Acceleration of bainitic transformation in 0.28C-3.8Mn-1.5Si steel utilizing chemical heterogeneity

This study proposes a novel method for accelerating bainitic transformation by utilizing Mn heterogeneity. The process involves intercritical annealing to create a heterogeneous distribution of Mn prior to full austenitizing. Heterogeneous Mn distribution leads to the rapid nucleation of bainitic ferrite in the Mn-depleted austenite region, and the rapidly formed bainitic ferrite acts as a nucleation site for Mn-enriched austenite region. Consequently, the required austempering time to achieve a similar fraction of bainitic ferrite is nearly halved compared to conventional process. The effectiveness of Mn heterogeneity in accelerating the transformation is comparable to other acceleration methods, such as prior austenite grain refinement and introducing prior martensite. Moreover, the proposed method has an additional advantage in increasing the fraction of retained austenite. The retained austenite is stabilized not only by C partitioning during austempering but also by pre-partitioned Mn during intercritical annealing.