AWS AWS Metallurgy & Materials 5 — Questions and Answers
Question 1: What microstructural feature is responsible for the high hardness of martensite in steel?
- Coarse pearlite lamellae
- Supersaturated solid solution of carbon in a BCT iron lattice (Correct answer)
- Large spheroidized carbides
- Equiaxed ferrite grains
Correct answer: Supersaturated solid solution of carbon in a BCT iron lattice
Martensite's extreme hardness results from carbon atoms being trapped in a distorted body-centered tetragonal lattice, creating high internal strain.
Question 2: In a copper-zinc (brass) alloy weld, which problem is most likely to occur due to the low boiling point of zinc?
- Hot tearing at the solidification front
- Porosity from zinc vaporization (Correct answer)
- Hydrogen embrittlement
- Sigma phase formation
Correct answer: Porosity from zinc vaporization
Zinc has a boiling point below typical welding temperatures, causing it to vaporize and create porosity and toxic fume hazards.
Question 3: What does a high carbon equivalent (CE) value indicate about a steel's weldability?
- Excellent weldability with no preheat required
- Poor weldability with increased risk of hydrogen cracking and HAZ hardening (Correct answer)
- High ductility in the as-welded condition
- Resistance to hot cracking
Correct answer: Poor weldability with increased risk of hydrogen cracking and HAZ hardening
A high CE indicates the steel will harden significantly in the HAZ and is susceptible to hydrogen-induced cracking, requiring preheat and low-hydrogen procedures.
Question 4: Which phenomenon occurs when two dissimilar metals with different electrochemical potentials are joined by welding and exposed to an electrolyte?
- Stress corrosion cracking
- Galvanic corrosion (Correct answer)
- Crevice corrosion
- Erosion corrosion
Correct answer: Galvanic corrosion
Galvanic corrosion occurs when dissimilar metals in electrical contact are exposed to an electrolyte, causing the more active (anodic) metal to corrode preferentially.
Question 5: What is the function of titanium or niobium additions in stabilized austenitic stainless steels used for welding?
- To increase the coefficient of thermal expansion
- To preferentially combine with carbon and prevent chromium carbide sensitization (Correct answer)
- To lower the austenite-to-ferrite transformation temperature
- To improve electrical conductivity
Correct answer: To preferentially combine with carbon and prevent chromium carbide sensitization
Ti and Nb have stronger affinity for carbon than chromium does, forming their own carbides and preventing the chromium depletion that causes sensitization.
Question 6: In welding aluminum alloys, what is the primary source of porosity in the weld metal?
- Nitrogen absorbed from the atmosphere
- Hydrogen dissolved from moisture in the oxide layer or filler wire (Correct answer)
- Carbon dioxide from the shielding gas
- Oxygen from incomplete fusion
Correct answer: Hydrogen dissolved from moisture in the oxide layer or filler wire
Aluminum's oxide layer readily absorbs moisture, and hydrogen solubility drops sharply upon solidification, causing hydrogen bubbles to form as porosity.
Question 7: Which heat treatment process is used to reduce residual stresses in a welded structure without significantly changing the microstructure?
- Full annealing
- Solution annealing
- Post-weld heat treatment (PWHT) / stress relief (Correct answer)
- Quench and temper
Correct answer: Post-weld heat treatment (PWHT) / stress relief
PWHT stress relief involves heating the weldment to a temperature below the transformation range and holding it to allow plastic flow and stress relaxation.
AWS AWS Metallurgy & Materials 5
What microstructural feature is responsible for the high hardness of martensite in steel?