AWS AWS Weld Metallurgy & Heat Treatment 4 — Questions and Answers
Question 1: What is the primary purpose of post-weld heat treatment (PWHT) on carbon steel weldments?
- Increase tensile strength
- Relieve residual stresses and temper the HAZ (Correct answer)
- Harden the weld metal
- Increase carbon content in the weld
Correct answer: Relieve residual stresses and temper the HAZ
PWHT relieves residual stresses induced during welding and tempers hard microstructures in the heat-affected zone to improve toughness.
Question 2: Which microstructure is most commonly associated with hydrogen-induced cracking (HIC) in the HAZ of high-strength steels?
- Ferrite
- Pearlite
- Martensite (Correct answer)
- Austenite
Correct answer: Martensite
Martensite is a hard, brittle microstructure susceptible to hydrogen-induced cracking because it traps hydrogen at grain boundaries and defects.
Question 3: What does the carbon equivalent (CE) formula help predict in welding?
- Weld bead width
- Preheat requirements and crack susceptibility (Correct answer)
- Electrode consumption rate
- Shielding gas flow rate
Correct answer: Preheat requirements and crack susceptibility
The carbon equivalent formula combines carbon and alloying element percentages to estimate a steel's hardenability and required preheat to prevent cracking.
Question 4: When welding austenitic stainless steel, sensitization occurs primarily in which temperature range?
- 200–400°F (93–204°C)
- 800–1500°F (427–816°C) (Correct answer)
- 2000–2200°F (1093–1204°C)
- Above 2500°F (1371°C)
Correct answer: 800–1500°F (427–816°C)
Sensitization in austenitic stainless steel occurs between approximately 800–1500°F as chromium carbides precipitate at grain boundaries, depleting chromium.
Question 5: In welding metallurgy, what does 'HAZ softening' refer to in precipitation-hardened aluminum alloys?
- Grain refinement near the fusion line
- Loss of strength in the heat-affected zone due to overaging or dissolution of strengthening precipitates (Correct answer)
- Increased hardness from rapid cooling
- Formation of martensite near the weld
Correct answer: Loss of strength in the heat-affected zone due to overaging or dissolution of strengthening precipitates
HAZ softening in precipitation-hardened aluminum alloys results from the welding heat overaging or dissolving the strengthening precipitates, reducing strength.
Question 6: What is the function of preheat when welding high-carbon or alloy steels?
- Increase welding speed
- Prevent porosity from nitrogen
- Slow the cooling rate to reduce hardness and minimize cracking (Correct answer)
- Remove mill scale from the base metal
Correct answer: Slow the cooling rate to reduce hardness and minimize cracking
Preheat slows the cooling rate through the critical temperature range, reducing the formation of hard martensite and lowering the risk of cold cracking.
Question 7: What is 'liquation cracking' (hot cracking) in the partially melted zone (PMZ)?
- Cracking caused by hydrogen diffusion at room temperature
- Cracking caused by low-melting-point constituents forming liquid films at grain boundaries during cooling (Correct answer)
- Cracking due to excessive preheat
- Cracking from improper shielding gas selection
Correct answer: Cracking caused by low-melting-point constituents forming liquid films at grain boundaries during cooling
Liquation cracking occurs when low-melting-point phases form liquid films at grain boundaries in the PMZ, which rupture under tensile stress as the weld cools.
AWS AWS Weld Metallurgy & Heat Treatment 4
What is the primary purpose of post-weld heat treatment (PWHT) on carbon steel weldments?