Microstructure and properties of laser welded IN738 alloy after post-weld heat treatment

Objective This study aims to investigate influence of post-weld heat treatment (PWHT) on microstructure and high-temperature properties of laser welded IN738 alloy. Methods IN738 alloy was subjected to a PWHT process of 1 125 ℃/2 h/air cooling + 850 ℃/24 h/air cooling. The effects of heat treatment on IN738 alloy were studied by analyzing microstructure, as well as the morphology and distribution of precipitated phases, combined with high-temperature stress rupture tensile testing. Results Weld zone of the as-welded specimen exhibited a typical columnar crystal structure, consisting of a γ-Ni matrix, numerous MC carbides, and a small amount of γ/γ′ eutectic structure. After PWHT, a large number of γ′ precipitates formed in weld zone, and chain-like M₂₃C₆ carbides segregated along grain boundaries. Following high-temperature stress rupture testing at 850 ℃ under 400 MPa, heat-treated specimen fractured in weld zone, with a rupture life of approximately 7.1 h. Chain-like M₂₃C₆ carbides were observed near secondary cracks in the fracture vicinity. Conclusion The presence of chain-like M₂₃C₆ carbides in the laser-welded and heat-treated microstructure of IN738 alloy readily acts as crack initiation sites under high temperature and stress, thereby degrading high-temperature mechanical properties of welded joints.