Improving fatigue strength of bainite/martensite dual-phase steels in very high cycle fatigue regime by refining microstructures

Very high cycle fatigue behaviors of two bainite/martensite dual-phase steels were investigated. One of the steels was cyclic rapid heat treated and its microstructures were refined. Fatigue strength of the steel is 225 MPa higher than that without refining. Observation of fracture surfaces show that the fatigue cracks initiate at bainites for non-refined steel and at non-metallic inclusions for the refined steel. The size of inclusions is much smaller than that of bainites which results in the improvement of fatigue strength.