MICROSTRUCTURAL EVOLUTION OF A LOW-CARBON STEEL DURING THE AUSTENITE DECOMPOSITION BELOW MS

The microstructural evolution of a low-carbon steel has been studied during the austenite decomposition, subsequent to initial martensite formation. Isothermal holding treatments after rapid cooling to various temperatures between martensite start (M-s) and martensite finish (M-f) temperatures were carried out in a dilatometer with the intention to form controlled volume fractions of initial martensite and austenite. The purpose of the above heat treatments was to create conditions in which the phase transformations during isothermal treatment between the Ms and Mf temperatures can take place. Different microstructures consisting of bainite, martensite, retained austenite and carbides were obtained depending on the isothermal holding temperature and time. The transformation kinetics was monitored by means of dilatometry and the microstructure was characterized by Scanning Electron Microscope (SEM), Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscope (TEM). Hardness measurements of the resulting microstructures were analysed. The results revealed that the microstructures formed below M-s are mainly formed by different fractions of tempered martensite and isothermal bainite with carbide precipitation.