Heat treatment of the SAE 9254 spring steel: Influence of cooling rate on the microstructure and microhardness

Abstract

Mn-Si spring steels are widely employed for manufacturing automotive components such as helical springs. To develop higher strength spring steels based on the concept of the composition design and heat treatments, the generation ultra-high strength spring steels can reach tensile strength over 2200 MPa increasing the C, Si and Cr concentrations and optimizing heat treatments routes Heat treatments of these steels usually comprise high-temperature austenitization followed by quenching and tempering. A number of heat treatment parameters strongly influence the final microstructures of these steels such as the cooling rate during quenching. In this research, phase transformations in SAE 9254 steel were investigated by quenching dilatometry. The samples were austenitized at 850 °C by 12 minutes followed by quenching with 7 different cooling rates ranging from 0.5 to 40 °C/s. Their microstructures were characterized by microscopy techniques and their mechanical properties assessed by microhardness measurements. The obtained results show that the microstructure was very sensitive to the cooling rate. Low cooling rates promoted the formation microstructures of ferrite and perlite while high cooling rates produced martensitic microstructures. It was demonstrated the intermediate cooling rates can produce multiphase microstructures composed by polygonal ferrite, perlite colonies, martensite islands and retained austenite. The complex microstructures contribute to enhance the mechanical properties of the steels.

Keywords

• Spring steels
• Quenching dilatometry
• Phase transformations
• Heat treatments