Response to thermal cycling of tool materials under steel thixoforming conditions

The principal failure mechanism of steel thixoforming dies is thermal fatigue owing to forging pressures much lower than those encountered in conventional forging. This makes a properly designed thermal fatigue test the best method to identify suitable tooling materials for the steel thixoforming environment. Samples of X32CrMoV33 hot work tool steel and CrNiCo alloy were cycled thermally between 450 and 750 degrees C, every 60 s for a total of 1500 cycles. While the thermal stresses generated at the surfaces of the two materials were very similar, their responses to thermal cycling were markedly different. The X32CrMoV33 steel was softened by nearly 40% after only 400 cycles, raising serious concerns over its temper resistance under steel thixoforming conditions. The extensive oxidation and subsequent spalling of oxide scales suffered by the X32CrMoV33 hot work tool steel is also a major shortcoming. The performance of the CrNiCo alloy, on the other hand, was judged to be satisfactory with a much thinner heat affected zone and a much better oxidation resistance. Lack of evidence for heat checking in this alloy after 1500 cycles is an encouraging sign.