Improving the empirical model for plasma nitrided AISI 316L corrosion resistance based on Mössbauer spectroscopy

Abstract

Traditional plasma nitriding treatments using temperatures ranging from approximately 650 to 730 K can improve wear, corrosion resistance and surface hardness on stainless steels. The nitrided layer consists of some iron nitrides: the cubic γ phase (Fe 4 N), the hexagonal phase ε (Fe 2−3 N) and a nitrogen supersatured solid phase γ N . An empirical model is proposed to explain the corrosion resistance of AISI 316L and ASTM F138 nitrided samples based on Mössbauer Spectroscopy results: the larger the ratio between ε and γ phase fractions of the sample, the better its resistance corrosion is. In this work, this model is examined using some new results of AISI 316L samples, nitrided under the same previous conditions of gas composition and temperature, but at different pressure, for 3, 4 and 5 h. The sample nitrided for 4 h, whose value for ε/γ is maximum (= 0.73), shows a slightly better response than the other two samples, nitrided for 5 and 3 h (ε/γ = 0.72 and 0.59, respectively). Moreover, these samples show very similar behavior. Therefore, this set of samples was not suitable to test the empirical model. However, the comparison between the present results of potentiodynamic polarization curves and those obtained previously at 4 and 4.5 torr, could indicated that the corrosion resistance of the sample which only presents the γ N phase was the worst of them. Moreover, the empirical model seems not to be ready to explain the response to corrosion and it should be improved including the γ N phase.

Influence of the Gas Pressure of Plasma Nitriding on the Structural, Mechanical and Tribological Surface Properties of AISI 316L

“…Our main objective is to find the best combination of the plasma nitriding parameters to optimize the properties of the nitrided layer. Our previous studies, using 80 % H 2 -20 % N 2 gas, investigated the influence of temperature and time, indicating the best combination: 4h, 400 ºC [13][14][15] . This research has shown that nitrided layer properties depend much more on pressure than on temperature or time.…”

Study of expanded austenite formed in plasma nitrided AISI 316L samples, using synchrotron radiation diffraction

“…Regarding the samples nitrided at 400 °C, they show magnetism and paramagnetism simultaneously 9 , and are magnetic near the surface and paramagnetic in the inner region, which can be observed in Figure 5. This figure indicates that the behavior of these samples is not uniform as is for the samples nitrided at 350 and 450 °C.…”

“…If the temperature is lower than 450 °C, an expanded austenite phase (g N ), which presents a large nitrogen concentration, is formed. This layer shows higher hardness and higher pitting corrosion resistance in comparison with the untreated austenite (g) [6][7][8][9] . Compared to g reflections, g N peaks are broader and shifted to lower diffraction angles.…”

Results of approbation of the innovative method of ion nitriding for steels with low temperatures of tempering

“…The ionic nitriding (in comparison with the furnace nitriding) also allows improving relative wear resistance of the nitrided layer [19]; it enlarges the possibilities to regulate phase composition [20,21] (by changing energy characteristics of the process or changing the ratio of working gases in the mixture (nitrogen and argon) [22]); as well as it makes it possible to eliminate harmful effect of ammonia [23]. In the process of saturation, under the action of ionic bombardment, excess concentration of tructural defects occurs in the material [24].…”