API 510 Material Evaluation 4 — Questions and Answers
Question 1: A 316L stainless steel vessel is suspected of sensitization from an improper heat treatment. Which nondestructive test can detect chromium carbide precipitation at grain boundaries?
- Standard Charpy impact testing
- Electrochemical potentiokinetic reactivation (EPR) testing (Correct answer)
- Standard hardness testing using Rockwell B scale
- Radiographic testing of weld zones
Correct answer: Electrochemical potentiokinetic reactivation (EPR) testing
EPR testing is an electrochemical NDE method specifically designed to detect sensitization in austenitic stainless steels by measuring chromium depletion at grain boundaries.
Question 2: When reviewing material specifications for a vessel repair, an inspector notes the replacement plate is SA-516-70 and the existing vessel shell is SA-285-C. What is the primary concern with this material combination?
- Galvanic corrosion between the two carbon steels
- Mismatch in tensile strength affecting weld joint efficiency calculations (Correct answer)
- Incompatibility in welding procedures requiring different filler metals
- Significant difference in thermal expansion coefficients
Correct answer: Mismatch in tensile strength affecting weld joint efficiency calculations
SA-516-70 (70 ksi tensile) and SA-285-C (55 ksi tensile) have different strength levels, requiring the engineer to base joint efficiency calculations on the weaker material.
Question 3: A vessel in amine service develops cracking that is confirmed as amine stress corrosion cracking. Which metallurgical condition of the base metal and welds is the most important risk factor?
- High residual tensile stresses from fabrication combined with hardness above 200 BHN (Correct answer)
- Low carbon content of the steel
- Excessive grain refinement from normalizing heat treatment
- High ferrite content in the weld deposit
Correct answer: High residual tensile stresses from fabrication combined with hardness above 200 BHN
Amine SCC requires tensile stress to propagate; high residual stresses from cold forming or welding combined with elevated hardness significantly increase susceptibility.
Question 4: An inspector identifies a section of vessel wall that has experienced creep damage. Which of the following microstructural features confirms advanced creep damage during metallurgical examination?
- Grain growth and spheroidization of carbides only
- Creep voids and microfissures along grain boundaries leading to grain boundary cracking (Correct answer)
- Oxide scale formation on external surfaces
- Decarburization of the outer surface layer
Correct answer: Creep voids and microfissures along grain boundaries leading to grain boundary cracking
Advanced creep damage is characterized by creep voids (cavities) forming at grain boundaries that link to form microfissures, ultimately leading to intergranular cracking and failure.
Question 5: A low-alloy steel (2.25Cr-1Mo) vessel has undergone long-term service at 850°F. The inspector is evaluating the material for temper embrittlement. Which test best quantifies the degree of embrittlement?
- Tensile testing at room temperature
- Charpy V-notch impact testing at 32°F vs. baseline impact values (Correct answer)
- Hardness survey across the vessel shell
- Step-cooling heat treatment followed by Charpy testing
Correct answer: Charpy V-notch impact testing at 32°F vs. baseline impact values
Charpy V-notch impact testing at a reference temperature compared to original baseline values directly measures the shift in DBTT caused by temper embrittlement.
Question 6: An API 510 inspector must evaluate a vessel with unknown wall thickness measurements that are less than the original nominal thickness minus the corrosion allowance. What is the first step in the fitness-for-service evaluation?
- Immediately recommend vessel retirement
- Calculate the required minimum thickness per the applicable design code at current operating conditions (Correct answer)
- Apply a 50% reduction in MAWP as a safety factor
- Schedule immediate shutdown for hydrotest
Correct answer: Calculate the required minimum thickness per the applicable design code at current operating conditions
The first step is calculating the required minimum thickness (t_min) per the original code formulas using current operating pressure and temperature to determine if remaining thickness is adequate.
Question 7: During material evaluation of a carbon steel vessel, the inspector finds spheroidized carbides throughout the microstructure. What does this indicate about the material's service history and current condition?
- The steel has been exposed to elevated temperatures causing carbide agglomeration, reducing strength and hardness (Correct answer)
- The steel has been cold-worked and is susceptible to strain-aging embrittlement
- The steel has undergone hydrogen embrittlement from cathodic protection
- The steel experienced rapid cooling from high temperature, increasing hardness
Correct answer: The steel has been exposed to elevated temperatures causing carbide agglomeration, reducing strength and hardness
Spheroidization results from prolonged elevated temperature exposure where lamellar carbides agglomerate into spheres, reducing tensile strength and hardness below original specification.
API 510 Material Evaluation 4
A 316L stainless steel vessel is suspected of sensitization from an improper heat treatment.
Which nondestructive test can detect chromium carbide precipitation at grain boundaries?