Abstract
In order to evaluate the fatigue strength in very high cycle regime of newly developed two kind of high-speed tool steels, such as, MHS5 (0.9C-2W-2V-3Mo) and MH2 (0.5C-0.5V-2Mo-0.1Nb), rotary bending fatigue test was carried out inlaboratoiy air at room temperature using a hourglass shaped specimen S-N curve obtained from MHS5-specimen appeared to have a stepwise shape with a horizontal asymptote changing from the surface carbide induced failure mode (S-mode) in high stress amplitude level to the subsurface carbide induced one with GBF area in low stress amplitude level. On the other hand, transition of failure mode in MHS2-specimen was occurred from the S-mode in high stress amplitude level to crystal-slip induced failure mode formed in the specimen surface. It was suggested that the difference of failure mode in very high cycle regime between tested materials was caused by the size of carbide particles precipitated in the matrix.
