Modelling of precipitation of carbides during tempering of martensite

Abstract

In steel industry, tempering is the final heat-treatment process for steel parts. The principal aims of tempering are: on one hand to get the microstructures that lead to service mechanical properties, and on the other hand to promote the relaxation of the residual stresses generated during quenching. The phase transformations during tempering process of martensite have been covered well by numerous experimental investigations. Based on this knowledge, efforts have been made to build models to predict the precipitation kinetics and the evolutions of the mechanical properties of tempered steels during the last decades. Most of these models are focused on the precipitation and coarsening stages of cementite that take place at high tempering temperatures. However, in many cases, to keep a high hardness and strength, the steel parts are tempered at lower temperatures and the precipitates in the steel are mainly -carbides. Thus, it is necessary to describe the precipitation of both -carbides and cementite. In our previous study [1,2], a model describing the global kinetics of tempering in low alloyed steels, i.e. the precipitation of -carbide, the decomposition of retained austenite and the precipitation of cementite, has been developed aiming at the prediction of the dimensional changes during tempering. But, in order to describe the changes in mechanical properties as well, a model predicting both the kinetics of precipitation and the microstructure evolutions is necessary. According to the present knowledge, the decomposition of Fe-C martensite is generally divided into three main stages: The first stage is recognized as the decomposition of martensite into the -carbide phase and a less tetragonal martensite, in which the carbon content corresponds to a metastable equilibrium with the -carbide. This stage takes place in the temperature range 100°C - 200°C. The composition and structure of metastable -carbide have been studied firstly by Jack [3]. He suggested that -carbide has an hexagonal structure and a composition in between Fe