Sour-Service Shell Repair: Hardness, PWHT and Release Authority
An Asian fabricator is building three sour-service shell-and-tube exchangers under a contract that invokes ASME Section VIII Div. 1 and NACE MR0175/ISO 15156. The channel shell is SA-516 Gr. 70N, 38 mm thick. After UT of a longitudinal SAW seam, two planar indications are excavated to a depth of 14 mm over a 90 mm length and repaired with SMAW. The qualified WPS specifies 150 °C preheat and post-weld heat treatment at 610 °C for 1 hour. The hardness survey reports base metal 180 HV10, weld 230 HV10, and HAZ 245 HV10. The contract sets a maximum hardness of 250 HV10 and requires compliance with NACE. The fabricator argues that 245 HV10 is acceptable, that the repair is only 14 mm deep, and that re-PWHT would distort the shell and delay shipment by five days. A previous repair exists 150 mm away in the same seam. The client's resident engineer is on leave; the fabricator asks the supervisor to sign the release.
1. Are the hardness results and PWHT acceptable as presented? State the technical basis and the disposition you would issue.
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They are not acceptable as presented. The PWHT holding time is deficient: for P-No. 1 material, ASME Section VIII Div. 1 requires about 1 hour per 25 mm of thickness, with a minimum of 1 hour. For a 38 mm shell this is approximately 1.5 hours at temperature, not 1 hour. The recorded 1 hour therefore does not validate the hardness survey.
The HAZ value of 245 HV10 is close to the 250 HV10 contract ceiling. Hardness conversion between HV10 and HRC is not exact near the NACE limit, and the HAZ is the critical region for sour service. A value within 5 HV10 of the limit, after an under-time PWHT, does not demonstrate compliance with NACE MR0175/ISO 15156. The argument that the repair is only 14 mm deep is also weak: 14 mm exceeds one-third of 38 mm, so a minor-repair PWHT exemption cannot be assumed, and the nearby prior repair means the cumulative HAZ is larger than a single isolated repair.
Disposition: reject the current PWHT record; require re-PWHT at the qualified temperature for at least 1.5 hours with continuous thermocouple records; repeat the hardness survey on weld, HAZ and base metal after final PWHT; require compliance with both 250 HV10 and the applicable NACE HRC limit; and re-run UT/MT after PWHT. Release should be withheld until the NCR is closed.
2. What NDT and documentation must be verified before the repair is accepted, and what is the supervisor's role in that verification?
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Before acceptance the supervisor should verify: the repair WPS and PQR cover the actual repair, including preheat, PWHT and hardness testing; the welder is qualified for the process and position; the excavation was checked by PT or MT before welding; the completed repair has appropriate UT or RT plus surface NDT after PWHT; delayed cracking checks are performed after the specified cooling period; and the final hardness map is traceable to the repaired area and the adjacent prior repair.
The documentation package should include the NCR, excavation sketch with dimensions, welding records, preheat and PWHT charts with thermocouple locations, consumable certificates, NDT reports with technician certification, hardness results, and the final as-built record. The supervisor should witness critical steps where the contract requires it and should audit calibration certificates for the hardness tester and NDT equipment.
The supervisor's role is verification and independent release, not performing production NDT or signing for the fabricator. The supervisor should confirm that the NDT technician's certification is valid and that the reported locations match the actual repair. If any record is missing or cannot be traced, the supervisor should not accept a verbal assurance; the correct action is to issue or keep open an NCR and require re-test.
3. The fabricator says the client's engineer gave a verbal instruction to ship without re-PWHT. How should the supervisor respond?
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The supervisor should not release the shell on a verbal instruction. A verbal waiver is not a controlled contract document, and the resident engineer who is on leave may not be the authorized person to approve a technical deviation. More importantly, the issue is not merely contractual: the repair would not be supported by compliant PWHT records or demonstrated hardness control for sour service.
The supervisor should immediately put the position in writing: the current PWHT duration is below the code-required holding time, the hardness margin is insufficient, and the repair is not accepted. The fabricator should be told to preserve the shell and not ship it. The supervisor should notify their own manager and the client's authorized representative, issue or confirm the NCR, and request a formal disposition. If the client wishes to consider a deviation, it must be evaluated by the responsible design authority and accepted in writing; the supervisor cannot sign a release that would imply code and NACE compliance.
If the fabricator ships anyway, the supervisor should document the shipment, withhold the release certificate, and escalate to the client and the inspection agency's quality function. A commercial promise or schedule pressure does not override the need for traceable compliance.