IMPROVEMENT OF PROOF FATIGUE WITH GRADIENT OF MECHANICAL PROPERTIES OF SURFACE LAYER

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Salt bath nitriding or plasma nitriding were applied to structural steel SS400 and low carbon steel S15CK respectively, and those fatigue properties and fatigue fracture mechanisms were investigated. The results obtained are as follows :(1) In the case of nitrided SS400, fatigue cracks are generated at the separated regions between the compound layer and the diffusion layer and propagate in the inner region of the diffusion region.(2) It is found that Fatigue strength of the nitrided steel is affected by the internal structures within the materials before nitriding treatment.(3) Dislocation structures before nitriding are reorganized by the nitriding treatment. Such structures are changed into cell structutes by the cyclic stress.(4) In the case of the plasmanitrided S15CK, fatigue cracks are generated along the needlelike nitrides and propagate in the connection of the nitrides.(6) It is found that the increase of the nitrogen content in the reaction gases and gas pressure are effective in the appropriate nitriding of the narrow gap. It is also found that the hollow cathode discharge are effective in the enhancement of the nitriding