Effect of Initial Microstructure, Heating Rate, and Austenitizing Temperature on the Subsequent Formation of Martensite and Its Microstructural Features in a QT Steel

Goal of this work is to study the effect of initial microstructure and different fast austenitization treatments with high heating rates and short dwell times on the transformation behavior of austenite to martensite and the resulting microstructural martensite features. A deeper understanding is gained regarding influences of the time temperature program during austenitization on strain within the martensite. Therefore, specimens with two microstructures, a ferritic/pearlitic and a soft annealed state, are heat treated with heating rates of 1-100 K s(-1) to different austenitization temperatures and held for 3 s. Distinct differences in initial microstructures is their cementite size and morphology. For the investigations of the martensitic transformation, its microstructure and its distortions dilatometry experiments are conducted and Electron Back Scattered Diffraction (EBSD) is used. The investigations show that strain and inhomogeneities within the austenite are passed to the martensitic microstructure and lead to higher distortions within the ferritic martensite crystals. Strain within the martensitic microstructure decreases with increasing austenitization temperature and decreasing heating rates. Especially, the carbon content in solution, which is increasing with increasing austenitizing temperature and smaller initial cementite size, is influencing martensitic features, such as martensite start temperature and block sizes.