Short duration aging heat treatment for microstructural and mechanical properties improvement of IN625 weldment

The high microsegregation of Nb and Mo elements in the fusion zone (FZ) of Inconel 625 (IN625) material leads to Laves phase formation. The Laves phase is hard and brittle, that initiates crack nucleation under stress and adversely impacts the mechanical properties. Therefore, in this study, direct aging heat treatment (AHT) was employed to dissolve the Laves phase and release the strengthening elements, thereby enhance the solid solution strengthening and precipitation strengthening abilities of the weld material. The impact of various aging temperatures and aging duration on the microstructure, hardness, and tensile behaviours of IN625 welded joints produced by plasma arc welding (PAW) process were studied in detail. To evaluate precipitates in the FZ, microstructural characterisation was performed using optical microscopy, field emission scanning electron microscopy, and transmission electron microscopy. The nucleation of gamma '' phase significantly increased after aging at 660 degrees C, which caused increase in hardness value in the FZ. However, the formation of gamma '' precipitate decreased whereas delta phase and secondary metal carbides (SMCs) increased above 660 degrees C aging temperature, which resulted in irregular hardness distribution and also drops in hardness in the FZ. The tensile test data showed that aging of welded specimens only at 660 degrees C improved the tensile properties. However, an aging duration of 24 hat 660 degrees C showed highest tensile strength but lower percentage of elongation. The fractography analysis revealed that the nucleation of SMCs, delta phases, and residual Laves phases induces the formation of high microvoids and secondary micro-cracks, thereby reduced the tensile properties.