus during welding?
- Fusion zone
- Partially melted zone (PMZ) (Correct answer)
- Heat-affected zone (HAZ)
- Unmixed zone
Correct answer: Partially melted zone (PMZ)
The partially melted zone (PMZ) experienced temperatures between the solidus and liquidus, resulting in partial melting at grain boundaries.
Question 2: What type of transformation occurs in plain carbon steel when rapidly cooled from the austenite region?
- Pearlitic transformation
- Martensitic transformation (Correct answer)
- Ferritic transformation
- Bainitic transformation at slow rates
Correct answer: Martensitic transformation
Rapid cooling (quenching) of austenite in plain carbon steel produces martensite, a hard and brittle body-centered tetragonal (BCT) structure.
Question 3: What is the role of interpass temperature control in multi-pass welding of low-alloy steels?
- Prevents weld bead from sagging
- Maintains heat input within acceptable limits to control microstructure and avoid excessive softening or cracking (Correct answer)
- Ensures the electrode remains hot
- Controls the weld pool viscosity only
Correct answer: Maintains heat input within acceptable limits to control microstructure and avoid excessive softening or cracking
Controlling interpass temperature prevents excessive cumulative heat input, which could cause grain coarsening, loss of toughness, or reheat cracking in certain steels.
Question 4: Sigma phase formation in stainless steel welds is most problematic because it:
- Increases corrosion resistance
- Causes weld porosity
- Reduces toughness and corrosion resistance, especially at elevated temperatures (Correct answer)
- Promotes weld solidification cracking
Correct answer: Reduces toughness and corrosion resistance, especially at elevated temperatures
Sigma phase is a hard, brittle intermetallic compound that forms in duplex and ferritic stainless steels, severely reducing impact toughness and corrosion resistance.
Question 5: In welding carbon steel, what is the purpose of a stress relief heat treatment performed below the lower critical temperature (Ac1)?
- To produce full annealing of the weld
- To reduce residual welding stresses without significantly altering the existing microstructure (Correct answer)
- To produce a martensitic microstructure
- To dissolve carbides uniformly in austenite
Correct answer: To reduce residual welding stresses without significantly altering the existing microstructure
Stress relief at sub-critical temperatures reduces residual stresses through plastic flow without causing phase transformation, preserving the existing microstructure.
Question 6: What is the primary cause of lamellar tearing in welded joints?
- Excessive preheat temperature
- Low through-thickness ductility in rolled plate due to planar inclusions, loaded in the Z-direction by weld shrinkage (Correct answer)
- High carbon content in the weld metal
- Improper shielding gas selection
Correct answer: Low through-thickness ductility in rolled plate due to planar inclusions, loaded in the Z-direction by weld shrinkage
Lamellar tearing occurs when weld shrinkage strains load rolled steel through its thickness, causing separation along planar sulfide or oxide inclusions with low Z-direction ductility.
Question 7: How does ferrite content in austenitic stainless steel weld metal help prevent solidification cracking?
- Ferrite increases weld fluidity and reduces spatter
- Ferrite dissolves harmful impurities like sulfur and phosphorus, reducing their segregation to grain boundaries (Correct answer)
- Ferrite increases tensile strength and carbon content
- Ferrite reduces the coefficient of thermal expansion
Correct answer: Ferrite dissolves harmful impurities like sulfur and phosphorus, reducing their segregation to grain boundaries
Ferrite in austenitic stainless steel weld metal dissolves and ties up sulfur and phosphorus, preventing the formation of low-melting-point films that cause solidification cracking.
AWS AWS Weld Metallurgy & Heat Treatment 5
Which weld zone is defined as the region that reached temperatures above the liquidus but below the solidus during welding?