Effect of austenitisation and tempering treatments on the mechanical properties of advanced high strength spring steel SAE 9254

Abstract

In response to increasingly stringent global regulations within the automotive industry, there is a growing demand for lightweight yet high-strength steel suspension coil springs. This study undertakes a comprehensive examination of the critical factors influencing the mechanical properties of SAE 9254, a well-established spring material, in its quenched and tempered (QT) state. The primary objective is to optimise its strength and ductility, especially in high-performance applications with an emphasis on lightweight design. The investigation encompasses a detailed exploration of the effects of varying austenitisation temperature (AT) and austenitisation time (At) on the microstructure and mechanical properties of the steel. Of particular interest is the study of tempering temperatures ranging between 300°C and 350°C to elucidate the mechanisms responsible for enhancing the steel's strength. Notably, the findings highlight dislocation density as the primary factor driving the strengthening of martensitic spring steel. Furthermore, the pivotal role of austenite grain size before quenching becomes evident, influencing toughness, ductility, martensite block size, and the percentage of retained austenite (γR). This study provides valuable insights into optimising the mechanical properties of SAE 9254 spring steel for high-performance applications, achieving ultimate tensile strengths (UTS) exceeding 2100 MPa and a reduction in area (RoA) exceeding 30%. Notably, the properties attained for this grade are comparable to those achieved with microalloying additions. Consequently, this research underscores the significance of controlling austenite grain size and dislocation density as critical factors for tailoring the properties of martensitic steels, positioning them as competitive materials, particularly in the pursuit of lightweight design.