An investigation of the repair weldability of Waspaloy and alloy 718

Time-efficient and effect-equivalent long term isothermal solution heat treatments were designed to simulate the metallurgical “damage” incurred by multiple weld repair/post weld heat treatment (PWHT) cycles. Gleeble hot ductility testing successfully quantified the degradation of weldability in Waspaloy and Alloy 718 subjected to those simulated multiple repair/PWHT cycles. HAZ liquation was the mechanism for the short time high temperature grain boundary (GB) weakening leading to intergranular (IG) failure.; Degradation of Waspaloy was correlated with carbide constitutional liquation and GB liquation. The effect of grain size and special GBs was evaluated based on a GB engineering concept to rationalize the weldability performance of Waspaloy after simulated multiple repair/PWHT cycles. The weldability degradation from 1079°C/40hr (equivalent 10 repair/PWHT cycles) treatment is associated to the coarsened grain size and a lower volume fraction of special GBs. The restored weldability by 1079°C/100hr (equivalent 25 cycles) is attributed to the increase of the fraction of special GBs, even though a coarse-grained microstructure existed. Based on this, a short time rejuvenation heat treatment (RHT) was proved to be experimentally practical for improvement of the weldability of Waspaloy, by increasing the fraction of special GBs.; Alloy 718 was characterized by abundant precipitation of δ phase in the γ-nickel matrix, which is the major metallurgical damage from simulated multiple repair/PWHT cycles (954°C/40hr–100hr, equivalent 20–50 cycles). The degradation of weldability is owing to the short time high temperature GB weakening caused by the combined effects of δ phase dissolution-promoted liquation, boron carbide constitutional liquation and GB liquation. A short time rejuvenation treatment (1010°C/2hr) for Alloy 718 effectively restored the degraded weldability by removing the adverse influence of δ phase by simple dissolution above the δ-phase solvus, and promoting a spontaneous grain refinement and increase of volume fraction of special GBs as a result of the elimination of δ phase pinning of GBs.