Transformação martensítica em aço inoxidável ASTM 2507 (UNS S32750) deformado em temperatura ambiente e criogênica

Abstract

In the present work, strain-induced martensite transformation phenomena on austenire was investigated in ASTM 2507 superduplex stainless steel, mainly epsilon martensite phase (ε, closed-packed hexagonal structure, HCP, paramagnetic). It was performed compression method to produces plastic deformation in two different conditions: at room and at liquid nitrogen temperature (cryogenic temperature). This aimed to investigate some specific features of strain-induced martensite formation, such as the influence of the plastic deformation method and test temperature by means of optical microscopy, scanning electron microscopy, X-ray diffraction, EBSD and Vickers microhardness. During the plastic deformation of the duplex stainless steels, the formation of ε martensite may occur. It seems that formation is more favorable in low stacking-fault energy stainless steels and also at low levels of deformation. These conditions was reached on the samples, where the strain was close to 5%, at both temperature conditions. X-ray diffraction lines and EBSD maps corroborate to presence of ε martensite in these samples and also the rotation of austenitic and ferritic grains throughout the compression process. In the samples with higher deformation levels, the ε martensite has been fully disappeared. It was observed a increasing in hardness along the increasing of deformation levels and temperature decreasing using Vickers microhardness measures. However, the ferritic phase showed negligible variation at room temperature, while austenite showed appreciable variation in both conditions. In additon, at cryogenic temperature, this increase is more appreciable. For example, the cryogenic temperature compressed sample of about 30% achieves hardness values up to 15% higher than the ambient temperature compressed sample with approximately 45% reduction.