Microstructural analysis of steel–nickel alloy clad interfaces

Abstract

The microstructures obtained by diffusion bonding a low alloy steel AISI 4130 to nickel alloys (Inconel 625 and Hastelloy G3), using different techniques (hot uniaxial pressing, hot plane strain compression, hot isostatic pressing (hipping), and coextrusion) have been studied by light microscopy, scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM). The interdiffusion of various elements through the interface has been determined using microanalysis in both the SEM and the STEM. In all instances profuse precipitation of M23C6 carbides, resulting from the diffusion of carbon from the steel into the superalloy, is clearly visible in a region close to the interface on the superalloy side. For specimens processed by hot pressing and coextrusion, this region shows a work hardened substructure composed of cells and dislocations. By contrast, in specimens processed by hipping a poorly developed substructure, with few dislocations associated with grain boundaries and carbides, is observed. The thickness of the precipitation region increases with increasing temperature and bonding time. Carbides precipitate preferentially at the grain and twin boundaries, although for the higher temperatures used in hipping, the precipitation inside the grains is also important. On the steel side, a precipitation free austenite band is observed in all specimens analysed. This band is the result of the diffusion of nickel and chromium from the superalloy to the steel, thus stabilising austenite at room temperature. From the experimental concentration profiles of different elements, the corresponding diffusion coefficients have been calculated using diffusion theory, and compared with data reported in the literature. Good agreement between these data and the experimental values can be observed.MST/3200