Effect of prior austenite grain size on the evolution of microstructure and mechanical properties of an intercritically annealed medium manganese steel

This paper presents the influence of prior austenite grain (PAG) size on the evolution of microstructure and mechanical properties in an intercritically annealed medium manganese steel (Fe- 8.1 Mn - 0.3 C- 1.1 Si- 1.5 Al). The hot forged alloy was austenitized at two different temperature-time combination to achieve the PAG size of similar to 20 mu m (sample 1) and similar to 40 mu m (sample 2). Both the samples retained a small amount of austenite after the austenitization treatment. The sample 1 with smaller PAG size stabilized a significantly higher amount of austenite (similar to 54.4 +/- 0.6 vol%) after the intercritical annealing treatment than the sample 2 (similar to 39.3 +/- 0.7 vol%). Composition as well as dimension of the austenite films were found to be different in the two samples. Sample 1 displayed a better combination of strength and ductility (1096 +/- 30 MPa and 32 +/- 4.2%, respectively) as compared to the sample 2 (1026 +/- 30 MPa and 22 +/- 1.9%, respectively). The superior mechanical properties of sample 1 were ascribed to the cumulative effect of the higher volume fraction and lower stability of retained austenite in sample 1 as compared to the sample 2. Fracture surface investigation revealed that the sample 1 experienced microvoid mediated ductile fracture, whereas sample 2 displayed brittle fracture facilitated by crack propagation along certain cleavage facets.