Modelling and experimental validation of austenite growth from As-quenched martensite during intercritical annealing of a medium-Mn steel

The growth of austenite from as-quenched martensite during intercritical annealing of a medium-Mn steel was studied by numerical simulation and experimental verification. The thickening of the austenite lath was numerically simulated under the assumption of local equilibrium. According to the presence or absence of cementite and the nucleation position of austenite, three models are proposed: austenite forming on the martensitic lath boundaries (Setup 1), on martensitic lath boundaries with far-away cementite location (Setup 2), and on the cementite/martensite interface (Setup 3). Result shows that the growth kinetics of austenite under the three models are slightly different, mainly because cementite is thermodynamically unstable in the two-phase zone. In the Setup 2, the growth of cementite and austenite occurs simultaneously at the early stage, in which the austenite grows slowly compared to Setup 1, and when cementite begins to dissolve, the austenite grows faster. In the Setup 3, cementite grows first and then dissolves to provide solute carbon for austenite growth, leading to the fastest austenite growth. In addition, with larger initial size of martensite, both growth kinetics of austenite and cementite and the dissolution kinetics of cementite are slowed down. Combining with microstructure characterization with EPMA, TEM and dilatometric measurement, the Setup 2 is considered to be the optimal model in which austenite mainly grows at the martensitic lath boundaries rather than at the ferrite/cementite interface inside the laths. Moreover, in order to elucidate the influence of cementite dissolution on the growth of austenite, a round tube austenite forming on the cementite rod (Setup 4) was proposed. The growth of austenite is accompanied with the continuous carbon supply from the dissolution of cementite, which could increase the stability of reverse transformed austenite but has little effect on the increase of austenite volume fraction.