Architecting on AWS Certification Welding Metallurgy & Materials 5 — Questions and Answers
Question 1: What is the 'nil ductility transition temperature' (NDT) and why is it important in welded structural design?
- The temperature below which fracture mode transitions from ductile to brittle, determining the minimum safe operating temperature for welded structures (Correct answer)
- The temperature at which weld metal loses all tensile strength during PWHT
- The temperature above which hydrogen cracking can no longer occur in the HAZ
- The temperature at which ferrite transforms to austenite in duplex stainless steel
Correct answer: The temperature below which fracture mode transitions from ductile to brittle, determining the minimum safe operating temperature for welded structures
The NDT is the temperature below which a material fractures without plastic deformation; structural components must not operate below this temperature to avoid catastrophic brittle fracture.
Question 2: Which alloying element in nickel-base superalloy filler metals is primarily responsible for their high-temperature strength and oxidation resistance?
- Molybdenum
- Chromium (Correct answer)
- Cobalt
- Tungsten
Correct answer: Chromium
Chromium forms a protective Cr2O3 oxide scale that provides oxidation resistance and also solid-solution strengthens the nickel matrix at elevated temperatures.
Question 3: What is 'solidification cracking' and during which phase of the welding thermal cycle does it occur?
- A crack that forms in the solid HAZ during cooling due to thermal contraction stresses
- A crack that forms in the weld metal during the final stages of solidification when liquid films are present between dendrites (Correct answer)
- A crack that propagates along the fusion boundary during PWHT
- A crack caused by hydrogen embrittlement in the weld metal after complete cooling
Correct answer: A crack that forms in the weld metal during the final stages of solidification when liquid films are present between dendrites
Solidification cracking (hot cracking) occurs when low-melting liquid films persist between dendrites near the solidus temperature and cannot withstand the tensile stresses from thermal contraction.
Question 4: In FCAW and SMAW, what is the primary function of the slag that forms on top of the weld bead?
- To add alloying elements uniformly across the entire weld width
- To protect the hot weld metal from atmospheric contamination and control cooling rate (Correct answer)
- To conduct heat away from the weld pool to increase travel speed
- To provide electrical conductivity for arc stabilization
Correct answer: To protect the hot weld metal from atmospheric contamination and control cooling rate
Slag shields the solidifying weld metal from oxygen and nitrogen, and its thermal insulation slows cooling, reducing susceptibility to martensite formation and improving mechanical properties.
Question 5: What is 'stress corrosion cracking' (SCC) in welded structures, and which combination of conditions is required for it to occur?
- Fatigue cracking accelerated by a corrosive environment, requiring only tensile stress and humidity
- Simultaneous presence of a susceptible material, a corrosive environment, and tensile stress (residual or applied) (Correct answer)
- Hydrogen-induced cracking in the HAZ requiring only high-strength steel and moisture
- Galvanic corrosion requiring dissimilar metals and an electrolyte
Correct answer: Simultaneous presence of a susceptible material, a corrosive environment, and tensile stress (residual or applied)
SCC requires the simultaneous combination of a susceptible material, a specific corrosive medium, and sufficient tensile stress; removing any one factor prevents cracking.
Question 6: What is the significance of the 'T8/5' cooling parameter in weld metallurgy?
- The time in seconds for the weld pool to cool from 800°C to 500°C, which controls HAZ microstructure in steels (Correct answer)
- The ratio of heat input at 800°C to heat input at 500°C for process comparison
- The temperature differential between the weld toe and the HAZ boundary at 500°C
- The minimum interpass temperature required for high-alloy steels
Correct answer: The time in seconds for the weld pool to cool from 800°C to 500°C, which controls HAZ microstructure in steels
T8/5 is the cooling time through the critical 800–500°C range where austenite transforms; fast T8/5 promotes martensite/bainite, while slow T8/5 promotes ferrite/pearlite in the HAZ.
Question 7: Which non-destructive testing (NDT) method is most effective for detecting planar, surface-breaking cracks at weld toes in ferritic steel?
- Radiographic testing (RT)
- Ultrasonic testing (UT) with phased array
- Magnetic particle testing (MT) (Correct answer)
- Liquid penetrant testing (PT)
Correct answer: Magnetic particle testing (MT)
Magnetic particle testing creates a leakage field at surface or near-surface discontinuities in ferromagnetic materials, making it highly sensitive to planar cracks at weld toes.
Architecting on AWS Certification Welding Metallurgy & Materials 5
What is the 'nil ductility transition temperature' (NDT) and why is it important in welded structural design?