Preheat Decision for a Thick Section Repair
A repair is needed on a 50 mm thick A516 Grade 70 pressure-vessel shell. The ambient shop temperature is 8 C. The existing WPS covers 12-25 mm thickness only and lists a 100 C minimum preheat. The repair cavity is 40 mm deep and will be filled with SMAW using E7018 electrodes from a holding oven at 120 C. The carbon equivalent of the plate, from the mill certificate, is 0.42. The vessel has been in service and is confirmed clean of process fluid. The contractor proposes to use the existing WPS by treating the repair as a 25 mm weld because each pass is thin, and to skip preheat because the shop is 'warm enough'.
1. Evaluate the contractor's proposal against qualification and metallurgical requirements.
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The proposal fails on two grounds. First, WPS qualification is by base-metal thickness, not by individual pass thickness. A 50 mm shell is outside the 12-25 mm qualified range, so the existing WPS does not cover the joint. A new PQR is required, or a qualified WPS covering the thickness. Second, preheat is not optional at 8 C ambient with a 0.42 CE steel and 50 mm section. The high thickness and carbon equivalent raise hardenability and restraint; a 100 C minimum preheat is a floor, and for this section the qualified preheat may need to be higher. The contractor's 'warm enough' argument is unsupported. The inspector should reject the proposal, require a qualified WPS with documented preheat and interpass control, and require preheat measurement with contact thermometers or temperature-indicating crayons before welding.
2. What else should be controlled during the repair fill, and why?
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Interpass temperature must be capped as well as preheat maintained. For a 50 mm restrained repair, excessive interpass temperature can degrade toughness, while insufficient preheat promotes hydrogen cracking. The E7018 electrodes must be kept in the holding oven and issued in limited quantities, with exposure time controlled, because moisture pickup raises diffusible hydrogen. The repair should use low-hydrogen practice, and the completed weld should get a post-weld hydrogen bake-out or controlled cooling per the qualified procedure. After welding, the cavity should be inspected by MT or PT at the root and final surface, and by UT or RT per the vessel code. Records of preheat, interpass, electrode lot, and NDE results should be retained. The controlling idea is that thickness, restraint, and CE together make this a high-risk repair.