Hot workability of three grades of tool steel

Three tool steels, a cold-work air-hardening grade, a hot-work die grade, and a high-speed type, were deformed by torsion in the range of 900 to 1100 °C at rates of 0.1 to 5 s^•1. In a series of continuous deformation tests the flow stress and ductility were determined. The exponent of the flow stress was proportional to the strain rate and to the temperature in a reciprocal Arrhenius relationship. In general the flow stress for a given deformation condition, the activation energy, and the strain for the start of dynamic recrystallization increased for the steels in the order listed above; however, the ductility of the hot-work grade is superior to the other two grades. Multistage tests were carried out on each steel to determine its softening behavior during intervals between passes. Each test was carried out under isothermal conditions with constant strain rate, pass strain, and interval duration. Softening occurred by both recovery and recrystallization with the amount increasing with temperature, strain rate, pass strain, and accumulated strain. The first two steels were similar in behavior having extensive softening at 1000 °C, whereas the high-speed steel experienced considerably less softening.