2009 Volume 95 Issue 9 Pages 655-659
Fatigue tests of W–Mo type high-speed steel specimens were carried out at 20 and 600°C under various stress amplitudes σ. Fatigue lives of the specimens tested at 20 and 600°C under each amplitude between 1100 and 1400 MPa were identical, but under each amplitude between 700 and 1000 MPa, those of the specimens tested at 600°C were shorter than those at 20°C. Fracture origins of the specimens fatigued at 20°C under all stress amplitudes and at 600°C under σ≧1100 MPa were large aggregates of 10~13 μm, comprising two or three M6C·MC carbides, and those tested at 600°C under σ≦1000 MPa were large isolated M6C·MC carbides of about 8 μm. When fatigued at 20°C under all stress amplitudes and at 600°C under σ≧1100 MPa, a crack propagated between two voids, produced by decohesion of the M6C/MC interfaces, in adjacent large two M6C·MC carbides which constitute a carbide aggregate, whereas in the specimens tested at 600°C under σ≦1000 MPa, cracking was initiated from the voids in large isolated M6C·MC carbides of about 8 μm. The fatigue lives of the specimens tested at 600°C under σ≦1000 MPa are shorter because the nucleation of cracks from isolated M6C·MC carbides occurs more rapidly than that of cracks in large M6C·MC carbide aggregates.