CWI Metallurgy and Weld Discontinuities 5 — Questions and Answers
Question 1: What is Post-Weld Heat Treatment (PWHT) primarily used to achieve in weld joints?
- Increase hardness of the weld
- Relieve residual stresses and improve toughness (Correct answer)
- Eliminate porosity
- Increase tensile strength
Correct answer: Relieve residual stresses and improve toughness
PWHT (stress relief heat treatment) reduces residual stresses from welding, softens the HAZ, improves toughness, and allows hydrogen to diffuse out of the weld.
Question 2: Which term describes the phenomenon where ferrite forms preferentially at prior austenite grain boundaries in the HAZ of carbon steel?
- Widmanstätten ferrite
- Acicular ferrite
- Grain boundary ferrite (Correct answer)
- Allotriomorphic ferrite
Correct answer: Grain boundary ferrite
Grain boundary ferrite (allotriomorphic ferrite) forms along prior austenite grain boundaries during cooling, which can reduce toughness when it forms a continuous network.
Question 3: A weld inspector observes cracks at the weld toe on a high-strength steel structure found during inspection 48 hours after welding. The most likely cause is:
- Hot cracking from solidification
- Hydrogen-induced delayed cracking (Correct answer)
- Liquation cracking
- Solidification shrinkage cracking
Correct answer: Hydrogen-induced delayed cracking
Delayed hydrogen-induced cracking (cold cracking) can occur hours or days after welding completion as hydrogen diffuses to areas of high stress concentration such as the weld toe.
Question 4: Which type of porosity consists of a single pore larger than the maximum allowable by code, isolated in the weld?
- Cluster porosity
- Piping porosity
- Scattered porosity
- Individual or isolated porosity (Correct answer)
Correct answer: Individual or isolated porosity
Isolated (individual) porosity refers to a single pore that is separated from other pores by a specified minimum distance, evaluated against size limits in the applicable code.
Question 5: What is 'burn-through' in welding?
- Excessive spatter on the weld face
- A hole or void completely through the weld or base metal caused by excessive heat (Correct answer)
- Undercut along the weld toe
- Excessive penetration below the root surface
Correct answer: A hole or void completely through the weld or base metal caused by excessive heat
Burn-through occurs when excessive heat input melts completely through the joint, creating a hole in the weld or base metal.
Question 6: The ductile-to-brittle transition temperature (DBTT) is a critical consideration for materials used in:
- High-temperature pressure vessel service
- Low-temperature or cryogenic service applications (Correct answer)
- High-current electrical applications
- High-humidity marine environments
Correct answer: Low-temperature or cryogenic service applications
DBTT is especially important for low-temperature and cryogenic applications where steel may operate below the transition temperature and fail in a brittle manner under impact loading.
Question 7: Which type of weld discontinuity is described as elongated or tubular pores aligned roughly parallel to the weld axis, often indicating moisture contamination?
- Cluster porosity
- Wormhole (piping) porosity (Correct answer)
- Scattered porosity
- Surface porosity
Correct answer: Wormhole (piping) porosity
Wormhole or piping porosity consists of elongated gas pores that form in a tubular shape, often caused by moisture or hydrocarbon contamination producing gas during solidification.
CWI Metallurgy and Weld Discontinuities 5
What is Post-Weld Heat Treatment (PWHT) primarily used to achieve in weld joints?