The effect of martensite volume fraction and tempering heat treatment on the fatigue behaviour of a dual phase steel

In this study, the fatigue behaviour of a low alloyed steel, which was intercritically quenched, was investigated. The quenching temperatures were 737 °C, 754 °C 779 °C and 900 °C corresponding to low, medium, high and full martensite structures respectively. In addition to the quenching conditions, the samples, which were tempered at 200 °C and 400 °C and full annealed at 900 °C, were prepeared, thereby the fatigue behaviors of 13 different heat treatment conditions of the steel were examined. The fatigue tests were carried out on a high cycle rotational bending machine and stress was used as a control variable. The fractured samples were investigated both at macro and micro scale . Experimental results indicated that the best fatigue behaviour was observed in the samples quenched at 754 °C, and the weakest fatigue behaviour was observed in the samples quenched at 900 °C. The tempering heat treatment decreased the fatigue life of the samples which were involving low and medium martensite volume fractions and increased the fatigue life of the samples, which were involving high martensite and wholly martensite samples. At macro scale, the samples, which were involving low martensite volume fractions, exihibited more ductile fracture behaviour relatively. Transgranular fracture was observed through ferrite grains at micro scale. The mechanism affecting the fracture through ferrite grains was estimated as the slip bands, which placed in ferrite-martensite grain boundaries and in the regions in ferrite grains close by ferrite grain boundary.Keywords: dual phase steel, fatigue behavior, fractography