Estudo da reversão martensítica em um aço inoxidável austenítico 201LN submetido à laminação criogênica

Abstract

With the progress of society, the demand for new materials that meet human needs is becoming increasingly greater. In this context, austenitic stainless steels (ASS) have attracted significant interest, primarily due to their excellent corrosion resistance. The 300 series ASS are the most commonly used among these steels. However, the microstructural refinement of 201LN steel, achieved through cryogenic deformation followed by heat treatment, enhances its mechanical properties, making it a cost-effective alternative to the 300 series, as nickel (Ni) is replaced by nitrogen (N) and manganese (Mn). The high susceptibility of ASS to the effect of transformation-induced plasticity (TRIP) when subjected to deformation justifies this observed improvement since 201LN ASS is more prone to this phenomenon than 304, due to its lower stacking fault energy (SFE). As a result, the transformation from the austenite phase to martensite (γ → α’) is facilitated. Thus, the goal was to induce the γ → α’ transformation through cryogenic rolling, in order to evaluate the effects of this transformation before and after the α’ → γ reversion. For this purpose, the sample was solution-treated, deformed at cryogenic temperature to thickness reductions of 10, 20, and 30%, and subsequently annealed for 30 minutes at 400, 500, 600, 700, and 800 ºC. To analyze the evolution of deformation-induced martensite (DIM) and its subsequent reversion, the following techniques were applied: metallography, X-ray diffraction (XRD), Vickers hardness, and Electron Backscatter Diffraction (EBSD). In summary, the results demonstrated that the greater the plastic deformation at cryogenic temperature, the higher the volume fraction of DIM in 201LN ASS, and consequently, the hardness increases. XRD data indicated the presence of approximately 43% α' after 30% cryogenic deformation, while the EBSD technique showed a volume fraction of around 50%. After annealing for 30 minutes at 800 ºC, the resulting grains exhibited a mean size reduced by almost 10 times, which resulted in an increase of about 2 times in hardness compared to the solution-treated sample.