Effect of Temperature on the Mechanical Properties of Stainless Steels with Silicon Additions

Details

Supervisors

Jacques, Pascal

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil en chimie et science des matériaux, à finalité spécialisée

Abstract

enMetastable austenitic stainless steels can combine high strength and ductility through deformation mechanisms such as Transformation-Induced Plasticity (TRIP) and Twinning-Induced Plasticity (TWIP). Their activation depends on several parameters, including chemical composition, temperature, and stacking fault energy (SFE). This work investigates the mechanical and microstructural behavior of Fe-Cr-Mn-Ni-Si alloys, with particular attention to the influence of silicon content and cryogenic temperature on these deformation mechanisms. Tensile tests were performed at room temperature and at 77 K, while phase constitution and microstructural evolution were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). Four alloys were successfully characterized, while two could not be investigated due to defects that occurred during cold rolling. Their microstructures were predominantly austenitic with different fractions of α′- and/or ϵ-martensite. Decreasing the testing temperature to 77 K increased both yield strength and ultimate tensile strength and modified the work-hardening behavior. XRD measurements showed that the TRIP effect was promoted under cryogenic deformation. At 77 K, the Mn-free alloys predominantly followed a direct γ → α′ transformation, whereas the Ni–Mn containing alloys exhibited a sequential γ → ϵ → α′ transformation. The balance between Ni and Mn therefore appears to play a role in determining the martensitic transformation pathway. In contrast, the influence of the Si content (2 and 5 at.%) does not appear to have modified the deformation mechanisms, and its specific influence could not be isolated from the current results. Although the tensile and work-hardening responses suggest the activation of additional deformation mechanisms, no direct evidence of deformation twinning was obtained by SEM or conventional EBSD. Consequently, the occurrence of a TWIP effect could neither be confirmed nor excluded. Overall, temperature was found to have a clear influence on the mechanical response and TRIP behavior of the investigated alloys, while the specific role of silicon requires further investigation.