AWS Welding Metallurgy & Heat Treatment 3 — Questions and Answers
Question 1: What welding metallurgy phenomenon causes the HAZ of certain stainless steels to become susceptible to intergranular corrosion?
- Sigma phase formation
- Sensitization from chromium carbide precipitation (Correct answer)
- Delta ferrite dissolution
- Grain boundary liquation
Correct answer: Sensitization from chromium carbide precipitation
Sensitization occurs when chromium carbides precipitate at grain boundaries in the 425–870°C range, depleting adjacent areas of chromium and reducing corrosion resistance.
Question 2: The liquidus temperature in a binary alloy phase diagram represents:
- Temperature at which the alloy is completely solid
- Temperature below which no liquid exists
- Temperature above which the alloy is completely liquid (Correct answer)
- Eutectic reaction temperature
Correct answer: Temperature above which the alloy is completely liquid
The liquidus is the temperature above which the alloy exists entirely as liquid; below this line, solidification begins.
Question 3: In welding metallurgy, 'dilution' refers to:
- Adding filler metal to reduce strength
- Reduction in weld metal properties due to base metal mixing (Correct answer)
- Thinning of the base metal by arc heat
- Reducing preheat temperature
Correct answer: Reduction in weld metal properties due to base metal mixing
Dilution is the proportion of base metal melted into the weld pool, which changes the weld metal chemistry and can affect mechanical properties and corrosion resistance.
Question 4: Which intermetallic phase in stainless steel welds severely embrittles the material at elevated service temperatures?
- Delta ferrite
- Martensite
- Sigma phase (Correct answer)
- Austenite
Correct answer: Sigma phase
Sigma phase, an iron-chromium intermetallic compound, forms between 600–900°C and causes severe embrittlement with near-zero impact toughness.
Question 5: The Schaeffler diagram is used in welding to predict the microstructure of:
- Carbon steel HAZ after cooling
- Stainless steel weld metal based on Cr and Ni equivalents (Correct answer)
- Aluminum weld pools under varying heat input
- Low-alloy steel transformation products
Correct answer: Stainless steel weld metal based on Cr and Ni equivalents
The Schaeffler (and WRC) diagram plots chromium equivalent vs. nickel equivalent to predict the ferrite, austenite, and martensite content of stainless steel weld deposits.
Question 6: What is the purpose of the 'interpass temperature' limit during multi-pass welding?
- Ensure complete austenite transformation before next pass
- Prevent overheating that degrades microstructure and mechanical properties (Correct answer)
- Maximize heat input to improve fusion
- Speed up welding productivity
Correct answer: Prevent overheating that degrades microstructure and mechanical properties
Limiting interpass temperature prevents excessive heat accumulation, which can coarsen grains, reduce toughness, and in some alloys cause sensitization or phase changes.
Question 7: Which welding defect is directly caused by the contraction stresses exceeding the ductility of the weld metal during solidification?
- Porosity
- Lack of fusion
- Solidification cracking (Correct answer)
- Undercut
Correct answer: Solidification cracking
Solidification cracking (hot cracking) occurs when tensile stresses during weld cooling and solidification exceed the ductility of the partially solidified weld metal.
AWS Welding Metallurgy & Heat Treatment 3
What welding metallurgy phenomenon causes the HAZ of certain stainless steels to become susceptible to intergranular corrosion?