Remalli, Nagarjuna
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Publication Access status: Open Access , An investigation of low temperature creep controlling mechanisms in a martensitic spring steel(2023)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.Source Type:Doctoral ThesisSeries:Schriftenreihe des Lehrstuhls für Werkstoffsysteme für den Fahrzeugleichtbau im Institut für WerkstofftechnikVolume number:45 8 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , On the low temperature creep controlling mechanism in a high strength spring steelDespite of the well-known fact that high strength steels are exhibiting low temperature creep deformation, its rate controlling mechanism is yet to be understood. The strain hardening theory and the exhaustion creep model were proposed almost seven decades ago to unravel the low temperature creep mechanism in a single phase homogeneous isotropic material. However, their applicability to low temperature creep deformation in a technical material like a modern high strength steel is still a matter of investigation owing to their nature of multi-phase, in-homogeneous, and an-isotropic behavior. The authors have grabbed this chance to experimentally validate the exhaustion creep model based on low temperature creep tests of the SAE 9254 spring steel.Source Type:Article297 173
