Heat Treatment Non-Conformance on Pressure Vessel
A pressure vessel is fabricated from SA-516 Gr. 70 steel, with a wall thickness of 50 mm. The welding procedure requires post-weld heat treatment (PWHT) at 620°C ± 10°C for 2 hours. The PWHT was performed, but the temperature chart shows that the furnace temperature overshot to 650°C for 30 minutes during the hold period. The vessel is now ready for hydrotest. The client's specification references ASME Section VIII Div. 1 and requires compliance with ISO 3834-2. The welding coordinator must decide whether to accept the PWHT record or take corrective action. The material's mechanical properties may be affected by the overtemperature. The coordinator has no immediate access to the original material test certificates.
1. What are the potential consequences of the PWHT overtemperature on the material and the vessel's compliance?
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The overtemperature to 650°C for 30 minutes exceeds the upper limit of the specified PWHT range (630°C). Potential consequences: (1) Material properties: SA-516 Gr. 70 may experience a decrease in tensile strength and an increase in ductile-to-brittle transition temperature due to over-tempering. The extent depends on the material's original condition and the time-temperature history. (2) The PWHT may still be acceptable if the material's properties remain within the required limits, but this must be verified. (3) Compliance: ASME Section VIII Div. 1 allows some tolerance on PWHT temperature, but the deviation may require evaluation. ISO 3834-2 requires that non-conformances be addressed and that the manufacturer demonstrate that the final product meets requirements. (4) The vessel cannot be accepted solely on the basis that PWHT was performed; the overtemperature is a non-conformance that must be dispositioned. (5) Without material test certificates, it is difficult to assess the impact; additional testing may be needed.
2. What corrective actions should the welding coordinator take to resolve the non-conformance?
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The coordinator should: (1) Review the PWHT chart and verify the actual temperature-time profile. (2) Obtain the material test certificates to determine the original mechanical properties and the effect of the overtemperature. (3) If the material is still within specification, perform additional testing (e.g., hardness testing, tensile testing on a coupon, or impact testing) to confirm properties. (4) Consult the client and the authorized inspector; propose a disposition such as accept-as-is with justification, re-PWHT if feasible (but note that re-PWHT may further affect properties), or repair/replace. (5) Document the non-conformance and the disposition in accordance with ISO 3834-2 requirements for control of non-conforming product. (6) If re-PWHT is chosen, ensure it is performed within the specified range and re-evaluate. (7) Implement corrective actions to prevent recurrence, such as calibration of furnace controllers and improved monitoring. The coordinator must not proceed to hydrotest until the non-conformance is resolved.