Phase transormations involving residual stresses during gaseous nitriding of steel

Abstract

Nitriding of steels is an important treatment in duplex hardening methods. This treatment is known in the art but some advances still need to be done in terms of residual stress understanding. Although residual stresses originate from volume micro-loading accompanying the precipitation of nitrides, questions about their in-depth distribution of a nitrided layer during the treatment are still a challenge. A chemico-thermo-micromechanical model has been developed on the volume change computation of secondary phase transformation. Supported by some experimental observations (TEM, X-ray analysis, Electron Probe Micro Analysis (EPMA), ...), this model gives some better understanding about nitriding. Residual stresses are mainly due to the precipitation of semi-coherent MN nitrides (M=Cr,V,Mo,. . . ). Moreover carbon, often not enough considered in the literature, is shown to be of importance in the process as it involves a second type of precipitation that is the transformation of carbides M23C6 and M7C3 into incoherent nitrides. The definition of the volume change of this transformation is a critical entry data of the mechanical modelling.