On the low temperature creep controlling mechanism in a high strength spring steel

Abstract

Despite 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. The ultimate tensile strength and yield strength σe of SAE 9254 were determined in a temperature range of 298K≤T≤353K at a constant strain rate of 2.5∙10−4s−1. Low temperature creep deformation behavior was studied at the above-mentioned temperatures T at a constant stress σ=1634MPa, and at stresses 1071MPa≤σ≤1634MPa at a constant temperature T=353K for a duration of 1hr, respectively, at each condition. The retained austenite quantity was determined prior and post low temperature creep testing by means of X-ray diffraction. A reworked exhaustion creep model well describes the stress σ and temperature T dependency of low temperature creep deformation behavior in SAE 9254. Consequently, our hypothesis of a low temperature creep rate controlling mechanism reads: Low temperature creep strain is predominantly contributed by dislocation glide in the retained austenite phase. The reduction in creep rate with time can then be attributed to the exhaustion of this specific deformation mechanism by strain induced martensitic transformation of the retained austenite.