API 653 Welding & Metallurgy 4 — Questions and Answers
Question 1: What is 'sensitization' in austenitic stainless steel, and at what approximate temperature range does it typically occur?
- Grain coarsening occurring above 1800°F (982°C)
- Chromium carbide precipitation at grain boundaries occurring between 800°F and 1500°F (427–816°C) (Correct answer)
- Sigma phase formation occurring above 1600°F (871°C)
- Hydrogen embrittlement occurring below 400°F (204°C)
Correct answer: Chromium carbide precipitation at grain boundaries occurring between 800°F and 1500°F (427–816°C)
Sensitization occurs when chromium carbides precipitate at grain boundaries in the 800–1500°F range, depleting adjacent areas of chromium and making them susceptible to intergranular corrosion.
Question 2: Per API 653 and ASME Section IX, which of the following is an 'essential variable' that requires weld procedure requalification when changed?
- Changing the welding travel speed by 10%
- Changing the base metal P-Number group (Correct answer)
- Increasing the arc voltage by 5 volts within the qualified range
- Changing the electrode brand but not classification
Correct answer: Changing the base metal P-Number group
P-Number is an essential variable in ASME Section IX; changing the base metal P-Number requires a new procedure qualification test because it affects the metallurgical properties of the weld.
Question 3: What characteristic of A36 steel versus A516 Grade 70 steel makes A516-70 preferable for storage tank construction subjected to low-temperature service?
- A516-70 has lower carbon content, providing better weldability
- A516-70 is specifically manufactured for pressure vessel service with controlled chemistry for improved notch toughness (Correct answer)
- A516-70 has higher sulfur content improving machinability for nozzle fabrication
- A516-70 has a higher thermal expansion coefficient reducing thermal stresses
Correct answer: A516-70 is specifically manufactured for pressure vessel service with controlled chemistry for improved notch toughness
A516 is a pressure vessel quality plate with controlled chemistry and specified toughness properties, making it superior to structural A36 for tanks requiring reliable performance at low temperatures.
Question 4: When evaluating a weld repair on a tank shell using magnetic particle testing (MT), what is the primary limitation of the wet fluorescent MT method?
- It can only detect subsurface defects, not surface-breaking cracks
- It requires demagnetization of the part before inspection can begin
- It cannot be used on austenitic stainless steel or aluminum alloys (Correct answer)
- It is not approved by API 653 for shell weld inspection
Correct answer: It cannot be used on austenitic stainless steel or aluminum alloys
MT relies on magnetic flux leakage and is only applicable to ferromagnetic materials; austenitic stainless steels and non-ferrous alloys cannot be inspected using this method.
Question 5: In API 653, what is the requirement for welder qualification when a welder has not welded using a qualified process for more than 6 months?
- The welder must requalify by completing a new performance qualification test (Correct answer)
- The welder may continue working but must be supervised by a certified welding inspector
- The welder's qualification remains valid indefinitely regardless of inactivity
- The welder must complete 8 hours of refresher training before resuming welding
Correct answer: The welder must requalify by completing a new performance qualification test
Per ASME Section IX (referenced by API 653), a welder who has not welded with a qualified process for more than 6 months must requalify before performing production welds.
Question 6: What is 'weld decay' as it applies to stainless steel tank repairs?
- General corrosion of the weld metal caused by the flux
- Intergranular corrosion in the HAZ of sensitized austenitic stainless steel occurring at a short distance from the fusion line (Correct answer)
- Erosion of the weld face caused by liquid impingement
- Galvanic corrosion between the weld and base metal due to potential difference
Correct answer: Intergranular corrosion in the HAZ of sensitized austenitic stainless steel occurring at a short distance from the fusion line
Weld decay is intergranular corrosion occurring in the sensitized HAZ zone (typically 1–3 mm from the fusion line) where chromium depletion leaves grain boundaries vulnerable to corrosive attack.
Question 7: Which API 653 repair method requires the most rigorous NDE verification because it introduces a full-penetration weld in a highly stressed region of the tank shell?
- Coating a corroded area with epoxy lining
- Installing a flush patch plate using full-penetration butt welds (Correct answer)
- Applying an external corrosion inhibitor compound
- Grinding smooth a surface scratch below the corrosion allowance
Correct answer: Installing a flush patch plate using full-penetration butt welds
A flush patch with full-penetration butt welds replaces the original shell plate and must meet the same quality requirements, necessitating radiography or UT of all new butt welds.
API 653 Welding & Metallurgy 4
What is 'sensitization' in austenitic stainless steel, and at what approximate temperature range does it typically occur?