WCL Metallurgy & Material Science 5 — Questions and Answers
Question 1: What is the primary metallurgical difference between 'ferrite' and 'austenite' in steel?
- Ferrite is FCC and magnetic; austenite is BCC and non-magnetic
- Ferrite is BCC and magnetic; austenite is FCC and non-magnetic (Correct answer)
- Ferrite has higher carbon solubility than austenite
- Ferrite forms only above 2200°F in plain carbon steel
Correct answer: Ferrite is BCC and magnetic; austenite is FCC and non-magnetic
Ferrite has a BCC crystal structure, is magnetic, and dissolves very little carbon; austenite is FCC, non-magnetic, and can dissolve significantly more carbon.
Question 2: Which titanium alloy type is considered most weldable?
- Beta titanium alloys
- Alpha-beta (Ti-6Al-4V) titanium alloys
- Commercially pure (CP) alpha titanium (Correct answer)
- Near-beta titanium alloys
Correct answer: Commercially pure (CP) alpha titanium
Commercially pure titanium (alpha phase) is the most weldable because it does not undergo the complex phase transformations that can embrittle alpha-beta and beta alloys during welding.
Question 3: When welding dissimilar metals with widely different coefficients of thermal expansion, which weld defect is most likely?
- Porosity from shielding gas contamination
- Undercut along the fusion line
- Cracking or delamination due to differential thermal stresses (Correct answer)
- Incomplete fusion at the root pass
Correct answer: Cracking or delamination due to differential thermal stresses
Mismatched thermal expansion causes differential contraction on cooling, generating residual stresses that can crack the joint or delaminate the bond at the fusion interface.
Question 4: What is 'weld dilution' and why does it matter when selecting filler metal?
- The reduction in joint strength caused by porosity
- The mixing of base metal into the weld pool, changing the deposit's composition (Correct answer)
- The loss of shielding gas during GMAW welding
- The decrease in travel speed due to increased deposition rate
Correct answer: The mixing of base metal into the weld pool, changing the deposit's composition
Dilution is the percentage of base metal that melts into the weld pool; high dilution can alter the chemical composition of the deposit away from the filler metal's nominal analysis, affecting corrosion resistance or strength.
Question 5: Which heat treatment process is used to remove brittleness from hardened steel by heating below the lower critical temperature?
- Annealing
- Normalizing
- Tempering (Correct answer)
- Solution annealing
Correct answer: Tempering
Tempering heats hardened steel to a sub-critical temperature (typically 300°F–1300°F), relieving internal stresses and reducing brittleness while retaining useful hardness.
Question 6: What type of corrosion occurs when two dissimilar metals are electrically connected in the presence of an electrolyte?
- Crevice corrosion
- Pitting corrosion
- Galvanic corrosion (Correct answer)
- Erosion-corrosion
Correct answer: Galvanic corrosion
Galvanic corrosion occurs when two metals with different electrochemical potentials are coupled in an electrolyte, accelerating corrosion of the more anodic (less noble) metal.
Question 7: In dual-phase (DP) advanced high-strength steel, which two microstructural phases are typically present?
- Pearlite and bainite
- Ferrite and martensite (Correct answer)
- Austenite and cementite
- Ferrite and pearlite
Correct answer: Ferrite and martensite
Dual-phase steels consist of a soft ferrite matrix with islands of hard martensite, combining good formability with high tensile strength—a combination important in automotive structural welding.
WCL Metallurgy & Material Science 5
What is the primary metallurgical difference between 'ferrite' and 'austenite' in steel?