API 571 Mechanical and Metallurgical Failures 2 — Questions and Answers
Question 1: Creep damage in metals is thermally activated and occurs when sustained stress is applied above what temperature fraction of absolute melting point?
- 10% of absolute melting temperature
- 25% of absolute melting temperature
- 40–50% of absolute melting temperature (Correct answer)
- 75% of absolute melting temperature
Correct answer: 40–50% of absolute melting temperature
Creep becomes significant when metals operate above approximately 40–50% of their absolute melting temperature (Tm) under sustained stress.
Question 2: Which stage of creep directly precedes and leads to fracture?
- Primary creep with decreasing strain rate
- Secondary (steady-state) creep with constant strain rate
- Tertiary creep with accelerating strain rate and void coalescence (Correct answer)
- Incubation period before measurable creep begins
Correct answer: Tertiary creep with accelerating strain rate and void coalescence
Tertiary creep is characterized by accelerating deformation due to microstructural damage accumulation — void growth, coalescence, and necking — that terminates in fracture.
Question 3: How does stress rupture differ from classic long-term creep?
- Stress rupture occurs at lower temperatures than creep
- Stress rupture occurs at higher stress levels with shorter time to failure (Correct answer)
- Stress rupture involves no measurable plastic deformation
- Stress rupture only affects non-ferrous alloys
Correct answer: Stress rupture occurs at higher stress levels with shorter time to failure
Stress rupture is a form of high-stress creep failure that occurs in a relatively short time at stresses near or above the yield strength at elevated temperature.
Question 4: 885°F (475°C) embrittlement in ferritic stainless steels is caused by precipitation of which phase?
- Sigma phase intermetallic
- Alpha-prime chromium-rich phase in the BCC matrix (Correct answer)
- Grain boundary chromium carbides
- Surface carburization layer
Correct answer: Alpha-prime chromium-rich phase in the BCC matrix
Alpha-prime, a chromium-rich BCC phase, precipitates in high-chromium ferritic stainless steels near 475°C through spinodal decomposition, causing severe embrittlement at room temperature.
Question 5: What microstructural change occurs during graphitization of carbon steel at elevated temperatures?
- Increases tensile strength through precipitation hardening
- Iron carbides transform to graphite nodules, reducing toughness and strength (Correct answer)
- Improves corrosion resistance by forming a carbon-rich surface
- Increases ductility by removing hard carbide phases
Correct answer: Iron carbides transform to graphite nodules, reducing toughness and strength
Graphitization at temperatures above ~800°F causes iron carbides (Fe3C) to decompose into graphite nodules, creating local weak points that reduce toughness and strength.
Question 6: Which mechanical test property is the best indicator of brittle fracture susceptibility and the ductile-to-brittle transition temperature?
- Ultimate tensile strength
- Charpy V-notch impact toughness (Correct answer)
- Vickers hardness (HV)
- 0.2% offset yield strength
Correct answer: Charpy V-notch impact toughness
Charpy V-notch testing measures impact toughness across a temperature range, directly determining the DBTT and quantifying brittle fracture susceptibility.
API 571 Mechanical and Metallurgical Failures 2
Creep damage in metals is thermally activated and occurs when sustained stress is applied above what temperature fraction of absolute melting point?