Evaluation of fatigue properties for new high speed steel nitrided was performed through a cantilever-type rotating bending fatigue test in air at room temperature in very high cycle fatigue (VHCF) regime. The high speed steel was made experimentally by control the chemical compositions to avoid formation of the GBF area near subsurface inclusion at crack origin, based on internal failure mechanism as ‘dispersive decohesion of spherical carbide’ model previously proposed by authors. Surface treatment was applied with both of radical nitriding and low-temperature nitriding. Fatigue strength was increased in the whole of applied stress amplitude levels by both nitridings. Fatigue failure mode was changed from a matrix cracking in surface of the untreated specimen to an internal-inclusion induced failure with GBF area of the nitrided specimens in VHCF regime, for the reason of surface layer having high hardness and high compressive residual stress. From the comparison with common high speed steel, JIS SKH51, failed by an internal- inclusion induced failure mode in VHCF regime, fatigue strength of nitrided specimens was increased about 35%. It was concluded that the GBF formation around a subsurface inclusion was restricted by decrease in distribution of small MC-carbide particles in the matrix and a large number of stress cycling were required for the formation of appropriate size for the GBF area to enable propagation as an ordinary crack.
