The Effect of Ferrite Grain Size on the Fatigue Behavior of Ferrite martensite Dual phase Steels

The effect of ferrite grain size on the fatigue and tensile properties of dual phase steels with a 0.25 volumefraction of martensite (Vm) under different heat treatments was investigated. The heat treatments werehomogenized at 1200 °C along with several subsequent normalizations at 910 °C, resulting in differentmicrostructures and mechanical properties. After heat treatment, the obtained steels, with differentferrite grain sizes, were heat treated to obtain a dual phase ferritic martensitic microstructure. In orderto process the dual phase steels, low carbon manganese steel was used. Fatigue tests were carried outat room temperature and the fracture surfaces of the fatigue specimens were studied by SEM. The dataobtained by the fatigue tests indicated that the fatigue strength at 107 cycles had a linear increase withdecreasing the grain size of ferrite, while higher applied stress had a little effect on the grain size andthe fatigue strength. The fracture surface of the fatigue specimens showed two distinct regions, namely,the fatigue fracture and the final fracture. Striation lines were clearly seen in the region of the fatiguefailure. Furthermore, for all microstructures, the final fracture was mainly brittle.