2.25Cr-1Mo header: high HAZ hardness after PWHT
A 2.25Cr-1Mo (SA-387 Gr.22 Cl.2) steam header, 75 mm wall, is fabricated by submerged arc welding with 3.2 mm wire and basic flux. The purchase specification requires post-weld heat treatment at 690 ± 10 °C for 4 h, HAZ hardness ≤ 240 HV10, and Charpy 27 J at 0 °C. The furnace chart and load thermocouples show 690 °C, but one thermocouple was attached to a support bracket rather than the test piece. After PWHT, the cross-weld hardness survey gives 255–265 HV10 in the coarse-grained HAZ at 2 mm below the fusion line; weld metal is 220 HV10. The Charpy tests are not yet done. The contractor proposes a second PWHT at 700 °C for 5 h. The client is concerned about reheat cracking and temper embrittlement because the header has a high J-factor. The fabrication schedule allows 48 h for rework and retest.
1. What is the most likely reason for the excessive HAZ hardness, and what checks would you make before accepting a second PWHT?
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The hardness is not a furnace chart problem alone. A single thermocouple on a support bracket does not prove the weld zone reached 690 °C; the 255–265 HV10 band at 2 mm below the fusion line suggests a local cold zone or untempered martensite. Before accepting a second PWHT I would:
- check thermocouple attachment, furnace survey and all PWHT channels;
- review the base metal certificate (Gr.22, not a modified grade), CE and welding heat input;
- repeat a hardness traverse across the weld and HAZ;
- perform MT/UT on the HAZ.
If the only issue is under-tempering, a second PWHT may be technically possible, but it must be justified against total time at temperature, J-factor and reheat-cracking risk. I would not accept the contractor's proposal until those checks are complete.
2. Evaluate the proposed second PWHT at 700 °C for 5 h. What risks and controls would you require?
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A second PWHT at 700 °C for 5 h will add tempering, but it also increases the total time in the temper-embrittlement range and the risk of reheat cracking in a thick 2.25Cr-1Mo header. Controls:
- measure and record the J-factor/X-bar;
- check the material's minimum PWHT temperature and maximum allowable;
- heat up slowly (≤50 °C/h) with thermocouples on the weld and HAZ;
- avoid local temperature gradients; cool slowly;
- perform MT/UT before and after;
- repeat hardness and Charpy.
If the steel is temper-embrittlement sensitive, require step-cooling or a client concession. The proposed 5 h is not automatically unacceptable, but it must be a qualified repair PWHT, not an uncontrolled repeat.
3. State the acceptance evidence you would require before signing off.
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Acceptance evidence:
- PWHT chart with thermocouples on the actual weld;
- hardness ≤240 HV10 at the specified locations;
- Charpy 27 J at 0 °C;
- MT/UT of the HAZ and weld;
- dimensional check and full traceability;
- a documented rationale for the second PWHT and client approval.
If any hardness reading remains above the limit, do not repeat PWHT indefinitely; investigate the material or thermal history.