Effects of quenching temperature on the microstructure and impact toughness of 50CrMnSiVNb spring steel

Abstract

50CrMnSiVNb spring steel was quenched at 760, 820, 880, 940 and 990 °C to obtain different microstructures and nucleation rates of tempered carbides. The results show that for the QT-760 sample, the microstructure is mainly pearlite due to incomplete austenization. The coarse lamellar cementite significantly deteriorated the impact toughness. For other conditions, the carbide is key factor affecting impact toughness ranging from 90.3 to 120.3 J/cm2. From the perspective of carbide size, the impact toughness is the result of the competition between matrix softening degree and carbide, thus, there should be a moderate optimal carbide size to obtain the optimal impact toughness. Additionally, increasing the aspect ratio of carbide can greatly reduce the optimal carbide size, making the carbides more likely to become a factor of impact toughness deterioration. This is unfavorable to impact toughness. Therefore, to obtain better impact toughness, the quenching temperature should be high enough to ensure that the microstructure is completely austenitized, and the quenching temperature should be adjusted to increase the carbide nucleation, so as to obtain carbides with a smaller aspect ratio and size closer to the optimum carbide size mentioned above. This study enriches the understanding of impact toughness of spring steel and provides a way to improve impact toughness in practical applications.