Effect of austempering treatment on the microstructure and mechanical properties of 0.4C–1.5Si-1.5Mn TRIP-aided bainitic ferrite steel

Abstract In this study, the effects of austempering treatment on the microstructure, mechanical properties, and fracture mechanisms of Fe-0.4C-1.5Si-1.5Mn transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steel were investigated. When the austempering time increased from 0 to 3600 s, the volume fraction of retained austenite and its carbon concentration initially increased and then decreased. The increase in carbon concentration during austempering may be attributed to the formation of fine bainitic ferrite, representing the matrix and in turn the formation of retained austenite with a film-type morphology. When the austempering time was increased from 0 to 1000 s, the TBF steel showed excellent mechanical properties such as high yield and tensile strengths, and uniform and total elongations. Meanwhile, void density increased significantly in the initial stages and then slightly decreased; in contrast, the trend observed for mean crack length was opposite to that of void density leading to the change in the fracture mode of the TBF steels from intergranular to quasi-cleavage and dimple fractures. It could be inferred that the observed improvement in mechanical properties, especially the good ductility of TBF steels, at the optimal austempering time is due to the suppression of crack propagation and crack blunting to form voids owing to the effective deformation-induced transformation of metastable retained austenite.