Instrumented oscillographic study of impact toughness of an axle steel DZ2 with different tempering temperatures

Compared with the conventional Charpy impact test method, the oscillographic impact test can help analyze the behaviors of the material during the fracture process. This study evaluated the trade-off relationship between strength and toughness of DZ2 axle steel at different tempering temperatures and the reason why impact toughness improved. It is recognized that the tempering process dramatically influenced carbide precipitation behavior, causing different aspect ratios of carbides. It is found that the improvement of impact toughness along with tempering temperature is mainly due to the increase in energy required during the impact crack propagation. The characteristics of the impact crack propagation process are studied through a comprehensive analysis of the stress distribution, oscilloscopic impact statistics, fracture morphology, and carbide morphology. The reason why impact toughness of low tempering temperature specimens is poor are the presence of more stress concentration points due to the morphology of carbides in the small plastic zone during the propagation process, resulting in a mixed distribution of brittle and ductile fractures on the fracture surface.