API 510 Material Evaluation and Selection 5 — Questions and Answers
Question 1: When inspecting a weld overlay (cladding) applied for corrosion protection in a pressure vessel, the most critical metallurgical concern is:
- Ensuring the overlay is a minimum of 3/16 inch thick at all locations
- Confirming the overlay P-number matches the base metal P-number exactly
- Verifying that overlay thickness is included in structural wall thickness calculations
- Excessive dilution of the overlay by the base metal, which reduces corrosion resistance (Correct answer)
Correct answer: Excessive dilution of the overlay by the base metal, which reduces corrosion resistance
High dilution of the overlay by the base metal reduces the alloy content (particularly chromium in stainless overlays), which can eliminate the corrosion protection the overlay was intended to provide.
Question 2: Post-weld heat treatment (PWHT) of carbon steel pressure vessel welds is performed primarily to:
- Increase weld metal tensile strength above the base metal level
- Eliminate porosity and slag inclusions from the weld fusion zone
- Reduce residual welding stresses and lower HAZ hardness to improve toughness and SCC resistance (Correct answer)
- Improve the weld surface finish for subsequent radiographic examination
Correct answer: Reduce residual welding stresses and lower HAZ hardness to improve toughness and SCC resistance
PWHT relieves thermally induced residual stresses and tempers hard martensite in the HAZ, reducing susceptibility to brittle fracture and environmentally assisted cracking.
Question 3: The risk of High-Temperature Hydrogen Attack (HTHA) in pressure vessels operating at elevated temperatures and hydrogen partial pressures is evaluated using:
- ASME Section VIII Division 2 design-by-analysis rules
- API RP 941 (Nelson Curves) (Correct answer)
- NACE MR0175/ISO 15156 hardness acceptance criteria
- API 579 Level 3 fitness-for-service assessment procedures
Correct answer: API RP 941 (Nelson Curves)
API RP 941 provides the Nelson Curves that define temperature and hydrogen partial pressure limits for carbon and alloy steels to avoid HTHA in refinery hydrogen service.
Question 4: Austenitic stainless steel pressure vessels in service above approximately 140°F (60°C) in the presence of chlorides are most likely to fail by:
- Chloride stress corrosion cracking (SCC) (Correct answer)
- High-temperature hydrogen attack in the base metal
- Sigma phase embrittlement after short-term thermal exposure
- Graphitization of weld metal in the fusion zone
Correct answer: Chloride stress corrosion cracking (SCC)
Chloride SCC is the dominant failure mode for austenitic stainless steels in chloride environments above roughly 140°F, requiring only tensile stress, chloride ions, and elevated temperature.
Question 5: Chrome-moly alloy steels such as 2.25Cr-1Mo (P22) are selected over carbon steel for service above approximately 850°F (454°C) primarily because they offer:
- Better pitting resistance in chloride-containing process streams
- Lower PWHT temperature requirements compared to carbon steel
- Superior weldability without preheat requirements
- Higher creep strength and greater oxidation resistance at elevated temperatures (Correct answer)
Correct answer: Higher creep strength and greater oxidation resistance at elevated temperatures
Chromium and molybdenum additions significantly raise the creep strength and oxidation resistance, allowing these steels to sustain design stresses where carbon steel would fail by creep deformation.
Question 6: When selecting a replacement material for a high-temperature pressure vessel shell, the allowable stress at the design temperature must be obtained from:
- API 510 Section 7 repair and alteration requirements
- ASME Section II Part D stress tables for the selected material (Correct answer)
- ASME Section IX welding qualification records for the repair procedure
- API 570 piping inspection code material property tables
Correct answer: ASME Section II Part D stress tables for the selected material
ASME Section II Part D contains the allowable stress tables for all code-approved pressure vessel materials across the full design temperature range.
Question 7: If the original ASME design specification required supplemental impact testing (e.g., S-2 supplementary requirements), replacement material used in a repair must:
- Meet at minimum the same supplemental impact test requirements as the original specification (Correct answer)
- Pass impact tests at a temperature 10°F colder than the original test temperature
- Satisfy only current base code requirements since the vessel is already in service
- Be upgraded to the next higher toughness material grade regardless of service conditions
Correct answer: Meet at minimum the same supplemental impact test requirements as the original specification
Replacement material must maintain the original design intent, including any supplemental testing requirements established by the original code of construction.
API 510 Material Evaluation and Selection 5
When inspecting a weld overlay (cladding) applied for corrosion protection in a pressure vessel, the most critical metallurgical concern is: