HAZ microstructural evolution in alloy 718 after multiple repair and PWHT cycles

Static high temperature gas turbine engine components consist of nickel-base superalloys, including Alloy 718. These components are subjected to periodic inspection, weld repair and postweld heat treatment (P)WHT) cycles. Multiple weld repair and PWHT cycles are reported to degrade weldability, purportedly due to an increased susceptibility to HAZ liquation cracking. Significant microstructural changes to base metals have been observed as a result of multiple repair and PWHT cycles. Base metal microstructures of both wrought and cast Alloy 718 after multiple repair and PWHT cycles exhibit a high volume fraction of δ phase. Little information is available regarding the effect of δ phase on HAZ liquation cracking. This study reports on HAZ microstructural evolution in both wrought and cast 718 containing δ phase, and its relation to cracking susceptibility.; Gleeble hot ductility testing was performed to assess HAZ liquation cracking susceptibility of Alloy 718 after multiple repair and PWHT cycles. Gleeble thermal simulation test was utilized to study the HAZ microstructural evolution in these alloys. The characteristic temperatures for δ phase dissolution, constitutional liquation of Nb-rich carbides and Laves were determined.; The results obtained demonstrated a significant increase in HAZ liquation cracking susceptibility in a laboratory heat cast alloy as a result of δ phase accumulation after 40 PWHT cycles. The δ phase was found to dissolve on heating during a typical weld repair cycle and participate in the HAZ liquation reaction. Both Laves and Nb-rich carbides were found to undergo constitutional liquation. The constitutional liquation of Nb-rich carbides dominates the liquation process in wrought alloys. In contrast, the constitutional liquation of Laves dominates the liquation process in cast alloys. The dissolution of δ phase contributes additional Nb to the liquation process and significantly influences the amount of liquid that forms. A unified theory is proposed for HAZ liquation cracking in Alloy 718 containing multiple phases including δ phase as a result of multiple repair and PWHT cycles.