抄録
Ultra-sonic fatigue tests were carried out for W-Mo type high-speed steel specimens containing 40 ppm N (low N), 160 ppm N (middle N) and 400 ppm N (high N). The fatigue strength of the high-N specimen was lower than those of the low-N and middle-N specimens, which were nearly identical. The increase in N content caused no changes in average sizes of MC carbides, M6C carbides and carbides consisting of M6C and MC (M6C•MC), but increased the size of large MC carbides, such as the maximum size from about 6.5 μm to 13 μm. The fracture origin of the high-N specimen was coarse MC carbides of about 13 μm, and those of the low-N and middle-N specimens were large aggregates of about 12 μm, consisting of two or three M6C•MC carbides. Voids formed by decohesion of the M6C/MC interfaces in M6C•MC carbides larger than about 5 μm in the low-N, middle-N and high-N specimens, and those formed by destruction of MC carbides larger than about 8 μm in the high-N specimen were observed after fatigue fracture under a stress amplitude of 1100 MPa. This result arises because of greater stress concentration due to more inhomogeneous deformation around the MC and M6C•MC carbides develops as the sizes of both the carbides increase. In the high-N specimen, cracking was initiated from the voids in coarse MC carbides, while in the low-N and middle-N specimens, a crack propagated between two voids in adjacent large M6C•MC carbides which constitute a carbide aggregate. The fatigue strength of the high-N specimen is lower because the nucleation of the cracks from coarse MC carbides occurs more rapidly than that of the cracks in large M6C•MC carbide aggregates of the same size as the coarse MC carbides.
