Role of heat-flux density and mechanical loading on the microscopic heat-checking of high temperature tool steels under thermal fatigue experiments

Thermal fatigue of a hot work tool steel (X38CrMoV5) is investigated under various test conditions. A microscopic interconnected heat-checking pattern forms on the oxidized surface of the specimen. The evolution of the crack density and morphology is characterized by Scanning Electron Microscopy (SEM) and image analysis. The effects of the initial hardness, the maximum temperature and the heating period of the thermal cycle are reported. It is shown that the saturated heat-checking density is independent of the maximum temperature of the thermal cycle, but is dependent on the heating rate. No significant effect of the initial hardness is observed. In the saturated regime, a linear relationship is established between the heat-checking density and the maximum heat-flux density applied to the specimen. The saturated crack density is explained by the difference between the thermo-mechanical strains of the oxide layer and the steel. A damage criterion, independent of the initial hardness of the steel, is proposed to describe the microscopic heat-checking life.