Phase-field modeling of microstructure evolution during complex thermal cycles: Application to martensitic stainless steels

A phase-field model is developed to simulate the microstructural evolution of a martensitic stainless steel, focusing on both martensite formation and simultaneous carbon partitioning during an experimental thermal cycle derived from a wire-arc additive manufacturing (WAAM) process. This directly incorporates the explicit equations of the CALPHAD formalism to determine the thermodynamic quantities necessary for simulating the microstructure of an Er410 NiMo steel across the entire range of temperatures experimentally measured during the thermal cycles, and taking into account local carbon content. The coupling of displacive transformation and diffusive carbon redistribution requires the development of a multi-time-step algorithm to handle both mechanisms involved. The resulting model yields several key insights, including the significant impact of the chosen elastic boundary conditions and noise parameters on the evolution of the martensite fraction. Under specific conditions, the martensite fraction exhibits a gradual increase when lowering the temperature, akin to Koistinen–Marburger curves. Simulations that couple martensitic transformation and carbon segregation reveal substantial partitioning during the initial cooling of a layer deposited by WAAM, with segregation amplitudes comparable to what has been observed experimentally. A pronounced dependence of carbon segregation on the propagation of the martensite/austenite interface during cooling and on the local arrangement of different martensite variants is observed. These results highlight the advantage of using a phase-field model, as it enables the consideration of complex morphologies such as those found in martensitic microstructures.