Influence of heat treatment on the microstructure and corrosion properties of the Inconel-625 clad deposited by microwave heating

Abstract

The present work explores the viability of using the microwave heating technique to apply Inconel-625 cladding to a stainless steel (SS-316) substrate. As-deposited clad specimens were subjected to various post-heat-treatments. These treatments included (1) a direct-aged (DA) treatment (2) solution at 981 °C followed by a DA treatment (981SHT) (3) solution at 1081 °C followed by DA treatment (1081SHT). Clad specimens displayed a gradual change in microstructure from planar to cellular to the dendritic depending upon the cooling rate experienced by the molten clad powder during the microwave cladding process. Due to precipitation of hardening particles (γ′), the heat-treated clad specimens displayed superior hardness than as-deposited clad specimen. In the interdendritic spaces/grain boundaries, heat-treatment at 981 °C induces a major reduction in the size of Laves process with a needle precipitation of δ (Ni3Nb) phase. Meanwhile, the heat-treatment at 1081 °C nearly dissolved the entire Laves phase in the microstructure. The corrosion test revealed that the coated samples at all conditions exhibit around 2 to 2.5 times more noble corrosion potential (corrosion resistance) than the uncoated samples. Moreover, the corrosion resistance of the post-treated sample decreases dramatically than that of the as-deposited clad sample, as heat-treatment improved the precipitate phase formation.