Abstract
This paper discusses the improvement of the fatigue limit of martensitic-ferritic dual phase steel (S20C) with high-cycle fatigue tests under both rotating bending and reversed torsion. Control of the morphology of the martensite second phase brings about higher fatigue strength through the restriction of the coalescence of small cracks than that by the conventional increase in the micro-Vickers hardness of the martensite second phase where the crack coalescence inevitably causes propagation of a fatal crack. This restriction can be attained by controlling the microstructure with ferrite encapsulated by martensite. The level of the fatigue limit depends on the degree of restriction of small crack propagation with this microstructure.
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