Direct decomposition of bainitic steel transformed below martensite start temperature: in situ S/TEM study

In-situ S/TEM was used to investigate tempering and reverse transformation in a nanocrystalline bainitic steel isothermally treated below the martensite start (M s ) temperature. Real-time observations reveal subtle differences in the thermal stability of bainitic ferrite and pre-martensite. Comparative decomposition pathways of bainitic ferrite and pre-martensite indicate the higher thermal stability of bainitic ferrite, while microstructural differences disappear above 600 °C. Austenite reversion initiates preferentially in regions containing blocky retained austenite and bainitic ferrite/pre-martensite interphase (type-A). Also, type-B austenite was revealed during reverse transformation in the region of adjacent dissolving carbides (see movie). These findings provide insight into the direct decomposition mechanism and reverse transformation in low-temperature bainitic steels, which lie beyond the capabilities of conventional techniques.