THE KINETICS OF PHASE TRANSFORMATIONS IN NEW HOT WORK TOOL STEEL

The reason for writing this paper was to describe the kinetics of phase transformation of undercooled austenite in the form of the CCT (Continuous Cooling Transformation) diagram as well as the kinetics phase transformation during heating from as-quenched state in the form of the CHT (Continuous Heating Transformation) diagram of the new hot work tool steel. In order to make the CCT diagram of the kinetics of phase transformation of undercooled austenite the samples were heated to a temperature of 940°C with a rate of 5C/s, hold for 20 minutes and then cooled with various rates (50÷0.02C/s) to a temperature of 20C. Numerically recorded dilatograms were differentiated to provide more precise readings of characteristic temperatures. In order to make the CHT diagram of the kinetics of phase transformations at continuous heating from as-quenched state, the previously quenched samples (TA = 940C, tA = 20 min, cooling with the rate of 30°C/s) were heated with the following rates: 0.05; 0.1; 0.5; 1; 5; 10; 15; 35°C/s to a temperature of 700°C, while changes in the samples elongation in dependence of the temperature were recorded. In this case, numerically recorded dilatograms were also differentiated for a more precise reading of characteristic temperatures. The CCT diagram is characterized by the range of diffusive changes shifted strongly to the right, it means to longer times (high hardenability). During continuous heating from the as-quenched state the obtained results confirmed the following order of phase transformations: precipitation of  carbide, transformations of retained austenite within of which transformations range precipitation of cementite take place, independent nucleation of MC carbides followed by the nucleation of M2C carbides. The increase of heating rate (from 0.05 to 35°C/s) results in the increase of temperatures of the beginnings and the ends of particular transformations.