Effects of Temperature and Loading Conditions (Monotonic/Cyclic) on the Microstructural Evolution and Phase Stability of Martempered SAE 9254 Spring Steel
摘要
The stability of retained austenite (γR) is critical to the mechanical performance of high-strength martensitic spring steels in electric vehicle (EV) suspension systems. This study examines SAE 9254 spring steel with low austenite stabiliser content to assess γR behaviour under different quenching temperatures (RT − 250 °C) and mechanical loading modes. Unlike previous studies, which primarily focussed on medium Mn steels or standard tensile testing, this work uniquely investigates γR transformation in low-Mn SAE 9254 under standard strain rate (10−4 s−1), slow strain rate (10−6 s−1), and cyclic loading conditions, providing new insights into fatigue behaviour relevant to suspension applications. Results showed that at standard strain rates, γR transformed to martensite, increasing strength by 10 pct but reducing ductility by 34 pct. At slow strain rates, γR stability improved with reduced transformation. Cyclic loading caused rapid transformation: γR content remained stable up to 10 cycles but dropped by 54 pct at 20 cycles and over 95 pct at 100 cycles, even within elastic stress limits. These findings highlight the critical role of interactions between quenching temperature, strain rates, and cyclic loading in governing γR transformation kinetics and overall steel performance. Key microstructural indicators such as γR morphology, dislocation density, and martensitic lath width were found to control phase stability and mechanical performance.
Graphical Abstract