Effect of Tempering Temperature on Microstructure and Impact Properties of Two High-strength Leaf Spring Steels

The effect of tempering temperature on the microstructure and impact toughness of two high-strength leaf spring steels 50CrMnSiVNb and 50CrMnMoVNb for automobile was comparatively studied by means of optical microscope, scanning electron microscope, transmission electron microscope and impact tester. The results show that compared with those of 50CrMnSiVNb steel, there are more segregation bands along with a larger proportion of large-angle grain boundaries in the microstructure of 50CrMnMoVNb steel, while the later steel shows less temper brittleness. When comparing the impact toughness of the two leaf spring steels, it is found that being quenched and then tempered in the range of 150~400℃ for the two steels, the 50CrMnSiVNb steel presents better impact toughness. The impact toughness of the steel tempered in this range is mainly affected by the degree of banded segregation, which is more prone to cleavage fracture and leads to a straighter impact crack propagation path; In the contrast, after the two steels were tempered in the range of 400~500℃, the 50CrMnMoVNb steel shows better impact toughness, and the impact toughness in this region is mainly affected by the tempering brittleness and the proportion of large-angle grain boundary. The tempering brittleness caused by the thin-film like carbides at the interface of the laths during tempering greatly worsens the impact toughness, while the large angle grain boundary has a stronger barrier effect to crack propagation and consumes more energy, leading to the improvement of the impact toughness. © 2023 Chinese Journal of Materials Research. All rights reserved.