Effect of hierarchical martensitic microstructure on fatigue crack growth behavior of ultra-high strength hot stamping steel

Highlights: • Higher hot stamping temperature results in lager substructure size. • Tensile strength decreased with stamping temperature, while fatigue life increased. • Block is the effective substructure to control the fatigue crack growth behavior. • Crack closure was observed in the fatigue fracture of rapid-growing region. Martensite is a typical hierarchical microstructure, but which substructure plays a major role on fatigue crack growth behavior is still not clear. Therefore, this paper studied the microstructure and mechanical properties of 22MnB5 hot stamping steels, the influence of hierarchical martensitic microstructure on fatigue crack growth behavior was analyzed. Results show that higher hot stamping temperature results in larger size of prior austenite grain and substructure, the tensile strength decreased with increasing temperature, while the fatigue life and critical crack length were higher. It was concluded that block is the effective substructure to control the fatigue crack growth behavior of the steel. The fatigue fracture of stable-growing region was typical fatigue striations along with secondary cracks, crack closure was observed in rapid-growing region.