Phase field study of heat treatment and strengthening in maraging steels

Martensite decomposition during heat treatment of maraging steels is a complex process, in which different precipitates as well as austenite phase can grow concurrently. Here, we formulate a phase field model to study the martensite decomposition by taking into account the nucleation and growth of phases as well as phase transformation-induced elastic and plastic deformations. To model 18Ni M300 maraging steel, a pseudo ternary Fe-Ni-Ti model is parameterized by using the thermodynamic and kinetic data of the alloy. The effect of transformation induced elastic and plastic strains is incorporated using an elastoplastic model. Using this model, we investigate the decomposition of the martensite phase during continuous heating as well isothermal ageing, focusing on the growth of precipitates as well that of the reverted austenite phase. The data generated from the simulations, in conjunction with an empirical model, are used to estimate the variation of yield strength as a function of temperature and ageing time. The temperature dependence of strength obtained is qualitatively consistent with the experimental data of maraging steel, and thus sheds light on how underlying strengthening mechanisms depend on the ageing temperature and time. In particular, the simulations reveal the important role played by reversed austenite on the temperature dependence of the strength.