Architecting on AWS Certification Welding Metallurgy & Materials 2 — Questions and Answers
Question 1: What is the primary cause of hydrogen-induced cracking (HIC) in high-strength steel welds?
- Excessive preheat temperature
- Diffusible hydrogen trapped in the heat-affected zone under residual stress (Correct answer)
- Insufficient filler metal deposition
- Overly rapid cooling through the austenite range
Correct answer: Diffusible hydrogen trapped in the heat-affected zone under residual stress
Hydrogen-induced cracking occurs when diffusible hydrogen migrates to the HAZ and combines with residual tensile stress and a susceptible microstructure.
Question 2: Which welding process is most commonly used for joining austenitic stainless steel to minimize sensitization?
- Shielded Metal Arc Welding (SMAW) with E308-16 electrode
- Gas Tungsten Arc Welding (GTAW) with low heat input (Correct answer)
- Flux-Cored Arc Welding (FCAW) with high interpass temperature
- Submerged Arc Welding (SAW) at high amperage
Correct answer: Gas Tungsten Arc Welding (GTAW) with low heat input
GTAW with low heat input minimizes the time spent in the sensitization temperature range (450–850°C), reducing chromium carbide precipitation at grain boundaries.
Question 3: The Schaeffler diagram is used to predict the weld metal microstructure of stainless steels based on which two parameters?
- Carbon equivalent and preheating temperature
- Chromium equivalent and nickel equivalent (Correct answer)
- Heat input and cooling rate
- Ferrite number and carbon content
Correct answer: Chromium equivalent and nickel equivalent
The Schaeffler diagram maps weld metal microstructure (austenite, ferrite, martensite) as a function of chromium equivalent (ferrite stabilizers) versus nickel equivalent (austenite stabilizers).
Question 4: What microstructural feature in low-alloy steel HAZ is most detrimental to toughness?
- Acicular ferrite
- Upper bainite (Correct answer)
- Grain boundary ferrite
- Widmanstätten ferrite
Correct answer: Upper bainite
Upper bainite contains coarse carbide films along lath boundaries that act as cleavage crack initiators, severely reducing Charpy impact toughness.
Question 5: Which phenomenon describes the loss of corrosion resistance in austenitic stainless steel due to chromium depletion near grain boundaries?
- Intergranular stress corrosion cracking
- Sensitization (Correct answer)
- Pitting corrosion
- Galvanic corrosion
Correct answer: Sensitization
Sensitization occurs when chromium carbides precipitate at grain boundaries during heating to 450–850°C, depleting the adjacent matrix of chromium below the passive threshold (~12%).
Question 6: In welding duplex stainless steel, what is the target ferrite content in the weld metal to balance corrosion resistance and toughness?
- 5–10%
- 30–60% (Correct answer)
- 70–85%
- 90–95%
Correct answer: 30–60%
A ferrite content of approximately 30–60% (FN 30–70) in duplex stainless steel weld metal provides the best combination of strength, toughness, and corrosion resistance.
Question 7: What is the purpose of post-weld heat treatment (PWHT) in carbon and low-alloy steel weldments?
- To increase the carbon content of the weld metal
- To relieve residual stresses and temper hard HAZ microstructures (Correct answer)
- To increase hardness by re-austenitizing the weld zone
- To dissolve carbide precipitates in the weld metal
Correct answer: To relieve residual stresses and temper hard HAZ microstructures
PWHT reduces residual stresses generated during welding and tempers brittle martensite or bainite in the HAZ, improving toughness and reducing susceptibility to hydrogen cracking.
Architecting on AWS Certification Welding Metallurgy & Materials 2
What is the primary cause of hydrogen-induced cracking (HIC) in high-strength steel welds?