Simulation and evaluation of thermal fatigue cracking of hot work tool steels

Abstract

Die casting is a very cost efficient method to manufacture near net-shaped and complex cast products. One limitation for further cost reduction is fatigue cracking of the tool due to thermal cycling, which is observed as a crack network on the tool surface. Hot work tool steels are commonly used as die material. In this study, an experimental test machine for simulation of thermal fatigue is described. The test is based on cyclic induction heating and internal cooling of hollow cylindrical test rods. The surface strain is continuously recorded during the thermal cycling through a non-contact laser speckle technique. The applicability of the test is demonstrated on two hot work tool steel grades, hardened and tempered to different conditions, and heat cycled between T min 170 °C and T max 600–850 °C. It is shown that the test method can simulate surface cracking of tools exposed to thermal fatigue. The surface strain recordings proved to give sufficient information to successfully deduce the strains and stresses behind the mechanism of thermal fatigue surface cracking, without knowledge of the temperature distribution below the surface. It was also found that low-cycle fatigue occurs for the tests with T max 600 and 700 °C, although the estimated tensile stress after cooling does not exceed the initial yield strength of the steel. Most probably, the reason is the gradual softening of the tool steels during the thermal cycling. Additionally, the presence of stress concentrators play a critical role during these conditions.