Thermal fatigue testing of chromium martensitic hot-work tool steel after different austenitizing treatments

Tools used in die casting, hot rolling, extrusion and hot forging are exposed to high cyclic temperatures and mechanical loads. Particularly designed hot-work tool steels are frequently used to meet the application requirements, regarding a high thermal fatigue resistance as one important tool property. To increase the tool performance, i.e. tool life length, the tool steel microstructural condition may be optimised. In the present study, the thermal fatigue properties of a new martensitic chromium hot-work tool steel alloy (Uddeholm designation DIEVAR) have been evaluated with regard to the austenitizing treatment. Using four different austenitizing temperatures, the material was quenched and tempered to the same hardness of 470 HV. Thermal fatigue was performed using a high frequency induction heating system, designed to simulate hot-work processing conditions, while recording the specimen surface strain with a laser speckle technique. It was found that the thermal fatigue crack growth in the tool steel was strongly dependent on the different austenitizing temperatures, as well as the maximum test temperature. Thermal fatigue resistance was assumed to rely on temper resistance at high maximum cycle temperature and on toughness and ductility at lower. The resistance to thermal fatigue could be increased by optimising the heat treatment. The surface mechanical strain during thermal cycling was estimated from the laser speckle measurements.