Impact failure of hot forging die occurrs frequently from surface cracks due to fatigue or thermal fatigue. However, useful information for failure analysis of hot forging dies is scarce because few data exist on the fracture behavior and fractography of hot forging die steels. Therefore, instrumented impact tests were conducted from room temperature to 500°C to get crack initiation and propagation energies as well as dynamic fracture toughness (K1d) of SKD 62 and SKT 4. Fractographic analyses were carried out by SEM. Measurements of the stretched zone width (SZW) on the fracture surface of hot forging die was found useful to determine the fracture toughness and temperature at the time of hot forging die failure by using the obtained relationship between K1d and temperature or SZW. Low cycle fatigue life tests on the notched round bar specimens with or without such surface hardening as ion nitriding and tuftriding were also conducted to clarify the influence of surface hardening on low cycle fatigue strength. of SKD 62. The effect of ion nitriding (500°C×30hr) was found at the stress lower than 80% ultimate tensile strength of this steel and after 3×103 cycles. Striation appeared on the fracture surface after 102 cycles.
