Effect of heat treatment on microstructure evolution and mechanical properties of Inconel 625 with 0.4 wt% boron modification fabricated by gas tungsten arc deposition

Abstract

Microstructure evolution and mechanical properties of IN625 wires modified with 0.4wt% B deposited by gas tungsten arc in shielding argon were studied in both as-deposited and heat treated condition. In the as-deposited condition, hardness heterogeneity was observed along the build. The upper section had higher hardness than the lower section. From SEM and EDS observation, this high hardness was attributed to the continuous eutectics (mainly Laves phase with few NbC) at the inter-dendritic regions. These eutectics in the lower section that were partially re-melted during the multi-pass process had formed non-continuous features contributing to a lower hardness. Moreover, fine isolated M5B3 borides that confirmed by TEM with SAD patterns precipitated out from the γ matrix in lower section as a result of the multiple thermal cycles. The Laves phases were totally re-melted during heat treatment, resulting in the formation of large size M5B3 borides from SEM and TEM analysis during heat treatment solidification. This led to elimination of hardness heterogeneity by homogenizing microstructure along the build after annealing and aging treatment. This microstructure evolution also eradicated the risk of strain-age cracks during heat treatment. A large amount of zigzag grain boundaries were formed attributed to the pinning effect of M5B3 and NbC. The tensile properties of heat treated IN625B deposits were tested at both ambient and elevated temperature. The results indicated that the ultimate tensile strength and yield strength of gas tungsten arc IN625B deposits were superior to the conventional IN625 in the solution annealed condition.