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New high speed tool steels, referred to as NF1 and NF2, were made experimentally by controlling the chemical compositions in order to improve the fatigue strength and evaluated with cantilever-type rotating bending fatigue test in air at room temperature in very high cycle regime. Larger size of MC-carbide particles were precipitated more in matrix of NF2 compared with those of NF1 and common high speed tool steel, JIS SKH51, due to the addition of more carbon and vanadium. From the experimental result of fatigue test, two kinds of the materials showed clear duplex S-N curve, composed of three or two types of failure mode, such as, a surface inclusion-induced failure mode (S-mode), a subsurface inclusion-induced failure mode without (I-mode) and with granular bright facet (GBF) area in the vicinity of inclusion (IG-mode) depending on the stress amplitude level. Fatigue strength of specimen failed by internal failure mode in the high cycle regime was the highest in NF2, followed by NF1 and JIS SKH51. As the result of discussion for the effect of microstructure of materials on subsurface crack initiation and growth behavior, size of inclusion at crack origin was the same among three kinds of the materials and crack propagation rate of NF2 was slower than those of NF1 and JIS SKH51, because the MC-carbide particles in the matrix acted as obstacle to crack growth. Also it was suggested that M6C-carbide particles distributed in the matrix did not affect for subsurface crack growth.

Evaluation of Very High Cycle Fatigue Properties of High-Speed Tool Steels Developed in order to Improve the Fatigue Strength