Study of the microstructure obtained after diffusion bonding inconel 625 to low alloy steel by hot uniaxial pressing or hipping

Abstract

The microstructures obtained by diffusion bonding Inconel 625 to AISI 4130 low alloy steel, using two different techniques (hot uniaxial pressing and hot isostatix pressing), have been studied by light microscopy, scanning electron microscopy, and transmission electron microscopy. The diffusion of the different elements across the interface and the chemical compositions of the different phases have been determined using EDS microanalysis. In specimens made by either method of bonding, a profuse precipitation of M 23C 6 carbides, resulting from the diffusion of carbon from the steel into the superalloy, is clearly visible. The higher temperatures used during the hipping, as compared to those during hot pressing, resulted in a broader region of carbide precipitation in the as-hipped specimen. The precipitation in the hot-pressed specimens occurred preferentially at the grain and twin boundaries. By contrast, although precipitation was observed at both locations in the as-hipped specimens, it was also found inside the grains, together with a precipitation-free zone around the grain boundaries. Additionally, large plate-like niobium carbides precipitated on the grain and twin boundaries in the as-hipped specimens.