API 510 Material Evaluation 3 — Questions and Answers
Question 1: An austenitic stainless steel vessel used in a chloride-containing service environment develops cracking that follows grain boundaries. What is the most likely damage mechanism?
- Intergranular stress corrosion cracking (IGSCC) (Correct answer)
- Pitting corrosion progressing to cracking
- Crevice corrosion at weld toes
- Transgranular stress corrosion cracking
Correct answer: Intergranular stress corrosion cracking (IGSCC)
Intergranular SCC in austenitic stainless steels occurs when sensitization depletes chromium at grain boundaries, and chloride stress corrosion follows these weakened zones.
Question 2: During material evaluation, an inspector finds hardness readings of 250 HB on a weld heat-affected zone in wet H2S service. What is the significance of this finding?
- Acceptable; hardness indicates good weld quality
- Concerning; NACE MR0103 limits HAZ hardness to 200 HB maximum for wet H2S service
- Concerning; hardness exceeds 248 HB limit per NACE MR0103/ISO 15156 (Correct answer)
- Irrelevant; hardness does not affect H2S cracking susceptibility
Correct answer: Concerning; hardness exceeds 248 HB limit per NACE MR0103/ISO 15156
NACE MR0103/ISO 15156 limits hardness to 248 HB (22 HRC) for carbon and low-alloy steels in wet H2S service to prevent sulfide stress cracking.
Question 3: A material evaluation reveals a carbon steel vessel component has experienced graphitization. Which condition is most likely responsible?
- Long-term service above 850°F (454°C)
- Extended service at temperatures between 650°F and 875°F (343-468°C) (Correct answer)
- Short-term overheating above 1400°F (760°C)
- Operation in cyclic temperature service below 400°F
Correct answer: Extended service at temperatures between 650°F and 875°F (343-468°C)
Graphitization occurs in carbon steels typically between 650-875°F over extended periods, where iron carbide decomposes to form graphite nodules, reducing toughness.
Question 4: When evaluating duplex stainless steel for fitness-for-service, which microstructural condition most severely degrades toughness and must be confirmed absent?
- Ferrite content below 40%
- Sigma phase formation from extended exposure between 600-1700°F (Correct answer)
- Sensitization at temperatures above 1500°F
- Martensite formation from cold work
Correct answer: Sigma phase formation from extended exposure between 600-1700°F
Sigma phase forms in duplex stainless steels exposed to 600-1700°F and causes severe embrittlement by consuming the ferrite phase.
Question 5: An inspector is evaluating a vessel that previously contained a caustic solution. Cracking is found in welds and heat-affected zones. What is the required assessment approach per API 579-1/ASME FFS-1?
- Level 1 assessment using remaining wall thickness only
- Environment-assisted cracking assessment per Part 9 of API 579 (Correct answer)
- Simple retirement based on presence of any cracking
- Creep damage assessment since caustic is a high-temperature service
Correct answer: Environment-assisted cracking assessment per Part 9 of API 579
Caustic SCC is an environment-assisted cracking mechanism evaluated under Part 9 of API 579-1/ASME FFS-1 for stress corrosion cracking fitness-for-service.
Question 6: A pressure vessel made of SA-516 Grade 70 carbon steel has been in service for 30 years at low temperature. An inspector is concerned about brittle fracture. Which factor is NOT a primary driver of brittle fracture susceptibility?
- Operating temperature relative to the DBTT
- Applied stress magnitude relative to yield strength
- Presence of sharp notches or crack-like flaws
- The vessel's original PWHT status (Correct answer)
Correct answer: The vessel's original PWHT status
While PWHT reduces residual stresses, brittle fracture susceptibility is primarily driven by temperature vs. DBTT, stress level, and flaw geometry rather than PWHT history alone.
Question 7: During a material traceability audit, an inspector discovers that weld filler metal certifications are missing for a repair performed five years ago. According to API 510, what is the appropriate course of action?
- Accept the repair as-is since it has been in service without incident
- Require PMI testing of the weld deposit to verify composition (Correct answer)
- Immediately declare the vessel unfit for service
- Request the contractor reconstruct records from memory
Correct answer: Require PMI testing of the weld deposit to verify composition
When weld filler metal certifications are unavailable, PMI testing of the actual weld deposit is the appropriate method to verify compositional compliance.
API 510 Material Evaluation 3
An austenitic stainless steel vessel used in a chloride-containing service environment develops cracking that follows grain boundaries.
What is the most likely damage mechanism?