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2579428 
Journal Article 
Effect of nitrogen on fatigue behavior of a high-speed steel 
Mitani, T; Hasegawa, T; Watanabe, C; Monzen, R 
2008 
Yes 
Journal of the Japan Institute of Metals/Nippon Kinzoku Gakkaishi
ISSN: 0021-4876
EISSN: 1880-6880 
72 
2 
105-110 
WOS:000254138000006 
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 center dot MC), but increased the size of large MC carbides, such as the maximum size from about 6.5 mu m to 13 mu m. The fracture origin of the high-N specimen was coarse MC carbides of about 13 mu m, and those of the low-N and middle-N specimens were large aggregates of about 12 mu m, consisting of two or three M6C center dot MC carbides. Voids formed by decohesion of the M6C/MC interfaces in M6C center dot MC carbides larger than about 5 pro in the low -N, middle-N and high-N specimens, and those formed by destruction of MC carbides larger than about 8 pm 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 center dot 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 center dot 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 center dot MC carbide aggregates of the same size as the coarse MC carbides. 
high-speed steel; nitrogen content; fracture origin; MC carbide; M6C.MC carbide; inhomogeneous deformation; stress concentration; crack initiation