API 510 Material Evaluation and Selection 4 — Questions and Answers
Question 1: How is the Minimum Design Metal Temperature (MDMT) defined for a pressure vessel?
- The coldest ambient air temperature at the vessel installation site
- The lowest temperature at which the vessel may be subjected to full design pressure (Correct answer)
- The minimum temperature at which in-service inspection may be performed
- The lowest operating temperature recorded during the previous year of service
Correct answer: The lowest temperature at which the vessel may be subjected to full design pressure
The MDMT is a design parameter stamped on the nameplate that sets the lower bound for pressurized operation, below which material toughness may be insufficient to prevent brittle fracture.
Question 2: Per ASME Section VIII Division 1 Figure UCS-66, impact testing exemptions for carbon steel are governed by which factors?
- Material heat treatment condition and chemical carbon content only
- Inspector's fitness-for-service evaluation and operating history
- The vessel's design pressure and temperature operating window
- Material thickness and the exemption curve (A through D) for the specific material specification (Correct answer)
Correct answer: Material thickness and the exemption curve (A through D) for the specific material specification
Figure UCS-66 assigns each carbon steel specification to a curve (A–D) based on inherent toughness, then relates nominal thickness to the minimum temperature at which impact testing is exempt.
Question 3: What is the minimum Charpy V-notch energy absorption required for weld metal and HAZ per ASME Section VIII Division 1 UG-84?
- 20 ft-lb (27 J) average with 15 ft-lb (20 J) minimum per specimen (Correct answer)
- 10 ft-lb (13.6 J) average with 7 ft-lb (9.5 J) minimum per specimen
- 15 ft-lb (20 J) average with 10 ft-lb (13.6 J) minimum per specimen
- 25 ft-lb (34 J) average with 18 ft-lb (24 J) minimum per specimen
Correct answer: 20 ft-lb (27 J) average with 15 ft-lb (20 J) minimum per specimen
ASME VIII-1 UG-84 requires a minimum average of 20 ft-lb for the three-specimen set with no single specimen below 15 ft-lb for weld procedure and material qualification.
Question 4: When performing a hydrostatic pressure test at temperatures below the vessel's MDMT, what is the primary risk the inspector must address?
- Accelerated internal corrosion from cold water contaminants
- Inaccurate pressure gauge readings due to thermal effects
- Collapse of internal components due to hydrostatic weight
- Brittle fracture of the pressure-containing shell or nozzles (Correct answer)
Correct answer: Brittle fracture of the pressure-containing shell or nozzles
Steel tested below its MDMT may have insufficient fracture toughness to resist the stress imposed by the hydrostatic test pressure, leading to brittle fracture.
Question 5: ASTM A516 Grade 70 is frequently selected for low-temperature pressure vessel applications primarily because it is:
- The highest-strength carbon steel plate available for ASME vessel fabrication
- Produced with killed, fine-grain practice that improves low-temperature toughness (Correct answer)
- Always exempt from impact testing requirements at any design temperature
- Exempt from PWHT requirements regardless of thickness per ASME Section VIII
Correct answer: Produced with killed, fine-grain practice that improves low-temperature toughness
Killed, fine-grain practice refines the grain size and reduces inclusion content, shifting the ductile-to-brittle transition temperature lower compared to non-fine-grain carbon steel plates.
Question 6: Which of the following does NOT directly influence the required Charpy impact test temperature for a pressure vessel component per ASME Section VIII?
- Nominal thickness of the component material
- The MDMT stamped on the vessel nameplate
- The ratio of required thickness to actual furnished thickness
- The color and type of external corrosion protection coating (Correct answer)
Correct answer: The color and type of external corrosion protection coating
External coating type and color have no bearing on the material's fracture toughness or the Code's minimum impact test temperature determination.
Question 7: When a set of Charpy V-notch specimens fails to meet the minimum energy requirement, which action is typically considered first per ASME Section VIII?
- Downgrading the MDMT on the vessel nameplate without re-testing
- Applying PWHT to improve toughness of already-fabricated components
- Re-testing with an additional set of specimens from the same heat or weld (Correct answer)
- Rejecting the entire heat of material and ordering new plate stock
Correct answer: Re-testing with an additional set of specimens from the same heat or weld
ASME Section VIII Division 1 UG-84 permits one additional set of three Charpy specimens if the initial set fails, provided the re-test set meets both average and minimum requirements.
API 510 Material Evaluation and Selection 4
How is the Minimum Design Metal Temperature (MDMT) defined for a pressure vessel?