WCL Metallurgy & Material Science 4 â Questions and Answers
Question 1: What is 'stress corrosion cracking' (SCC) and which condition promotes it?
- Cracking from rapid thermal cycling alone
- Cracking resulting from simultaneous tensile stress and a corrosive environment (Correct answer)
- Cracking due to hydrogen embrittlement in dry environments
- Cracking from impact loading at low temperatures
Correct answer: Cracking resulting from simultaneous tensile stress and a corrosive environment
SCC requires the simultaneous presence of a susceptible material, tensile stress (residual or applied), and a specific corrosive environment to propagate.
Question 2: Why does welding austenitic stainless steel with high heat input increase risk of sensitization?
- High heat input causes rapid solidification that traps oxygen
- High heat input keeps the metal in the sensitization temperature range longer (Correct answer)
- High heat input reduces the chromium content of the filler metal
- High heat input promotes ferrite formation in the weld
Correct answer: High heat input keeps the metal in the sensitization temperature range longer
Greater heat input extends time in the 800°Fâ1500°F sensitization range, giving more opportunity for chromium carbides to precipitate at grain boundaries.
Question 3: What is the primary advantage of using low-hydrogen (EXX18) electrodes when welding high-strength steel?
- They produce deeper penetration than cellulosic electrodes
- They minimize diffusible hydrogen in the weld, reducing cold cracking risk (Correct answer)
- They operate at lower amperages, reducing distortion
- They do not require preheat under any conditions
Correct answer: They minimize diffusible hydrogen in the weld, reducing cold cracking risk
Low-hydrogen electrodes have moisture-resistant coatings that limit diffusible hydrogen pickup in the weld metal, directly reducing hydrogen-induced cold cracking susceptibility.
Question 4: Which crystal structure do most common austenitic stainless steels (e.g., 304, 316) exhibit 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)
Nickel stabilizes the austenite phase, which has an FCC crystal structure, in these stainless steels even at room temperature.
Question 5: What metallurgical problem can arise when welding aluminum alloys in the 6xxx series (e.g., 6061)?
- Hot shortness due to zinc vaporization
- Loss of precipitation hardening strength in the HAZ (Correct answer)
- Formation of brittle sigma phase in the HAZ
- Intergranular corrosion identical to stainless sensitization
Correct answer: Loss of precipitation hardening strength in the HAZ
Heat from welding dissolves the Mg2Si precipitates that provide 6xxx-series strength, overaging or annealing the HAZ and significantly reducing its mechanical properties.
Question 6: In the iron-carbon phase diagram, at what approximate carbon content does the eutectic reaction occur, producing cast iron microstructures?
- 0.022% C
- 0.77% C
- 4.3% C (Correct answer)
- 6.67% C
Correct answer: 4.3% C
The iron-carbon eutectic point occurs at approximately 4.3% carbon and 2097°F (1147°C), where liquid iron solidifies into a mixture of austenite and cementite (ledeburite).
Question 7: What does 'Charpy impact testing' measure in welded joints?
- Tensile strength of the weld metal at room temperature
- Absorbed energy during fracture, indicating toughness at specified temperatures (Correct answer)
- Hardness of the HAZ after PWHT
- Fatigue life under cyclic loading conditions
Correct answer: Absorbed energy during fracture, indicating toughness at specified temperatures
The Charpy V-notch test measures the energy absorbed when a notched specimen is struck by a pendulum, quantifying toughness and ductile-to-brittle transition behavior.
WCL Metallurgy & Material Science 4
What is 'stress corrosion cracking' (SCC) and which condition promotes it?