Filler Metal Mismatch on a Low-Alloy Pressure Vessel
A shop is fabricating a pressure vessel from 19 mm ASTM A387 Gr.11 Class 2 chrome-moly plate. The WPS calls for GTAW root and SMAW fill using an ER80S-B2 root and E8018-B2 covered electrodes, with preheat of 200 C and post-weld heat treatment at 690 C. The store room is out of E8018-B2 and issues E7018-1 instead, arguing that the tensile strength is similar and the vessel will be PWHT anyway. The welder completes the root and two fill passes before the QC inspector notices the substitution.
1. Assess the consequences of substituting E7018-1 for E8018-B2 in this joint.
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E7018-1 is a carbon-manganese low-hydrogen electrode, not a chrome-moly electrode. The base metal is 1.25% Cr, 0.5% Mo steel, and the WPS requires matching composition to provide creep strength and to match the base metal's response to PWHT. Substituting E7018-1 creates an undermatched weld deposit with essentially no chromium or molybdenum, so the weld metal will have lower elevated-temperature strength and different hardenability. Under the 200 C preheat the carbon-manganese deposit may still avoid cracking, but the joint will not meet the design basis for a chrome-moly vessel. PWHT at 690 C will temper the deposit but cannot add alloying elements. The substitution is a filler metal essential variable violation and the completed passes are nonconforming. The strength similarity at room temperature is irrelevant because the vessel is designed for elevated-temperature service.
2. What disposition and corrective actions are appropriate?
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The two fill passes and root made with E7018-1 must be removed. Grind or arc-gouge out the nonconforming weld metal back to sound base metal, with preheat maintained, and verify removal by PT or MT before re-welding. Re-weld with the correct ER80S-B2 root and E8018-B2 fill per the WPS, maintaining preheat and interpass limits, then perform the specified PWHT and required NDE. Investigate the store room issue: the substitution happened because a similar-strength electrode was treated as interchangeable. Corrective actions include segregating chrome-moly filler metals, requiring QC sign-off for any filler substitution, and training store staff that filler metal selection is governed by the WPS, not by tensile strength alone. Document the nonconformance, the repair, and the retest results.