Influência do fluxo intermitente de nitrogênio nas propriedades superficiais do aço inoxidável austenítico 316L nitretado por plasma

Abstract

Plasma nitriding is an effective thermochemical treatment to increase the surface hardness of austenitic stainless steels while preserving or enhancing their corrosion resistance when performed at temperatures around 400°C. However, the process can generate high residual stresses, cracks, and nitride precipitation, compromising the integrity of the formed layer. In this context, intermittent nitrogen flow emerges as a strategy to control nitrogen concentration, reducing the incidence of cracks and nitride precipitation. The present study evaluated the influence of this method on the corrosion and scratch resistance of AISI 316L austenitic stainless steel nitrided at 425 °C, aiming to understand the microstructural and electrochemical effects of different nitriding conditions. Scanning electron microscopy (SEM), X-ray diffraction (XRD), nanoindentation, open circuit potential (OCP), potentiodynamic polarization (PP), electrochemical impedance spectroscopy (EIS), and linear scratching tests were performed. The nitrided condition with continuous flow exhibited superior performance, particularly in terms of localized corrosion resistance and scratch resistance. Nitriding with intermittent nitrogen flow significantly impacted the nitriding kinetics, altering the layer thickness, present phases, surface hardness, corrosion resistance, and scratch resistance. The use of intermittent nitrogen flow subjected the samples to a denitriding phenomenon during the absence of nitrogen, especially in the last nitriding period. Since this period lasts only a few minutes, this phenomenon may affect a few atomic layers, leading to nitrogen depletion from the extreme surface. Reducing the duty cycle to 50% resulted in behavior similar to that observed with continuous nitrogen flow in terms of layer thickness, surface hardness, and scratch resistance. Although it exhibited good corrosion resistance, its protection was compromised at high potentials due to variations in nitrogen concentration, depending on grain orientation, and the high density of dislocations generated during the nitriding process. Nitriding with a 10% duty cycle promoted surface hardening similar to that observed in the other nitrided conditions. However, the combination of a reduced γn layer thickness and an increased γc layer led to a sharp drop in hardness with increasing contact depth. During linear scratching tests, crack nucleation and propagation were observed even under low loads. Additionally, its corrosion resistance was significantly compromised, yielding results inferior to the untreated condition. Despite the sample nitrided with a 50% duty cycle exhibiting slightly inferior performance compared to the sample treated under continuous nitrogen flow, the results were superior to those of the untreated sample. A 3.6-fold increase in static hardness and a 1.4- fold increase in scratch hardness were observed, while corrosion resistance was maintained. Thus, the intermittent flow approach presents itself as a potential alternative, although further optimization of the processing parameters is still required.