The effect of rejuvenation heat treatments on the repair weldability of wrought Alloy 718

Extensive precipitation of needle- and plate-shaped δ-phase in the γ-nickel matrix of wrought Alloy 718 is the major microstructural change resulting from multiple weld repair/post weld heat treatment (PWHT) cycles. Isothermal heat treatments at 954 °C for times up to 100 h were used to simulate the multiple PWHTs in laboratory samples. Grain size did not change appreciably during these heat treatments owing to δ-phase pinning of the grain boundaries (GBs). The susceptibility of Alloy 718 to heat-affected-zone (HAZ) liquation cracking degraded as a result of these heat treatments. This degradation is due to the short time, high temperature GB liquation caused by the combined effects of δ-phase dissolution, boron carbide constitutional liquation, and GB segregation. A rejuvenation treatment (1010 °C/2 h) effectively restored the degraded weldability by removing the adverse influence of δ-phase through dissolution above the δ-phase solvus. This heat treatment also promoted a spontaneous grain refinement and increase in the fraction of special GBs owing to the elimination of δ-phase pinning of GBs. The combined effects of δ-phase, grain size and fraction of special GBs are discussed in the context of HAZ liquation cracking that occurs during repair welding of Alloy 718.