Different Cooling Rates and Their Effect on Morphology and Transformation Kinetics of Martensite

The characteristics of martensitic transformation is strongly dependent on the cooling rate applied to the material. For a quenched and tempered steel, the martensitic transformation occurs below 500 °C, but in industry, cooling rates are normally characterized for cooling in the temperature regime between 800 and 500 °C. The effects of different cooling rates in the lower temperature regime were thus, not intensively investigated in the past. To this end, a 50CrMo4 steel is quenched in a dilatometer applying varying cooling rates below 500 °C. The martensite microstructure is analyzed by APT, TEM and EBSD in regard to carbon distribution, lath width and block sizes. Additionally, hardness measurements are carried out and martensite start temperatures as well as the retained austenite phase fractions are evaluated. It can be shown, that lowering the cooling rate leads to increased carbon segregation within the martensitic matrix. The main effect is a decrease in martensite hardness. Also the block size increases with lower cooling rate.