Influence from temperature variations in stacking fault energy on the mechanical properties of stainless steels

Sammanfattning: This paper investigates the mechanical properties and deformation mechanisms of austenitic stainless steels and how they relate to the material property of stacking fault energy (SFE) and its relation to temperature and nickel content. Austenitic stainless steels are commonly used and well known for good mechanical properties and deformation characteristics. Austenitic stainless steels are primarily defined by their face centered cubic (FCC) structures. Austenitic stainless steels typically include alloying elements with contents of >11wt-% Cr, >50wt-% Fe, <1.2wt-% C as well as other alloying elements. The steels in this report were provided by Alleima AB were 904L and 316L whose primary difference is the Ni content of 25wt-% and 11.5wt-% respectively. SFE is a microscopic material parameter that can influence what deformation hardening phenomena will occur. SFE is, according to the literature, temperature-dependent, and composition-dependent. The deformation hardening mechanisms will change across the temperature range and different nickel contents. At low SFE, the strain hardening mechanism of transformation-induced plasticity (TRIP) will occur. TRIP will be seen through the transformation of the austenitic phase into body centred tetragonal (BCT) structured α’-martensite, or hexagonal close-packed (HCP) structured ε-martensite. When the temperature is raised, the SFE increases. At a certain point, the strain hardening mechanism will change to twinning-induced plasticity instead (TWIP). Through mechanical deformation at different temperatures, the mechanical properties of the steels can be improved by different strain hardening mechanisms which is explored in this report. Tensile tests were performed at five temperatures: 500℃, 200℃, 23℃, -80℃ and -196℃. Each tensile test produces stress-strain curves and by comparing the results from the tensile tests at the different temperatures material characteristics and strain hardening were studied. The results implied the phenomena of TRIP and TWIP had occurred during strain hardening which would be further confirmed through microscopy examination of the samples. The samples were also examined through the two microscope methods light optical microscopy (LOM) and electron backscatter diffraction (EBSD). By cutting the samples 5mm from the fracture, just below necking, the surface of interest was revealed. Polishing and etching the samples for LOM allows the surface to be examined and to determine possible present phases in the steel after the tensile test. To create an examination of the phases present, the scanning electron microscope technique, EBSD, is used to determine the present phases and grain orientation of the samples. By examining the microstructure of the samples, the temperature dependence of SFE is developed. The results of the study indicated that SFE is indeed correlated to temperature and Ni content. The stress-strain curves that were produced from the tensile testing machine showed clear characteristics of strain hardening at low and cryogenic temperatures. The findings from the stress-strain curves were corroborated by the LOM images and the EBSD maps where at cryogenic temperatures deformation twins and martensite formation was clearly visible. This was shown not to be the case across the temperature range. 316L showed TRIP at cryogenic temperatures which then changed to TWIP in the middle of the temperature range, and finally neither when the temperature was further raised. The difference in the amount of martensite formation and the formation of deformation twins at the different temperatures indicated that the SFE increases with the temperature. 316L showed greater strain hardening characteristics and more TWIP and TRIP transformation compared with 904L in the different tested temperature ranges. The results confirmed the positively correlated relationship between SFE and temperature as well as that the addition of Ni increases the SFE of the alloy.