The nitriding conditions used for low alloy steels or tool steels cannot be employed for stainless steels, since the treatment temperatures (approx. 500 degrees C or higher) cause the formation of large amounts of chromium nitride, CrN. As a consequence, together with an increase of surface hardness, a marked decrease of corrosion resistance in usually observed, due to the depletion of Cr atoms from the matrix. CrN precipitation can be inhibited with the low temperature nitriding process. For austenitic stainless steels nitriding at temperatures lower than 450 degrees C produces modified surface layers with a double layer structure, having a thicker outer layer, in which a supersaturated solid solution of nitrogen in the expanded and distorted f.c.c. austenite lattice, known as S phase or expanded austenite, is present, and a thinner inner layer, which consists of a solid solution of interstitial atoms (nitrogen, carbon) in austenite. The S phase has a nitrogen content up to about 10 wt. %, and it shows very high hardness (up to 1500 HV) and improved corrosion resistance in chloride-ion containing solutions. Treatment temperature and time are known to influence the characteristics of the modified surface layers, in particular regarding nitride precipitation; however, also treatment pressure plays an important role, especially when the glow-discharge process is employed for nitriding. In the present research the influence of treatment parameters (temperature, pressure, time) on the microstructural, microhardness and corrosion resistance characteristics of low temperature glow-discharge nitrided AISI 304L was studied. Prismatic samples (30x17x3 mm) were cut from an annealed bar (diameter: 60 mm) and then they were ground and polished up to 6-pm diamond suspension. Before the nitriding treatment the samples were heated up 380 degrees C by means of a cathodic sputtering performed at 1.3 mbar with 80 vol. % N-2 and 20 vol. % H-2. After this step temperature and pressure were increased up to their nominal value. Nitriding treatments were carried out at temperatures chosen in the range 400 - 500 degrees C, at pressures in the range 2.5 -10 mbar and for times from 1 to 8 h, using a gas mixture of 80 vol. % N-2 and 20 vol. % H-2. The characteristics of the modified surface layers depend on treatment conditions. The surface of the nitrided samples has an etched appearance, delineating the austenitic microstructure with the characteristic twins; moreover, shear lines are observable within the grains and reliefs are present at grain boundaries (Fig. 1). These features are due to both the sputtering and nitriding processes and local plastic deformations caused by the formation of the modified surface layers, and they are more noticeable as the treatment temperature and time are higher, or the pressure is lower. The modified surface layers have a double layer microstructure (Fig. 2), consisting of a thicker outer layer, in which the S phase is detected, and a thinner inner layer, in which a solid solution of interstitial atoms (nitrogen, carbon) in f.c.c. austenite lattice, gamma(N,C), is present. In the outer layer, the presence of further phases together with S phase depends on treatment conditions (Figs. 3, 4; Table 1). Local plastic deformations cause the formation of a solid solution of nitrogen in h.c.p. martensite, epsilon(N).. Precipitation of chromium, CrN (c.f.c.), and iron-based, epsilon-M2-3 N (hex.), gamma'-M4N (c.f.c.) (M = Fe, Cr, Ni, Mn), nitrides tends to increase as the treatment temperature and time are higher, or the pressure is lower. The thickness of the modified layers as a whole tends to increase as the treatment temperature and time increase, due to larger nitrogen diffusion, or the pressure decreases, since at lower pressures higher discharge voltage and mean free path occur, and they cause the increase of ion and fast neutral energy and thus a more efficient nitriding process (Table 1). All nitrided sample types have higher surface microhardness in comparison with the untreated steel, and the hardness values tend to increase as the modified layers are thicker and the amount of nitride precipitates is larger (Table 1). In the modified layers microhardness values are very high, and then they steeply decrease to matrix values (Fig. 5). The thickness of the hardened layers is in accordance with morphology observations. Corrosion behaviour of untreated and nitrided samples, tested in a 5 % NaCl aerated solution using the potentiodynamic method, is typical of passive materials subjected to localized corrosion when potential value is higher than a threshold (E-pit) (Fig. 6). All the nitrided sample types have corrosion potential values higher than that of untreated AISI 304L, but the passive potential range and surface damage depend on the thickness of the modified surface layers and the amount of nitride precipitates, which are influenced by treatment conditions (Fig. 7). Thin modified surface layers, as those obtained when nitriding is performed at 400 degrees C for 5 h or at 430 degrees C for 1 h, do not allow a marked increase of c in comparison with that of the untreated steel. Large nitride amounts, as those observed when the treatment temperature is 450 degrees C or higher, or the pressure is 5 mbar or lower, cause a decrease of corrosion resistance. On the other hand, modified surface layers, which have an adequate thickness and a fairly small amount of nitride precipitates, as those of samples nitrided at 430 degrees C, 10 mbar for 5 and 8 h, allow to significantly increase the corrosion resistance in comparison with that of untreated AISI 304L.
