Creep-rupture behavior of alloy 740H weldment with alloy 263 filler metal.

Abstract

The creep-rupture behavior of weldments of alloy 740H fabricated using shielded metal arc welding with a commercially available filler metal based on alloy 263 and a post-weld heat treatment of 800 °C/4 h was studied at 650, 750, and 850 °C. Stress levels were chosen to reach extended rupture lifetimes (up to over 71,000 h) relevant for long-term applications and pressure vessels and piping code qualifications. All ruptures occurred within the weld zones of the cross-weld specimens except for one case at 850 °C for which the ruptured region covered both the weld and heat-affected zone. The effect of the welding process on creep lifetime was quantitatively evaluated using weld strength reduction factors (WSRFs) which represented, for a given rupture time and temperature, the ratio of the applied creep-rupture stress of the weldment to that of alloy 740H base metal. These factors were 0.78-0.88, 0.82-0.89, and 0.64-0.75 at 650, 750 and 850 °C, respectively. Accordingly, with two exceptions at 850 °C with lower applied stresses, the creep failures were attributed to the lower strength of the weld zone relative to the alloy 740H base metal. Importantly, the failure location and WSRF depended on the microconstituents, microstructure, and stabilities of the weld and base metals at the creep temperatures, rather than welding-induced chemical inhomogeneities or defects. The weld strength reduction of these weldments was very similar to the expected ratio of creep-rupture strength of alloy 263 to that of alloy 740H.