Austenite formation from a steel microstructure containing martensite/austenite and bainite bands

In many commercial steel processing routes, steel microstructures are reverted to an austenitic condition prior to the final processing steps. Understanding the microstructure development during austenitization is crucial for improving the performance and reliability of the microstructure that forms from austenite. In this work, austenite formation in a high-C steel (0.85 wt%) from a microstructure containing martensite/austenite and bainite bands is investigated. It is shown that austenite formation from bainite results in a refined austenite grain structure, and the martensite matrix thus obtained on quenching has a homogeneous distribution of carbides with a relatively low fraction of retained austenite (24%). On the other hand, a coarser austenite microstructure is obtained when austenite forms from a mixture of martensite and retained austenite. The reason for the coarse austenite grains is argued to be a memory effect, which is substantiated by in situ X-ray diffraction analysis. After quenching, an inhomogeneous carbide distribution and a higher retained austenite fraction (30%) are observed in the regions that were initially martensite/austenite. The global microstructure, hence, has a bimodal size distribution of prior austenite grains and carbide-dense bands. The causes for these heterogeneities are discussed with the help of interrupted quench experiments, equilibrium phase calculations, and DICTRA simulations.