Pivotal role of retained austenite as a low temperature creep controlling mechanism in a martensitic spring steel

The rising demand for electric vehicles urges automotive suppliers to push their limits on the sag resistance of suspension coil springs. However, the mechanism contributing to sag loss in a coil spring or low temperature creep (LTC) in a spring steel is still unclear. Furthermore, the LTC rate-controlling mechanism remains elusive. In the current study, an attempt has been made to unfold the LTC controlling mechanism in a SAE 9254 steel grade. A combined analysis by means of (i) a mechanism-based exhaustion creep model (ECM) and (ii) advanced microstructural characterization prior to and post LTC suggests that dislocation glide, mainly localized in the metastable gamma phase, is considered as one LTC contributing mechanism in martempered SAE 9254. Furthermore, stress-assisted martensitic transformation (SAMT) is considered as additional LTC contributing mechanism in SAE 9254 up to the temperature T <= 323 K. Beyond that temperature strain-induced martensitic transformation (SIMT) takes the place of SAMT. Our approach includes that the LTC rate-controlling mechanisms are stressassisted recovery (SAR), SAMT, and SIMT especially at elevated temperature T.