Relationship between Fatigue Strength and Case Hardness Distribution in Casecarburized and Hardened Steel

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  • 浸炭表面硬化鋼における疲労強度と硬度分布の関係
  • シンタン ヒョウメン コウカ コウ ニ オケル ヒロウ キョウド ト コウド

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The rotating bending fatigue tests were performed on the carburized SCM21 steel specimes, and the relation between the fatigue strength and the case hardness distribution was examined on two series of specimens having different hardness distributions. The plain and notched specimens were carburized at 930°C, under 0.9% carbon potential atmosphere for 4hrs, and oil-quenched from 830°C. Tempering was carried out at 180°C for 2hrs.<br>The results obtained are as follows.<br>(1) In the case of the plain specimen with a gradual stress gradient, fatigue damage seems to progress in a wide field of the carburized layer. So the initiation of fatigue cracks may occur either on the surface or inside of the carburized layer. The present test results show that the fatigue cracks initiate on the surface of the carburized layer, and the specimen having higher case hardness possesses lower fatigue strength than the one of lower case hardness.<br>(2) In the case of the notched specimen with a steep stress gradient, the progress of fatigue damage is limited to the notch root portion or the crack tip. So the crack initiating stress σw1 and the crack propagating stress σw2 of the specimen seem to be decided by the hardness and the residual stress of the critical portion where the fatigue fracture occurs. The present test results show that the fatigue crack initiating stress of the notched specimens is related to the effective case depth. The fatigue crack propagating stress of the notched specimen is proportional to the hardness of the crack tip, and the stress range between σw1 and σw2 is almost constant for both series of the specimens having different effective case depth.

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