Preservation conditions of hot work hardening in die steel with regulated austenitic transformation during exploitation

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Die steels with regulated austenitic transformation during exploitation (RATE steels) are a new class of tungsten-free steels for hot forming at operating temperatures up to 750 – 800 °C. High durability of the pressing tool and its long service life are ensured by the ability of these steels to preservation of hot work hardening. This circumstance distinguishes RATE steels from traditional alloy steels, which are prone to softening at high temperatures. However, the temperature ranges for the preservation of hot hardening in RATE steels was not systematically studied, which makes it difficult to use a pressing tool more efficiently. In this paper, we study the mechanical behavior of RATE die steel during thermo-mechanical treatment in a wide temperature range, including the stage of preliminary deformation at lower temperatures and the stage of main deformation at higher temperatures corresponding to operating temperatures of the pressing tool. The thermo-mechanical treatment was carried out using a hardening-deformation dilatometer DIL 805 A/D according to the compression mode. We obtained the true stress-strain curves and determined the mechanical characteristics and strain hardening index. Size of the former austenite grain in the steel structure after thermo-mechanical treatment was measured. The temperature-force conditions for enhancing hot hardening or stabilizing hot hardening, or softening, were established. It is shown that the hardening achieved at the stage of preliminary deformation at a temperature of 450 °C is enhanced at the stage of main deformation at temperatures in the range from 550 to 800 °C, while in this temperature range the tendency to increase hot hardening is weakened.