Isothermal transformations in advanced high strength steels below martensite start temperature

The transformation products in advanced high strength steels have been studied during the isothermal decomposition of austenite, subsequent to initial martensite formation. Rapid cooling to various temperatures below martensite start was carried out in a dilatometer with the intention to form controlled volume fractions of initial martensite and austenite, followed by isothermal holding. The transformation kinetics was monitored by means of dilatometry and microstructural characterisation by scanning electron microscopy, electron backscatter diffraction and X-ray diffraction. Hardness measurements of the resulting microstructures were analysed. The results revealed that the microstructures formed below M-S are mainly composed of different fractions of tempered martensite, isothermal bainite with carbide precipitation and retained austenite.