Variation of Structure and Hardness of Tool Steel for Hot Pressing upon Heating after Isothermal Holding in the Bainite Range

This work is devoted to transformations in tool steel for hot pressing after isothermal holding in the bainite range and heating in the range of 500–780°C. Medium carbon steel (0.44% C) is investigated: Si–Cr–Ni–Mn–Mo–V–Ti–Nb. It is shown that, at heating to 630°C after isothermal holding, when the amount of newly formed austenite is comparatively low, the properties of steel are determined mainly by structural variations in bainite sectors and its hardness continuously decreases. At higher heating temperatures, the amount of austenite sharply increases, and its subsequent decomposition upon cooling leads to a significant increase in steel hardness.