A high-strength and high-hardness steel generally shows a duplex or a stepwise S-N curve due to change in fracture mechanism from surface-inclusion induced failure mode in high stress amplitude level to subsurface-inclusion induced one in low-stress amplitude and very high cycle fatigue (VHCF) regime. GBF area is formed around a subsurface-inclusion at crack origin and its formation mechanism was previously proposed as 'dispersive decohesion of spherical carbide' model by the authors. It is possible to control the appearance of subsurface-inclusion induced failure in VHCF regime by means of the reduction in size and number of spherical carbide particles around an inclusion according to the proposed model. New high speed steel was made experimentally by control the chemical compositions and evaluated with.cantilever-type rotating bending fatigue tests. From the experimental results, fatigue crack initiation mode changed from large carbide in surface layer in low-cycle regime to matrix crack in surface in high-cycle regime, and subsurface-inclusion induced failure never appeared in VHCF regime. This behavior could be caused by decrease in distribution of small MC-carbide particles in the matrix and restriction of the GBF area formation around a subsurface inclusion.
