In Situ Observation of Bainite Transformation and Simultaneous Carbon Enrichment in Austenite in Low-Alloyed TRIP Steel Using Time-of-Flight Neutron Diffraction Techniques

An in situ neutron diffraction experiment during austempering of low-alloyed transformation-induced plasticity steel, Fe-1.48Si-1.52Mn-0.15C, in wt pct was conducted. In this study, time-of-flight neutron diffractometer with a large detector coverage, iMATERIA at J-PARC MLF, was employed. The phase fraction and carbon concentration in austenite could be quantitatively determined with a time resolution 1 minute although considerable textures existed for both ferrite and austenite. The carbon concentration in austenite during austempering was found to be inhomogeneous, resulting in a bimodal concentration distribution. The low-carbon region was consumed by bainite transformation whereas the high-carbon austenite slightly increased and even survived the final cooling to room temperature, forming a retained austenite. The rate of bainite transformation was affected by the state prior to the start of austempering. Consequently, different morphological features of the retained austenite were formed. More block-shaped austenite was observed in the case of slower bainite transformation, and it was determined that film-shaped austenite could also exist. The average carbon concentration was similar to that of high-carbon austenite during austempering. Hence, the film and block shapes of the retained austenite do not necessarily reflect the difference in carbon concentration.