API 510 Material Evaluation and Selection 3 — Questions and Answers
Question 1: The Nelson curves published in API RP 941 define safe operating limits to prevent which damage mechanism?
- Wet H2S stress corrosion cracking in carbon steel
- High-temperature hydrogen attack (HTHA) in carbon and alloy steels (Correct answer)
- Temper embrittlement in Cr-Mo pressure vessel steels
- Chloride SCC in austenitic stainless steel
Correct answer: High-temperature hydrogen attack (HTHA) in carbon and alloy steels
API RP 941 provides temperature versus hydrogen partial pressure curves that define the boundary above which HTHA becomes a risk for carbon and low-alloy steels.
Question 2: Sigma phase embrittlement in 316 stainless steel typically develops after extended exposure in which temperature range?
- Below -50°F (-46°C) during low-temperature excursions
- During rapid quenching from solution annealing temperatures
- Between 1000°F and 1700°F (538°C and 927°C) during service (Correct answer)
- Only in cold-worked material above 500°F (260°C)
Correct answer: Between 1000°F and 1700°F (538°C and 927°C) during service
Sigma phase, an intermetallic compound that severely reduces toughness, precipitates from the ferrite phase of austenitic and duplex stainless steels within this temperature band.
Question 3: Which metallurgical damage mechanism reduces the impact toughness of 2.25Cr-1Mo steel after long-term service in the 650–1070°F (343–577°C) range?
- Graphitization of carbides in the base metal
- High-temperature hydrogen attack above the Nelson curve
- Sulfidation corrosion at the outer vessel surface
- Temper embrittlement due to tramp element grain-boundary segregation (Correct answer)
Correct answer: Temper embrittlement due to tramp element grain-boundary segregation
Temper embrittlement results from grain-boundary segregation of tramp elements (P, Sn, As, Sb), raising the ductile-to-brittle transition temperature of the alloy steel.
Question 4: API RP 571 recommends which measure as the primary mitigation for alkaline stress corrosion cracking (caustic SCC) in carbon steel pressure vessels?
- Applying PWHT to all carbon steel welds above caustic SCC threshold conditions (Correct answer)
- Using 316L stainless steel for all wetted components in caustic service
- Maintaining vessel wall temperature below 100°F during all operations
- Adding inhibitors to neutralize the caustic solution concentration
Correct answer: Applying PWHT to all carbon steel welds above caustic SCC threshold conditions
PWHT reduces residual welding stresses that, combined with caustic concentration and temperature, are the driving force for stress corrosion cracking in carbon steel.
Question 5: In wet H2S service, what is the maximum hardness specified for carbon steel weld heat-affected zones per NACE MR0175/ISO 15156 to prevent sulfide stress cracking?
- HBW 150 (HRC 10)
- HBW 300 (HRC 31)
- HBW 250 (HRC 26)
- HBW 237 (HRC 22) (Correct answer)
Correct answer: HBW 237 (HRC 22)
NACE MR0175/ISO 15156 limits carbon and low-alloy steel hardness to HRC 22 (approximately HBW 237) to prevent sulfide stress cracking in sour service.
Question 6: Amine stress corrosion cracking (amine SCC) in pressure vessels most commonly affects which material-condition combination?
- 304L stainless steel with incomplete solution annealing after fabrication
- Carbon steel operating above 500°F (260°C) in rich amine service
- Carbon steel with high residual welding stresses and inadequate PWHT (Correct answer)
- Vessels hydrostatically tested at pressures above MAWP prior to commissioning
Correct answer: Carbon steel with high residual welding stresses and inadequate PWHT
Amine SCC initiates in residual stress fields at and near welds in carbon steel vessels, and is effectively prevented by full PWHT per API 945.
Question 7: Above which approximate temperature does creep become a significant design consideration for carbon steel pressure vessel components?
- 750°F (399°C) (Correct answer)
- 500°F (260°C)
- 650°F (343°C)
- 900°F (482°C)
Correct answer: 750°F (399°C)
Carbon steel enters the creep range above approximately 700–750°F (371–399°C), where time-dependent plastic deformation must be considered in design and fitness-for-service evaluations.
API 510 Material Evaluation and Selection 3
The Nelson curves published in API RP 941 define safe operating limits to prevent which damage mechanism?