AWS AWS Metallurgy & Materials 4 — 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 tensile stress (Correct answer)
- Insufficient filler metal deposition
- Oxygen contamination from the shielding gas
Correct answer: Diffusible hydrogen trapped in the heat-affected zone under tensile stress
HIC occurs when diffusible hydrogen becomes trapped in the HAZ or weld metal under tensile stress, causing delayed cracking.
Question 2: Which crystal structure is associated with austenitic stainless steel at room temperature?
- Body-centered cubic (BCC)
- Face-centered cubic (FCC) (Correct answer)
- Hexagonal close-packed (HCP)
- Body-centered tetragonal (BCT)
Correct answer: Face-centered cubic (FCC)
Austenitic stainless steels retain an FCC crystal structure at room temperature due to sufficient austenite-stabilizing elements like nickel.
Question 3: In welding metallurgy, what does the term 'eutectic' refer to?
- The highest melting point of an alloy system
- A specific alloy composition that melts and solidifies at a single temperature (Correct answer)
- The temperature at which martensite forms
- A heat treatment process for stress relief
Correct answer: A specific alloy composition that melts and solidifies at a single temperature
A eutectic composition is the specific alloy mixture that has the lowest melting point and transforms directly between liquid and solid at a constant temperature.
Question 4: What is the effect of increasing carbon content on the hardenability of steel?
- Decreases hardenability and lowers the Ms temperature
- Increases hardenability and raises the Ms temperature
- Increases hardenability and lowers the Ms temperature (Correct answer)
- Has no effect on hardenability
Correct answer: Increases hardenability and lowers the Ms temperature
Higher carbon content increases hardenability (the ability to form martensite) while simultaneously lowering the martensite start (Ms) temperature.
Question 5: Which type of corrosion is most commonly associated with sensitized austenitic stainless steel welds?
- Uniform corrosion
- Intergranular corrosion (Correct answer)
- Pitting corrosion
- Galvanic corrosion
Correct answer: Intergranular corrosion
Sensitization causes chromium carbide precipitation at grain boundaries, depleting chromium and making the steel susceptible to intergranular corrosion.
Question 6: What is the purpose of normalizing a steel weld joint after welding?
- To increase surface hardness through rapid cooling
- To refine the grain structure and relieve residual stresses by heating above the upper critical temperature and air cooling (Correct answer)
- To dissolve carbides and retain a supersaturated solution
- To convert retained austenite to martensite
Correct answer: To refine the grain structure and relieve residual stresses by heating above the upper critical temperature and air cooling
Normalizing refines coarse grain structures in the HAZ and weld metal by austenitizing and allowing controlled air cooling.
Question 7: Which alloying element is most effective at improving the creep resistance of steel used in high-temperature welded applications?
- Sulfur
- Phosphorus
- Molybdenum (Correct answer)
- Lead
Correct answer: Molybdenum
Molybdenum significantly improves creep resistance by forming stable carbides and strengthening grain boundaries at elevated temperatures.
AWS AWS Metallurgy & Materials 4
What is the primary cause of hydrogen-induced cracking (HIC) in high-strength steel welds?