肌焼鋼の疲れ強さに及ぼす浸炭粒度の影響

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A study was made on the effect of austenite grain size on the fatigue properties of carburizing steels SCM420 and SCr420. Rotating-bending fatigue strength obtained with smooth specimens was found to increase with decreasing austenite grain size. A coarse -grained steel with grain size number 3, for example, showed the 60% of fatigue strength as that of the fine-grained steel with grain size number 8. In case of notched specimens, on the other hand, fatigue strength was hardly influenced by austenite grain size.Microfractography of the specimens revealed that the fatigue cracks originated along austenite grain boundaries, propagated in transgranular mode and were followed by the dimple patterned final fracture. No intergranular fracture was observed in notched specimens. Consequently austenite grain size is considered to affect the fatigue crack initiation stage in rotating-bending fatigue properties.Furthermore, rolling contact fatigue strength was almost independent of the austenite grain size.
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