Influence of high temperature deformation and double tempering on the microstructure of a H21 tool steel

An axisymmetric compression test was used to study the effect of hot deformation and double tempering on the microstructure and hardness of a H21 tool steel. The compression tests used a constant true strain rate of 0.01 s−1, and were performed in the temperature range 1000-1100 °C after austenising at temperatures of either 1100 or 1250 °C. The double tempering was carried out at 650, 750 and 800 °C, with air cooling in between the first and second temper. An overview of the flow curves and the characterisation of microstructures showed no evidence of dynamic recrystallisation. The increase in flow stress with decreasing austenising and deformation temperature was attributed to dislocation movement and the presence of fine and dispersed carbides causing a Zener pinning effect. Without double tempering, the highest hardness was measured after austenising at 1250 °C, followed by deformation at 1000 °C and water quenching (617 HV). No secondary hardening phenomena occurred after double tempering for samples that were first subjected to hot deformation, and the highest double tempered hardness (354 HV) occurred after a double temper at 650 °C following an austenising temperature of 1250 °C and subsequent deformation at temperature of 1000 °C.