AWS Welding Metallurgy & Heat Treatment 2 — Questions and Answers
Question 1: What is the primary purpose of preheating before welding high-carbon steel?
- Increase tensile strength
- Reduce the risk of hydrogen-induced cracking (Correct answer)
- Improve weld bead appearance
- Decrease penetration depth
Correct answer: Reduce the risk of hydrogen-induced cracking
Preheating slows the cooling rate, allowing hydrogen to diffuse out and reducing the risk of hydrogen-induced (cold) cracking in the heat-affected zone.
Question 2: Which microstructural phase in steel is hardest and most brittle, often forming in rapidly cooled welds?
- Ferrite
- Pearlite
- Martensite (Correct answer)
- Austenite
Correct answer: Martensite
Martensite forms when austenite is rapidly quenched, trapping carbon in a distorted body-centered tetragonal lattice, resulting in high hardness and brittleness.
Question 3: What does the term 'carbon equivalent' (CE) indicate in welding?
- Amount of carbon burned off during welding
- Weldability based on carbon and alloying element content (Correct answer)
- Carbon content of the filler metal
- Post-weld carbon migration tendency
Correct answer: Weldability based on carbon and alloying element content
Carbon equivalent (CE) is a formula that quantifies a steel's weldability by accounting for the combined effect of carbon and other alloying elements on hardenability.
Question 4: Post-weld heat treatment (PWHT) of carbon steel welds is primarily performed to:
- Increase yield strength
- Refine grain size and relieve residual stresses (Correct answer)
- Add carbon to the weld zone
- Increase hardness of the weld metal
Correct answer: Refine grain size and relieve residual stresses
PWHT (stress relief) reduces residual stresses, tempers hard microstructures, and improves ductility and toughness in the weld and HAZ.
Question 5: Which region of the heat-affected zone experiences the highest peak temperature during welding?
- Subcritical zone
- Intercritical zone
- Coarse-grained HAZ (Correct answer)
- Fine-grained HAZ
Correct answer: Coarse-grained HAZ
The coarse-grained HAZ is nearest the fusion line and experiences temperatures well above the upper critical temperature, causing significant austenite grain growth.
Question 6: Hot cracking in austenitic stainless steel welds is most commonly associated with:
- Low interpass temperatures
- Excess ferrite content
- Low-melting-point segregates at grain boundaries (Correct answer)
- Hydrogen absorption from flux
Correct answer: Low-melting-point segregates at grain boundaries
Hot cracking (solidification cracking) occurs when low-melting-point impurities (sulfur, phosphorus) segregate to grain boundaries and remain liquid as the weld contracts.
Question 7: Which heat treatment process involves heating steel above its upper critical temperature and then cooling in still air to refine grain structure?
- Annealing
- Stress relieving
- Normalizing (Correct answer)
- Tempering
Correct answer: Normalizing
Normalizing heats steel above the upper critical (Ac3) temperature and cools it in air, producing a uniform, refined grain structure with improved mechanical properties.
AWS Welding Metallurgy & Heat Treatment 2
What is the primary purpose of preheating before welding high-carbon steel?