WCL Metallurgy & Material Science 3 — Questions and Answers
Question 1: What is the significance of the 'carbon equivalent' (CE) formula in welding?
- It predicts the tensile strength of the completed weld
- It estimates hardenability and cold cracking susceptibility of the base metal (Correct answer)
- It determines the required electrode diameter for the job
- It calculates the amount of filler metal needed
Correct answer: It estimates hardenability and cold cracking susceptibility of the base metal
The CE formula combines carbon and alloying element percentages to estimate how hardenable a steel is and thus how susceptible it is to hydrogen-induced cold cracking.
Question 2: Interpass temperature control during multi-pass welding primarily prevents which metallurgical problem?
- Undercut at the weld toes
- Excessive grain growth and reduced toughness in the HAZ (Correct answer)
- Incomplete fusion between passes
- Slag inclusion on the root pass
Correct answer: Excessive grain growth and reduced toughness in the HAZ
Allowing interpass temperature to become too high causes prolonged exposure to elevated temperatures, promoting coarse grain growth and degrading HAZ toughness.
Question 3: Which type of stainless steel is NOT hardenable by heat treatment?
- Martensitic stainless steel
- Precipitation-hardening stainless steel
- Austenitic stainless steel (Correct answer)
- Ferritic stainless steel is not hardenable by heat treatment
Correct answer: Austenitic stainless steel
Austenitic stainless steels cannot be hardened by heat treatment because they do not undergo a martensitic transformation; they can only be work-hardened.
Question 4: What is 'hot cracking' (solidification cracking) in welds, and what promotes it?
- Cracking in the HAZ due to hydrogen absorption after welding
- Cracking in the fusion zone during solidification due to low-melting-point segregates (Correct answer)
- Cracking caused by thermal fatigue during service
- Cracking at the root due to lack of fusion
Correct answer: Cracking in the fusion zone during solidification due to low-melting-point segregates
Hot cracking occurs when low-melting-point compounds (e.g., iron sulfides) segregate to grain boundaries during solidification, forming liquid films that tear under shrinkage stresses.
Question 5: In welding metallurgy, what is the 'heat-affected zone' (HAZ)?
- The weld metal that has been melted and resolidified
- The base metal region heated enough to alter its microstructure without melting (Correct answer)
- The area where flux residue remains after welding
- The zone that experiences no temperature change during welding
Correct answer: The base metal region heated enough to alter its microstructure without melting
The HAZ is the portion of the base metal adjacent to the fusion zone that is heated sufficiently to cause microstructural changes but does not reach the melting point.
Question 6: What purpose does post-weld heat treatment (PWHT) serve in pressure vessel welding?
- It increases the carbon content of the weld metal
- It relieves residual stresses and tempers hard HAZ microstructures (Correct answer)
- It increases yield strength by precipitation hardening
- It eliminates porosity trapped during solidification
Correct answer: It relieves residual stresses and tempers hard HAZ microstructures
PWHT tempers martensite, reduces residual welding stresses, and improves toughness and ductility in the HAZ, meeting code requirements for pressure-containing welds.
Question 7: Which copper alloy is considered most weldable and commonly joined by GTAW?
- Beryllium copper
- Leaded copper (free-machining)
- Deoxidized copper (phosphorus-deoxidized) (Correct answer)
- High-zinc brass (>30% Zn)
Correct answer: Deoxidized copper (phosphorus-deoxidized)
Phosphorus-deoxidized copper contains minimal oxygen, preventing porosity during welding and making it the most readily weldable copper alloy.
WCL Metallurgy & Material Science 3
What is the significance of the 'carbon equivalent' (CE) formula in welding?