Structural Characterization of Ion Nitrided 316L Austenitic Stainless Steel: Influence of Treatment Temperature and Time

Abstract

The ion nitriding behavior of AISI 316L austenite stainless steel was investigated at different nitriding times (2 h, 4 h, and 9 h) and temperatures (450 °C, 500 °C, and 550 °C). The structural characterization has been assessed by several considerations which can be listed: (i) the evaluation of phase distribution through Rietveld analysis of X-ray diffraction patterns and accompanying peak fitting process, (ii) hardness profile and related nitride layer thickness by microhardness and microscopic measurements, and (iii) displacement measurements to assess the residual stress accumulation. The diffusion of nitrogen atomic species into the sample surface caused a transformation of the γ phase matrix into an expanded austenite (γN) phase, which is recognized with its high hardness and wear resistance. Furthermore, depending on the nitriding condition, chromium nitride (Cr1-2N) and iron nitride (ε-Fe2-3N and γ′-Fe4N) phases were detected, which can be detrimental to the corrosion resistance of the 316L austenite stainless steel. The γN phase was observed in all nitriding conditions, resulting in a significant increase in the surface hardness. However, decomposition of the γN phase with an increase in nitriding temperature eventually altered the surface hardness distribution in the nitriding layer. Considering the phase-type and distribution with the consequent hardness characteristics in the nitride layer, to our best knowledge, this is the first report in which an ion-nitriding temperature of 500 °C (higher than 450 °C) and time of 9 h can be proposed as ideal processing parameters leading to optimal phase composition and hardness distribution for 316L austenite stainless steels particularly for the applications requiring a combination of both wear and corrosion resistance.

Corrosion and Micro-Abrasion Properties of an AISI 316L Austenitic Stainless Steel after Low-Temperature Plasma Nitriding

“…On samples nitrided at 470-500 • C, in addition to CrN nitride, γ'-Fe4N nitride also becomes evident. The thickness and hardness of the nitrided layer range from 4 µm/373 HK0.1 after nitriding at 400 • C to 47 µm/1605 HK0.1 after processing at 500 • C. In reference [17], AISI 316L steel was nitrided with the following parameters: a gas flow ratio H 2 /N 2 = 80%:20%, a vacuum pressure of 1000 Pa, nitriding temperatures ranging from 450 to 550 • C, and nitriding times of 2, 4, and 9 h. The presence of the S-phase, along with precipitates of iron (Fe 4 N) and chromium (CrN, Cr 2 N) nitrides, was observed on samples nitrided at 450 • C for 2 h, resulting in a layer with a thickness of about 15 µm. After nitriding at temperatures higher than 450 • C for 9 h, a thin film composed of γ'-Fe 4 N and ε-Fe 2-3 N nitrides forms on the surface above the S-phase layer.…”

“…The hardness of nitride phases and CrN precipitates can reach around 2000 HV. While a nitriding temperature of around 450 • C is recommended to achieve the highest hardness, good wear resistance, and maintain good corrosion resistance, the appearance of ε-Fe 2-3 N nitride precipitates in the S-phase and the formation of a surface film of hard nitrides after nitriding at 550 • C can lead to embrittlement, cracks, and delamination of the surface layer [17]. Summarizing all the above research results for low-temperature plasma nitriding of AISI 316L AS steel, the optimal process temperature is between 420 and 450 • C can be recommended for producing a nitride layer with the maximum thickness composed from the S-phase without significant precipitation of iron or chromium nitrides along the grain boundaries.…”

“…In PN process with the parameters H 2 /N 2 = 4:1, a vacuum of 650 Pa, a nitriding temperature of 400 • C, and time of 20 h, the nitrided layer with S-phase was 15 µm thick [14]. For the PN process carried out with the same parameters of gas flow and temperature of 450 • C in a shorter time of 4 h, with a vacuum of 1000 Pa, a thickness of the nitrided layer of 45 µm was achieved [17].…”

“…Nitriding at temperatures between 430 and 450 • C proved to be optimal for increasing resistance to pitting and crevice corrosion. Plasma nitriding at temperatures above 470-500 • C reduces the resistance to intergranular corrosion and localized corrosion on the surface in comparison with untreated AS steel or steel nitrided at lower-temperature PN process [16,17,29].…”

“…Additional improvement in the wear resistance of AS steel can be achieved by forming an S-phase layer covered by a thin film of Fe 2-4 N, Cr 1-2 N, and (Fe,Cr) 4 N nitrides, with a surface hardness of 800-1200 HV [5,17,29]. In reference [30], a long-duration micro-abrasion test involved a fixed rotating steel ball pressed against a test sample without abrasive slurry to assess the wear resistance of a thin nitrided and nitrocarburized layer on AISI 316L AS steel after treatment at 450 • C for 5 h and 10 h. The test was performed using a 25.4 mm ball diameter at a rotational speed of 500 rpm under dry sliding conditions, with a normal load of 2.45 N. The test durations ranged from 5 to 20 min.…”