Alternated high- and low-pressure nitriding of austenitic stainless steel: Mechanisms and results (original) (raw)
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Performance enhancement by plasma nitriding at low gas pressure for 304 austenitic stainless steel
Vacuum, 2017
Plasma nitriding was conducted at low gas pressure and low temperature of 400 ℃ for 304 austenitic stainless steel. The combined performance of the treated specimens was evaluated by scanning electronic microscopy (SEM), X-ray diffractometer (XRD), microhardness tester, ball-on-disc tribometer and electrochemical polarization. The results showed that an expanded austenite (γ N), also called S phase layer was formed after plasma nitriding at low gas pressure and low temperature of 400 ℃, and the nitriding efficiency was significantly improved at lower gas pressure; maximum expanded austenite layer of 51.7 µm and effective hardening layer of 72 µm were obtained at low gas pressure of 100 Pa. Surface hardness and wear resistance were enhanced dramatically by plasma nitriding at 100 Pa, and the weight loss after wear test decreased from 0.102 g to the minimum of 0.013 g. Meanwhile, the corrosion resistance was improved after plasma nitriding at 100 Pa, the minimum corrosion current of 0.009 µA•cm-2 and the maximum corrosion potential of-361.9mV are obtained.
Glow-discharge nitriding of AISI 316L austenitic stainless steel: influence of treatment temperature
Nitriding treatments of austenitic stainless steels can be performed only at relatively low temperatures in order to avoid a decrease of corrosion resistance due to chromium nitride formation. These conditions promote the formation of the so-called S phase, which shows high hardness and good corrosion resistance. In the present paper, the influence of the treatment temperature of glow-discharge nitriding process on the microstructural and mechanical characteristics of AISI 316L steel samples was evaluated. Glow-discharge nitriding treatments were performed at temperatures in the range 673-773 K for 5 h at 10 3 Pa. The modified surface layer of the nitrided samples consists mainly of the S phase and, according to metallographic technique analysis, it seems to be essentially a modification of the austenite matrix. All the nitrided sample types show a peculiar surface morphology due to both plasma etching during nitriding and the presence of slip steps and relieves at grain boundaries, the latter features presumably due to the formation of the nitrided layer. X-ray diffraction analysis shows that for the samples nitrided at temperatures up to 723 K, besides the S phase, small chromium nitride precipitates are present at the surface, while using higher treatment temperatures both chromium (CrN) and iron (g'-Fe 4 N) nitrides precipitate along the grain boundaries and in the middle of the grains, and their amount increases as treatment temperature increases. High hardness values (from~1450 to~1550 HK 0.01 , depending on nitriding conditions) are observed in the modified layer with a steep decrease to matrix values. Preliminary corrosion resistance tests, carried out in 5% NaCl aerated solution with the potentiodynamic method, show that with the used treatment parameters a substantial improvement of corrosion resistance can be achieved when glow-discharge nitriding treatments are performed at temperatures in the range 703-723 K. D
A series of experiments have been conducted on AISI 304 stainless steel using a hollow cathode discharge assisted plasma nitriding apparatus. Specimens were nitrided at high temperatures (520-560°C) in order to produce nitrogen expanded austenite phase within a short time. The nitrided specimen was characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, potentiodynamic polarization and microhardness tester. The corrosion properties of nitrided samples were evaluated using anodic polarization tests in 3.5% NaCl solution. The nitrided layer was shown to consist of nitrogen expanded austenite and possibly a small amount of CrN precipitates and iron nitrides. The results indicated that rapid nitriding assisted hollow cathode discharge not only increased the surface hardness but also improved the corrosion resistance of the untreated substrate.
Low temperature nitriding, nitrocarburising and carburising of AISI 316L austenitic stainless steel
International Heat Treatment and Surface Engineering, 2011
AISI 316L grade ASTM F138 austenitic stainless steel specimens were low temperature plasma nitrided (LTPN), nitrocarburised (LTPNC) and carburised (LTPC) using different gas mixtures. Different expanded austenite layers formed after each thermochemical treatment. LTPN and LTPCN led to formation of nitrogen supersaturated expanded austenite (c N). After LTPN, a second carbon expanded austenite (c C) layer was formed beneath the nitrogen expanded austenite layer (c N). LTPC led to formation of a carbon supersaturated expanded austenite (c C). Scanning electron microscopy, XRD and microhardness were used to characterise the expanded austenite layers formed on the surface of the specimens. Different mechanisms of formation and growth of the layers are pointed out. XRD results show that the lattice parameter of nitrogen expanded austenite (c N) is higher than that calculated for carbon expanded austenite c C. As a consequence, the lattice expansion Da/a for the nitrogen rich (c N) phase is higher than the one observed for the (c C) phase and the nitrogen rich expanded austenite layer displays higher hardness than the carbon rich expanded austenite layer. The LTPNC bilayer displays a less steep hardness gradient, indicating that the carbon rich expanded austenite layer can grant mechanical support to the harder nitrogen rich layer.
Journal of ASTM International, 2012
High chromium content is responsible for the formation of a protective passive surface layer on austenitic stainless steels (ASS). Due to their larger amounts of chromium, superaustenitic stainless steels (SASS) can be chosen for applications with higher corrosion resistance requirements. However, both of them present low hardness and wear resistance that has limited their use for mechanical parts fabrication. Plasma nitriding is a very effective surface treatment for producing harder and wear resistant surface layers on these steel grades, without harming their corrosion resistance if low processing temperatures are employed. In this work UNS S31600 and UNS S31254 SASS samples were plasma nitrided in temperatures from 400 C to 500 C for 5 h with 80 % H 2 -20 % N 2 atmosphere at 600 Pa. Nitrided layers were analyzed by optical (OM) and transmission electron microscopy (TEM), x-ray diffraction (XRD), and Vickers microhardness testing. Observations made by optical microscopy showed that N-rich layers were uniform but their thicknesses increased with higher nitriding temperatures. XRD analyses showed that lower temperature layers are mainly composed by expanded austenite, a metastable nitrogen supersaturated phase with excellent corrosion and tribological properties. Samples nitrided at 400 C produced a 5 lm thick expanded austenite layer. The nitrided layer reached 25 lm in specimens treated at 500 C. There are indications that other phases are formed during higher temperature nitriding but XRD analysis was not able to determine that phases are iron and/or chromium nitrides, which are responsible for increasing hardness from 850 up to 1100 HV. In fact, observations made by TEM have indicated that formation of fine nitrides, virtually not identified by XRD technique, can begin at lower temperatures and their growth is affected by both thermodynamical and kinetics reasons.
Vacuum, 2006
Attempts have been made in the present work to investigate the influence of carbon in the treatment atmosphere on the lowtemperature plasma nitriding characteristics of austenitic stainless steels. It was found that treatment gas composition has a significant effect on the structural characteristics of the nitrided layer. The addition of a small amount of carbon-containing gas such as methane (CH 4) to the treatment atmosphere can alter the structural development in the alloyed zone and offer several beneficial effects to the nitriding process. Based on this discovery, a new process has been developed, which involves the simultaneous incorporation of both nitrogen and carbon into the alloyed zone to form a dual-layer structure, which is free from chromium nitride, and carbide precipitates. Such a hybrid structure not only possesses larger layer thickness, high hardness, and more favourable hardness gradient than nitridedalone layers, but also exhibits much improved corrosion resistance.
Nitriding using cathodic cage technique of austenitic stainless steel AISI 316 with addition of CH4
Materials Science and Engineering: A, 2008
Samples of austenitic stainless steel AISI 316 were nitrided using the cathodic cage technique with the addition of methane in the nitriding atmosphere. The aim was to study the influence of this technique in reducing the precipitation of chromium nitrite and in improving the wear resistance. The results show that there was a significant improvement in such properties when compared to the results of ionic plasma nitriding. Formation of a double layer, one more internal composed of carbon and another with high nitrogen content, was confirmed by Scanning Electron Microscopy (SEM). The microhardness profile of the layer showed an increase in hardness values and a larger uniformity, while X-ray analysis showed less chromium nitriding precipitation when compared with results obtained for samples treated using ionic plasma nitriding.
Surface modification of austenitic steel by various glow-discharge nitriding methods
Materials Science, 2015
The article presents a characterisation of nitrided layers produced on austenitic X2CrNiMo17-12-2 (AISI 316L) stainless steel in the course of glow-discharge nitriding at cathodic potential, at plasma potential, and at cathodic potential incorporating an active screen. All processes were carried out at 440 °C under DC glow-discharge conditions and in 100 kHz frequency pulsed current. The layers were examined in terms of their microstructure, phase and chemical composition, morphology, surface roughness, hardness, wear, and corrosion resistance. Studies have shown a strong influence of the type of nitriding method used and of the electrical conditions on the microstructure and properties of the diffusion layers formed.
Metals
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 resis...