An investigation of low temperature creep controlling mechanisms 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 mechanisms contributing to sag loss in a coil spring or low temperature creep (LTC) in martensitic spring steel (MSS) are still unclear. Furthermore, the LTC rate-controlling mechanisms in MSS remain elusive. In the current research, an attempt has been made to unfold the LTC rate-controlling mechanisms in SAE 9254 steel grade.
First and foremost, the LTC mechanism-based models, i.e. strain hardening theory (SHT), and exhaustion creep model (ECM) are developed to make them capable of describing the stress σ and temperature T dependent LTC strain of a material. Subsequently, the modified SHT and reworked ECM are verified and validated at the example of SAE 9254. The temperature-dependent LTC behavior of inductively quenched and tempered (IQT) SAE 9254 and martempered SAE 9254 are studied at the temperatures 298 K ≤ T ≤ 353 K for a duration of 1 hr at each condition. The stress σ dependent LTC behavior of IQT SAE 9254 is studied at the stresses 1071 MPa ≤ σ ≤ 1634 MPa, and the stress σ dependent LTC behavior of martempered SAE 9254 is studied at the stresses 421 MPa ≤ σ ≤ 632 MPa.
A combined analysis by means of (i) the reworked ECM, and (ii) advanced microstructural characterization prior to and post LTC suggests that dislocation glide, mainly localized in the metastable retained austenite phase, is one of the basic LTC contributing mechanisms in SAE 9254. Stress-assisted martensitic transformation (SAMT) and, at elevated temperatures, strain-induced martensitic transformation (SIMT) are the additional LTC contributing mechanisms. Furthermore, the considered approach suggests that the LTC rate-controlling mechanisms are stress-assisted recovery (SAR), SAMT, and, at elevated temperatures, SIMT.