Abstract
Axial fatigue tests were conducted for plane bar specimens of SKD61 steel and 0.65 mass% carbon matrix high speed steel, YXR3 in order to investigate ion nitride effect. In higher stress amplitude of S-N diagrams ion nitride effect was not observed for both steels. Ion nitride effect was observed in stress amplitude lower than 71% of ultimate tensile strength for SKD61 steel and in that lower than 56 % of ultimate tensile strength for YKR3 steel. The stress amplitude expected for ion nitride effect decreased with an increase in the Vickers hardness number of hot forging die steel. In higher stress amplitude fatigue cracks initiated from ion nitride layer at specimen surface, while in lower stress amplitude fatigue cracks initiated from non metallic inclusions located in sub-surface. Ultrasonic fatigue test with frequency of 20 kHz was also conducted to investigate ion nitride effect in higher fatigue life up to 10^7 cycles. Ion nitride effect was not observed for all tested specimens failed in ultrasonic frequencies because of sub-surface crack initiation from non metallic inclusions located at sub surface.
