The microstructural modelling of austempered ductile iron camshafts
Austempered ductile iron (AD!) is a material which is receiving increasing interest from the manufacturers of automotive components such as camshafts due to its superior mechanical properties, and in particular excellent wear resistance, compared with other grades of cast iron. ADI is produced from a spheroidal graphite casting using a two-stage heattreatment process. During the first stage of the heat-treatment the matrix is transformed to austenite, and then in the second austempering stage, some of the austenite is transformed to bainitic ferrite. The final microstructure is therefore complex, consisting of graphite, bainitic ferrite, austenite, carbides and possibly martensite. The major focus of this work has been to develop a novel method of predicting the effect of composition and heat-treatment parameters on the major constituents of the microstructure. This has resulted in a single model which can predict a 'microstructural map' of ADI and will assist the foundry industry in reducing lead times for component manufacture. The high temperature equilibrium between graphite and austenite was investigated using Gibbs free energy minimisation in conjunction with critically assessed thermodynamic data. Having established the carbon concentration in austenite at the start of the austempering process, the volume fraction of bainitic ferrite was established from prediction of the limiting carbon content for the diffusionless transformation. The kinetics of the bainite transformation were determined by making modifications to a model which was originally developed for low alloy steels. The predictions were compared with experimental data obtained, both during the course of this research and available in the literature, using dilatometric and X-ray diffraction techniques. The kinetics of the austenitisation were investigated through consideration of a diffusion couple between graphite and austenite. The degree of segregation and formation of primary carbides, in the original ductile iron casting, was calculated using a Scheil approach to solidification. The effect of this segregation was subsequently accounted for by making microstructural predictions on a number of individual 'shells' of material between two graphite nodules. Finally, complete microstructure predictions were compared with reported mechanical properties for a range of compositions and heat treatments of austempered ductile irons.