Heat Treatment Simulation and Characterization of Experimental Multi-Constituent Austenite-Containing Steels

Multi-constituent austenite-containing (MCA) steels consist of a ferrite/bainite/martensite matrix with retained austenite, which is stabilized by elements such as carbon (C), manganese (Mn), and nickel (Ni). Inherent features of these microstructures can contribute to relatively high combined strength-toughness performance. The duplex-type microstructure inhibits coarsening of microconstituents during heat treatments and the presence of retained austenite can contribute to an array of work-hardening behavior. In this study, twelve experimental MCA alloys with varying Mn and Ni concentrations were produced to investigate a variety of MCA microstructures that can be produced through different heat treatments. Thermo-Calc^Ⓡ equilibrium simulations were used to model austenite fraction and composition across three heat treatments: intercritical annealing (IA), quench-lamellarize-temper (QLT), and double soaking (DS). Alloys underwent select physical heat treatments to validate the computational predictions and identify discrepancies between practical processing timescales and equilibrium modeling. Dilatometry enabled in situ measurement of austenite fraction during heat treatments as well as the martensite start temperature associated with the newly formed austenite. Microstructures were characterized using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD), which enabled direct comparison to the as-received conditions and revealed the microstructural evolution associated with each heat treatment.