In situ study of AISI 304 and AISI 430 stainless steel using synchrotron light

Abstract

The cryogenic deformation has been extensively studied due to the possible improvements of generated properties, such as an increase in mechanical resistance and ductility. The gains occur as a consequence of the partial suppression of dynamic recovery, which produces microstructural refining and increases microstrain, due to an increase of internal defect density. Two stainless steels, AISI 304 and AISI 430, were used for the study under four strain-rates. The XTMS experimental station allowed tensile tests simultaneously to X-ray diffraction, providing in situ information of phenomena and microstructural changes that occur during deformation. The analysis of fracture surfaces was essential to understand the ductile-brittle transition phenomenon on AISI 430 steel, which indicated conditions at the threshold of the ductile-brittle transition. An additional strain rate was used to overcome the transition point. The mechanical results showed that the decrease in temperature has a similar effect on the increase in strain rate, and these changes generated an increase in tensile strength for AISI 430 steel. However, the four initials strain rates, there was no significant impact on the mechanical result, but when the fracture surface was analysed, there was a competition between the cleavage and ductile fracture mechanisms. For the AISI 304 steel, at cryogenic temperature, it was possible to verify a change in the shape of the stress-strain curve, with loss of ductility and increase of resistant, being an effect of the martensitic transformation. For the AISI 304 steel, phase transformation at room temperature occurs with higher strain than for the cryogenic temperature, due to the effect of the adiabatic heating, which stabilises austenite. The microstrain in the austenite planes decreased during the plastic deformation, indicating the presence of dynamic recovery. For the AISI 430, the crystallite size indicated that at cryogenic temperature there was greater microstructural refining.