Enhanced toughness of Fe–12Cr–5.5Ni–Mo-deposited metals through formation of fine reversed austenite

Abstract

To overcome the strength–toughness trade-off in Fe–12Cr–5.5Ni–Mo-deposited metals, post-weld heat treatment (PWHT) was performed at the intercritical temperature, and the formation mechanisms of reversed austenite were investigated. The microstructures were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy and electron backscattered diffraction techniques. It is found that lathy reversed austenite and nanometre-scale carbides are embedded in the martensite matrix after PWHT at 620 °C. The reversed austenite prefers to nucleate at multiple lath boundary junctions, and a subset forms adjacent to the M23C6 carbides. The growth of reversed austenite in the manner of martensite–austenite grain boundary migration and austenite–austenite grain boundary mergence is governed by Ni diffusion. The deposited metals exhibit a good combination of strength, ductility and toughness after PWHT at 620 °C for 1 h. However, the impact toughness and strength do not significantly change with a longer holding time, from 1 to 4 h. These phenomena are attributed to the combined effects of reversed austenite toughening, martensite matrix softening and M23C6 carbide precipitation strengthening. Moreover, the formation mechanisms of reversed austenite are discussed and proposed based on two-spherical-cap nucleation model, offering guidance for strength–toughness balance of low-carbon martensitic metals.