On the role of liquated γ′ precipitates in weld heat affected zone microfissuring of a nickel-based superalloy

The role of γ′ precipitate particles, which is the principal strengthening phase of most of the precipitation hardened nickel base superalloys, on weld heat affected zone (HAZ) cracking was investigated. The HAZ microstructures around welds in a commercial nickel-based superalloy IN 738LC were simulated using Gleeble thermomechanical simulation system. Microstructural examination of the simulated HAZs and those present in actual tungsten inert gas (TIG) welded specimens showed that γ′ particles persisted during heating to the welding simulation temperatures, where they reacted with the surrounding γ matrix producing liquid film by a eutectic-type reaction, which subsequently infiltrated the grain boundary regions. The on-cooling ductility of the alloy was significantly reduced as shown by the comparatively low ductility recovery temperature. Correlation of simulated HAZ microstructures with hot ductility properties of the alloy revealed that HAZ cracking resistance was damaged by liquation reaction involving the γ′ precipitate particles. The results support previously reported observations which indicated that γ′ precipitate particles could contribute to HAZ microfissuring in superalloys through liquation reaction besides the generally reported rapid solid-state re-precipitation of γ′ phase.