Instrumented oscillographic study on 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 in the behavioral analysis of materials during the fracture process. In this study, the trade-off relationship between the strength and toughness of a DZ2 axle steel at various tempering temperatures and the cause of the improvement in impact toughness was evaluated. The tempering process dramatically influenced carbide precipitation behavior, which resulted in different aspect ratios of carbides. Impact toughness improved along with the rise in tempering temperature mainly due to the increase in energy required in impact crack propagation. The characteristics of the impact crack propagation process were studied through a comprehensive analysis of stress distribution, oscilloscopic impact statistics, fracture morphology, and carbide morphology. The poor impact toughness of low-tempering-temperature specimens was attributed to the increased number of stress concentration points caused by carbide morphology in the small plastic zone during the propagation process, which resulted in a mixed distribution of brittle and ductile fractures on the fracture surface.