BALA Piotr AGH University of Science and Technology

This work contains a detailed description of the kinetics of phase transformations occurring during tempering of two Cr-Mo-V tool steel with different carbon content (0.37 and 1.22%). The temperature ranges of the phase transformations occurring during heating from the as-quenched state (tempering) were determined from dilatometric tests. An interpretation of dilatograms of heating from the as-quenched state was also presented. The temperature ranges of the observed transformations during tempering of investigated steel were presented on CHT (Continuous Heating Transformations) diagrams. Moreover, the microstructure development in tested samples, reflecting the extend of the phase transformations during tempering, was discussed too. The change in microstructure of the investigated steels during heating from the as-quenched state results from precipitation of epsilon and M3C carbides, and transformation of different amount of retained austenite to tempered martensite. The most preferential sites for precipitation of cementite, are martensite strips, indicating that its nucleation is independent. It was also noticed, that precipitation of cementite takes place only when precipitations of metastable epsilon carbide are present in the microstructure. The morphology of the obtained microstructures is similar to that of lower bainite, suggesting that it is a likely reason for a temper brittleness. Precipitation of MC and M2C type alloy carbides have not been detected although the investigated steels contain molybdenum and vanadium, and 0.37 or 1.22% carbon.