Short fatigue crack growth behaviour in ferrite-bainite dual-phase steels

The influence of bainite content on the short crack growth behaviour has been examined in a series of ferrite-bainite dual phase steels containing 50.3-90.3% bainite. Fatigue tests have been carried out using an earlier reported specimen configuration with the help of a rotating bending machine. These tests have been supplemented by characterization of the generated microstructures, determination of their hardness and tensile properties, and examinations of the nature of association of fatigue crack paths with microstructural constituents. The characteristic critical crack lengths above which linear elastic fracture mechanics may be applicable, the fatigue threshold values at the transition of short to long cracks, and the influence of microstructure on the crack path have been discussed. The maximum value of short crack fatigue threshold indicates a marginally increasing trend with increased amount of bainite. The fatigue crack path is found to be predominantly intra-granular for steels containing < 70% bainite while it is predominantly inter-granular for steels containing > 70% bainite.