Investigation of high temperature microstructure stability and tensile rupture behavior of ultrahigh strength hot-work tool steel

This paper reports the investigation of microstructure stability of an ultrahigh strength Dievar tool steel subjected to tensile loading at high temperature. The results showed that the ambient temperature have a significant influence on the endurance performance of Dievar steel. Below 580°C, the main defect feature was the generation of intergranular cavities that distributed along the PAGBs, and the GB intersections and the triple GB were the preferential sites. Over 600°C, the failure mechanism of this material gradually shifted from the intergranular damage mode to the intragranular damage. Under 620°C, the large volumetric defects have appeared inside the grains, which prompted the premature failure of this material. The EBSD results revealed that the misorientation parameter was significantly higher inside the grains of the 620°C fracture specimen, which indicates that the lattice rotation level was larger at the high temperature condition. The MD simulation results showed that the dislocation motion became stronger at the 620°C, which manifested that the thermal activation of intragranular defects were easier as the temperature increased. These factors may contribute to the transition of failure mechanism from intergranular to the intragranular mode.