Architecting on AWS Certification Welding Metallurgy & Materials 4 — Questions and Answers
Question 1: Which shielding gas mixture is preferred for GMAW of austenitic stainless steel to minimize carbon pickup and carbide precipitation?
- 100% CO2
- Ar + 5% O2 or Ar + 2% CO2 (Correct answer)
- He + 25% CO2
- 100% Ar
Correct answer: Ar + 5% O2 or Ar + 2% CO2
Argon with a small addition of O2 or CO2 stabilizes the arc and improves wetting without introducing significant carbon that could cause sensitization in the weld metal.
Question 2: What is the primary metallurgical reason for preheating high-carbon or alloy steels before welding?
- To increase the tensile strength of the weld metal
- To slow the cooling rate of the HAZ, reducing martensite formation and hydrogen diffusion (Correct answer)
- To reduce the heat input required to achieve full penetration
- To stabilize the austenite phase during solidification
Correct answer: To slow the cooling rate of the HAZ, reducing martensite formation and hydrogen diffusion
Preheat slows HAZ cooling, reducing martensite formation and keeping the steel above the temperature at which hydrogen diffusion stalls, allowing hydrogen to escape before cracking can initiate.
Question 3: What type of weld discontinuity is described as a planar defect running parallel to the fusion line, caused by poor inter-run fusion during multi-pass welding?
- Porosity
- Slag inclusion
- Lack of inter-run fusion (cold lap) (Correct answer)
- Hot crack
Correct answer: Lack of inter-run fusion (cold lap)
Cold lap (lack of inter-run fusion) occurs when the previous weld bead is not fully remelted, creating a planar, unbonded interface that acts as a severe stress concentrator.
Question 4: The toughness of ferritic stainless steel welds is generally poor because of which microstructural characteristic?
- Presence of retained austenite at grain boundaries
- Coarse ferritic grain structure that cannot be refined by heat treatment since there is no allotropic transformation (Correct answer)
- High carbon content leading to massive carbide precipitation
- Formation of sigma phase during welding thermal cycle
Correct answer: Coarse ferritic grain structure that cannot be refined by heat treatment since there is no allotropic transformation
Ferritic stainless steels do not undergo the austenite-to-ferrite transformation on cooling, so grain growth in the HAZ and weld metal is irreversible and leads to inherently poor toughness.
Question 5: What is the Ferrite Number (FN) used to measure in weld metal, and which instrument is used to measure it?
- Carbon content, measured by optical emission spectrometry
- Residual austenite content, measured by X-ray diffraction
- Delta ferrite content, measured by a calibrated magnetic instrument (ferritscope) (Correct answer)
- Martensite content, measured by hardness testing
Correct answer: Delta ferrite content, measured by a calibrated magnetic instrument (ferritscope)
The Ferrite Number quantifies delta ferrite in austenitic and duplex stainless steel weld metal; a ferritscope measures the magnetic response of the ferrite phase to determine FN.
Question 6: In welding aluminum alloys, what is the main cause of porosity in the weld metal?
- Nitrogen absorption from the atmosphere
- Hydrogen dissolved in the molten pool that becomes insoluble upon solidification (Correct answer)
- Oxygen reacting with aluminum to form Al2O3 bubbles
- CO2 from flux decomposition trapped in the weld
Correct answer: Hydrogen dissolved in the molten pool that becomes insoluble upon solidification
Hydrogen is highly soluble in molten aluminum but nearly insoluble in solid aluminum; upon solidification, the excess hydrogen forms pores that are trapped in the weld metal.
Question 7: What is the purpose of using an E309 filler metal when welding carbon steel to austenitic stainless steel (dissimilar metal weld)?
- E309 provides higher tensile strength than E308 for critical joints
- E309's higher Cr and Ni content dilutes the carbon steel contribution while maintaining an austenitic weld metal (Correct answer)
- E309 eliminates the need for post-weld heat treatment on the carbon steel side
- E309 prevents galvanic corrosion at the dissimilar metal interface
Correct answer: E309's higher Cr and Ni content dilutes the carbon steel contribution while maintaining an austenitic weld metal
E309 contains more Cr and Ni than E308 so that even after significant dilution from the carbon steel base metal, the final weld deposit remains austenitic without martensite formation.
Architecting on AWS Certification Welding Metallurgy & Materials 4
Which shielding gas mixture is preferred for GMAW of austenitic stainless steel to minimize carbon pickup and carbide precipitation?