Effect of temperature on straining of AISI 304 austenitic stainless steel

Resumo

The study examined the influence of plastic strain on AISI 304 austenitic stainless steel at room (298 K) and cryogenic (93 K) temperatures. Initially, seven samples were performed in different levels of engineering strain (0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 0.875 mm/mm) at both temperatures. Their results were compared to an as-received undeformed sample, and the analysis of the microstructure evolution involved techniques such as optical microscopy, microhardness measurements, and ferritescopy. Subsequently, complete mechanical tests were conducted until the sample failed at each temperature, generating stress-strain curves. These experiments were performed using the Thermomechanical Simulation System (XTMS) located at the XRD1 experimental station within the National Synchrotron Light Laboratory (LNLS) at the National Center for Research in Energy and Materials (CNPEM). The findings indicated that the quantity of strain-induced martensite α' (SIM-α') increased with the strain level, particularly in samples performed at low temperatures. At an engineering strain of 0.875 mm/mm, the amount of SIM-α' reached 96% at 93 K, whereas it only reached 60% at room temperature under the same conditions. Additionally, cryogenic straining resulted in higher transformation rates of SIM-α' and greater microhardness values. Moreover, cryogenic straining led to a significant increase in yield strength (205% higher) and tensile strength (96% higher), with a minimal decrease in uniform elongation (4.3% less) compared with room temperature straining. These effects were attributed to the partial suppression of dynamic recovery and the transformation of austenite into SIM-α'. The results suggest the occurrence of the Transformation Induced Plasticity (TRIP) effect, contributing to the improvement of mechanical strength. This study demonstrated that straining at cryogenic temperatures induces favorable changes in the mechanical properties of austenitic stainless steel by transforming austenite into SIM-α' and inhibiting dynamic recovery.