WCL Metallurgy & Material Science 2 — Questions and Answers
Question 1: What is the primary reason carbon content is limited in structural steel used for welding applications?
- Higher carbon increases brittleness and cold cracking risk (Correct answer)
- Higher carbon reduces tensile strength significantly
- Higher carbon causes porosity in the weld pool
- Higher carbon lowers the melting point of steel
Correct answer: Higher carbon increases brittleness and cold cracking risk
Elevated carbon content raises hardenability and increases susceptibility to hydrogen-induced cold cracking in the heat-affected zone.
Question 2: Which phase transformation occurs when austenite is rapidly quenched and results in a hard, brittle microstructure?
- Pearlite formation
- Bainite formation
- Martensite formation (Correct answer)
- Ferrite formation
Correct answer: Martensite formation
Rapid quenching traps carbon in a body-centered tetragonal lattice, forming martensite—the hardest and most brittle steel microstructure.
Question 3: What does the term 'preheat' accomplish metallurgically in high-carbon steel welding?
- It eliminates all residual stresses permanently
- It slows the cooling rate to reduce martensite formation (Correct answer)
- It increases the carbon equivalent of the base metal
- It promotes porosity expulsion from the weld
Correct answer: It slows the cooling rate to reduce martensite formation
Preheat raises the base metal temperature, slowing post-weld cooling and reducing the likelihood of martensite formation in the HAZ.
Question 4: Which alloying element in stainless steel is primarily responsible for corrosion resistance?
- Molybdenum
- Nickel
- Chromium (Correct answer)
- Manganese
Correct answer: Chromium
Chromium (minimum ~10.5%) forms a passive chromium oxide film on the surface that protects stainless steel from oxidation and corrosion.
Question 5: What is 'sensitization' in austenitic stainless steel and when does it occur?
- Grain growth at temperatures above 1200°F
- Chromium carbide precipitation at grain boundaries between 800°F–1500°F (Correct answer)
- Sigma phase formation above 1600°F
- Martensite formation during rapid cooling
Correct answer: Chromium carbide precipitation at grain boundaries between 800°F–1500°F
Sensitization occurs when chromium carbides precipitate at grain boundaries, depleting adjacent zones of chromium and making them susceptible to intergranular corrosion.
Question 6: A welder is joining 4140 alloy steel. Which element in this steel most significantly raises its carbon equivalent (CE)?
- Silicon
- Chromium (Correct answer)
- Sulfur
- Copper
Correct answer: Chromium
Chromium has a high CE contribution factor; in 4140 steel it is present at ~0.8–1.1% and substantially raises the overall carbon equivalent.
Question 7: What microstructural feature makes cast iron extremely difficult to weld without special procedures?
- High sulfur content causing hot cracking
- Very high carbon content forming brittle martensite or white iron in the HAZ (Correct answer)
- Low melting point causing burn-through
- Absence of ferrite in the microstructure
Correct answer: Very high carbon content forming brittle martensite or white iron in the HAZ
Cast iron's high carbon content (2–4%) promotes formation of extremely hard, brittle white iron or martensite in the rapidly cooled heat-affected zone.
WCL Metallurgy & Material Science 2
What is the primary reason carbon content is limited in structural steel used for welding applications?