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Synopsis: The rotating bending fatigue and static bending tests were carried out to know the roles of surface nonmartensitic structure and sub-zero treatment and the effects of retained austenite and carbon contents in the carburized case on the fatigue strength properties of carburized steel. The main results obtained are as follows: (1) For both tests of rotating bending fatigue and static bending, the existence of surface non-martensitic structure decreased the mechanical properties of carburized steel when fracture occured at the specimen surface. On the other hand, the fatigue limit was independent of the existence of the structure if fracture did not occur at the specimen surface. (2) For the specimen with the carburized case containing carbon of about 0.9% (0.9%C specimen), sub-zero treatment followed by oil—quenching reduced the fatigue limit of about 15 kgf/mm2 by comparison with the case of no sub-zero treatment. Also, the maximum fatigue limit was obtained for the specimen with the carburized case containing austenite of about 20-25%. (3) The fatigue limit, the value of the fatigue crack initiating stress, the fatigue crack propagating stress, the nominal fatigue crack propagating stress at the tip of the maximum non-propagating crack, and the static bending strength and deflection up to rupture for 0.5%C specimen were higher than those for 0.9%C specimen. (4) The existence of the non-propagating crack at 107 cycles was observed in the case of the notched specimen with the notch root radius of 0.16 mm.

Effects of Microstructure and Carbon Content in the Case on the Fatigue Strength Properties of Carburized Steel