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

Wu, Shipin, Wang, Dongpo, Zhao, Chen, Zhang, Zhi, Li, Chengning, Di, Xinjie
Journal of materials science 2018 v.53 no.22 pp. 15679-15693
X-ray diffraction, carbides, heat treatment, microstructure, models, nickel, scanning electron microscopy, temperature, transmission electron microscopy
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.