Investigating the correlation between some of the properties of plasma nitrided AISI 316L stainless steel

Abstract

When AISI 316L stainless steels are submitted to the nitriding process at temperatures lower than 450 °C, a high nitrogen content expanded austenite phase is formed, which shows higher hardness and higher pitting corrosion resistance compared to the untreated material. As a result, this material becomes adequate for biomedical application. The conditions of the nitriding technique, such as gas mixture, pressure, time and temperature, play an important role in some properties of the modified layer, including: thickness, hardness and N concentration along the layer. This paper explores a set of six samples of AISI 316L, nitrided at different times and temperatures, whose properties show important differences. The aim of this research is to investigate the correlation between the nitrided layer thickness (in the range of 0.77 to 11 µm) with both X-ray patterns characteristics and hardness measurements, which used two distinct loads. The results of this study show that: whereas the 3.6 gf load was suitable to measure the real hardness for four of the nitrided layers showing thickness ≥ 2.9 µm, the 50 gf load measured a substrate contribution, probably even for the highest thickness, 11 µm. Moreover, analyzing different reflections of the X-ray patterns showed evidence of the clear consistency between the X-Ray depths and the nitrided layer thicknesses: if the layer thickness is lower than the penetration depth of X-rays, two phases (austenite and expanded substrate) are present. If the layer thickness is higher, only the austenite is observed. Finally, concerning the citotoxicity property, all the samples, nitrided or not, were approved in the test for biocompatibility, indicating their potential use for biomedical applications.

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

“…According to the authors, this process is similar to plasma nitriding; regarding the current results, certainly 7 Torr produces a higher current density than P≤ 6 Torr. Moreover, the present result for the P6 central region hardness (H= 167 (±12) x10 HV 0.036 ) is consistent with the previous value 23 measured for the samples nitrided at 6 Torr, 400 ºC, 3 and 5h (H=172 (±17) x10 HV 0.036 ). Finally, Fig 3(b) shows that not only for P3 and P4, but also for P5 in the intermediate region there is some substrate contribution to the hardness, because their thicknesses are not thick enough to measure the real hardness 23 .…”

Nitrogen Surface Enrichment of Austenitic Stainless Steel ISO 5832-1: SHTPN vs Low-temperature Plasma Nitriding

“…This fact, allied to low hardness and tribological properties, limits their applicability. Several techniques are being used to improve the surface characteristics of these steels, like plasma nitriding at low temperatures [1][2][3][4] , ion implantation 5 , and high temperature gas nitriding (HTGN) 6 . More recently, the so-called "Solution Heat Treatment after Plasma Nitriding" (SHTPN) process was proposed as a new method for surface enrichment of nitrogen in solid solution, with good results regarding improvement of corrosion resistance of stainless steels 7,8 .…”

“…Hence, the use of plasma nitriding is limited to the low temperature range, where only the formation of the S-phase (also called expanded austenite -γ N ) occurs 1,12,13 . The S-phase formation on austenitic stainless steels by advanced nitriding treatments has been the subject of several studies [1][2][3][4][5][6][7][8][9][10][11][12][13] . Theo focus of this work is not the study of the S-phase itself, rather the SHTPN process as an alternative to low temperature plasma nitriding.…”

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

“…Details of the preparation of the samples, which were plasma nitrided using 80% H 2 -20% N 2 , at 6 Torr, for 3 and 5 h using different temperatures (350, 400 and 450 °C), have been given previously 17 , and the nitrided layer thickness (t) values are presented in Table 1. Here results for samples nitrided for 4h, using 400 and 450 °C are presented as well.…”