Influenza dei parametri del processo di nitrurazione ionica sulla resistenza a corrosione di acciai inox

Nitriding is a thermochemical treatment of surface hardening developed to improve the resistance of ferrous alloys to wear and fatigue. The nitriding process object of this work is of ionic type, which involves the use of a plasma generated on the substrate, or in proximity to it, to facilitate the atomization of the gaseous nitrogen present in the chamber and its diffusion into the metallic lattice 1. Aim of this study is the evaluation of the influence of process parameters, such as temperature2-3, time4, composition, and atmosphere, of the nature of the nitrided layer, and the consequent resistance to corrosion of stainless steels of six different microstructures. The composition of the stainless steels is shown in Table 1. These are two martensitic stainless steels AISI 410 and AISI 420, an austenitic AISI 316L, a duplex AISI 329, a maraging steel and an austenitic precipitation-hardening 17-4 PH. The nitriding treatment is used in different applications like building, automotive, energy technologies and industrial machinery, often in potentially aggressive environments, which can give rise to corrosion phenomena. Although the process of nitriding has been known for decades, there has been a growing interest in the development of this thermochemical treatment in recent years since, in addition to good mechanical and tribological properties, adequate characteristics of corrosion resistance are increasingly required. Stainless steels are subject to the phenomenon of sensibilization5, namely the precipitation of secondary phases as a result of permanence for long time in a specific range of temperatures (450-900 degrees C). The typical nitriding temperatures fall in this range and the thermochemical treatment can cause an important decrease of corrosion resistance. With the aim to identify the best operating conditions, several cycles of ion nitriding were performed, in a traditional ion nitriding oven and in an oven with a different technology, called "H oven" The specimens were obtained from bars with different geometric sections and polished to the papers up to 600 grit. Vickers surface hardness measures were performed to determine the nitriding depth. Metallographic analysis allowed to study the microstructure and to evaluate the thickness of the nitriding layer, where present. The electrochemical behavior (corrosion) was investigated by recording cyclic potentiodynamic polarization tests in 0.01 M NaCl solution. The concentration was chosen on the basis of preliminary tests. Finally, X-ray diffraction conducted on some of the most significant samples allowed to obtain information on the nature of the phases in the nitrided layer. Stainless steels were subjected to nitriding conditions but in the absence of nitriding atmosphere (test T) to evaluate only the effect of the sensibilization. Table 2 shows the operating conditions chosen for the different tests of nitriding. Each cycle of nitriding is preceded by hydrogen sputtering at 350 degrees C for 2 hours, except Test T, Test H, and the Test H2, for which was performed argon sputtering. Cyclic polarizations show in general worsening of corrosion resistance of nitrided steel samples compared to untreated samples. The intensity of worsening appears to be in accordance with the different microstructure of steels and the operating conditions of the processes. Active behavior for the martensitic steels AISI 410, AISI 420, and 17-4 PH is observed in different cycles of nitriding. About austenitic stainless steel AISI 316L, and maraging steel tendency is observed to assume active corrosion behavior for nitriding cycles in traditional oven, with better corrosion resistance than untreated samples as regards the cycle in H oven. Bi-phase steel AISI 329 shows significant worsening of corrosion resistance for cycles at higher temperatures and longer process times. It is however necessary to observe together the corrosion resistance of steels that the main objective of the ion nitriding is the increase of the surface hardness,. Results of hardness measurements are reported in Table 3. Surface hardness shows a general increase, according to the operating parameters used, and the nature of the substrate. The highest increase of surface hardness corresponds to nitriding cycles conducted at higher temperatures and longer times. However, the most intense worsening of corrosion resistance is observed at this operating conditions. Metallographic analysis showed that the diffusion layer is not always present for all cycles of nitriding. Furthermore, the presence of the edge effect is noticed on most of the treated samples namely a very deep thickness of nitriding caused by non-uniform distribution of temperature and plasma geometry in that point. XRD analysis, performed on nitrided AISI 329 SS, confirms the presence of the discontinuous layer of nitriding on the surface as determined by the appearance of specific peaks. These peaks correspond to a phase consisting of nitrogen diffused in the iron lattice. This discontinuity is in accord with process parameters. In particular, it is observed the presence of typical phases of nitriding is observed for treated steel in test 6, in test H, and test H2. Based on the obtained results, it can be stated that prolonged exposure to nitriding temperatures and times allows surface hardness to increase significantly although it can lead to sensitization of the stainless steel substrate. The quality of ion nitriding changes with the type of steel and process parameters, first of all temperature, followed by exposure time and nitriding atmosphere. The range of sample treatment were the broadest possible in this work. Common condition for all types of steels was however not obtained. It would be necessary to determine the best conditions, and, to perform a single treatment for each material. That would be however impracticable, so few possibilities of intervention are lefte. XRD analysis allows to attribute the decreasing of corrosion resistance to the presence of chromium nitride, appearing in the cycles using higher temperatures and longer process times. The oven H technology, where plasma is not generated directly onto the piece, and the pressure is lower and more controllable, allows to perform a nitriding process with the best compromise between increase of surface hardness and decrease in corrosion resistance.