Microstructural Evolution of 31CrMoV9 Steel upon Controlled Gaseous Nitriding Treatment

Abstract

Abstract The evolution of the microstructure and the surface residual stress upon controlled gaseous nitriding of quenched and tempered 31CrMoV9 steel was investigated by varying the nitriding potential and the nitriding time at the nitriding temperature of 550 °C. A graded microstructure developed as a function of depth: a surface adjacent decarburized-nitrided zone, in which all tempering induced carbides had trans-formed to nitrides, under loss of the released carbon through the surface, is followed by a nitrided zone in which the carbon released from the carbide-to-nitride transformation had diffused to grain boundaries and precipitated there as (new) carbides. The extent of decarburization showed a strong dependence on whether or not an outer iron-nitride compound layer was present, as well as on the type of iron nitride present at the specimen surface. The surface residual stresses varied as a function of nitriding time and nitriding potential which has been discussed in terms of the rates of the carbide-to-nitride transformation, the decarburization and/or the carbide pre-cipitation along the grain boundaries.