Phase formation in selected surface-roughened plasma-nitrided 304 austenite stainless steel (original) (raw)
Related papers
Surface and Coatings Technology, 2006
Experiments were performed with an aim of studying an effect of initial surface roughness with different gas compositions in plasma nitriding, using pulse D.C. glow discharge plasma in presence of nitrogen and hydrogen gas mixtures. Samples were prepared with different mechanical treatments: polishing, rough polishing, machining and grinding. Plasma nitriding was carried out on AISI 304 stainless steel at 560°C under 4mbar pressures for 24 h in presence of N 2 : H 2 in 20 : 80 and 80 : 20 ratios. After plasma nitriding, surface roughness, micro hardness, case depth and phase formation were evaluated by using stylus profilometer, Vickers micro hardness tester, optical microscope and X-ray diffraction techniques, respectively. After plasma nitriding, hardness and case depth variation are observed with variation in surface roughness as well as gas compositions. Maximum hardness i.e. 1325 HV and case depth i.e. 110 μm are achieved on mirror polished samples at 80N 2 : 20H 2 . The diffraction patterns show the most dominant phase formation of CrN, Fe 4 N and Fe 3 N which is responsible for this increase.
On the materials properties of thin film plasma-nitrided austenitic stainless steel
Surface and Coatings Technology, 2006
A series of experiments were performed to study the composition and mechanical properties of the surface layers formed on the austenitic stainless steel after plasma nitriding in the temperature range of 400-500 8C with different N 2 -H 2 gas mixtures. The thin layer at the nitrided surface was examined by the glancing-angle XRD and the differential load penetration from the microhardness measurements. The formation of the expanded austenite phase was detected at temperatures below 450 8C and/or 10% N 2 in the treatment gas. The distortion of equivalent lattice constant after plasma nitriding was as high as 10%. The modulus of elasticity in the nitrided surface was increased by 33% after the plasma nitriding. The coating-only hardness was measured and it was equivalent to the hardness of coating containing CrN. Thus, it is possible to obtain thin coatings with superior resistance to corrosion and high hardness on the austenitic stainless steel. D
Effect of Cooling Rate on The Plasma Nitriding Process of 304 Austenitic Stainless Steel
American Research Journal of Physics, 2018
The aim of this work is to study the influence of the cooling rate on the properties of the modified surface layer of AISI 304 steel after rf plasma nitriding. The nitrided samples were characterized by glow discharge optical spectroscopy, x-ray diffraction, optical microscopy, scanning electron microscopy and Vickers microhardness measurements. The results revealed that microstructure, nitriding rate and surface microhardness values were found to be cooling rate dependent. The treated layer is mainly composed of nitrogen expanded austenite (γ N), iron nitride (γ'-Fe 4 N) and chromium nitride (CrN). A maximum thickness of treated layer (19.9 µm) is achieved for sample treated at medium cooling rate of 900 Cm 3 /min. It has a maximum surface hardness and nitriding rate of 1402 HV0.1 and 0.66 μm 2 /s, respectively.
Advances in Materials Physics and Chemistry
The purpose of this study is to improve the surface properties of austenitic stainless steel using the double-folded electrode screen plasma nitriding (SPN) process. In general, the S-phase is well-known for its excellent properties such as improved hardness and wear resistance along with sustained corrosion resistance. The concentrated nitrogen via SPN process was injected to form S-phase with time at 713 K. This study was carried out under the conditions of 44 at% of nitrogen injection, which was higher than 25 at% known as the condition of no precipitation of S-phase formed by the SPN process, and 20 K higher than the maximum temperature without precipitation phase. The hardness analysis of stainless steel sample treated by the SPN process at 713 K showed a much higher value than the typical nitriding hardness at a depth of lower nitrogen than the maximum nitrogen concentration. The SPN 20 hr treated specimen showed the average value of 2339 HV while 40 hr showed the average value of 2215 HV. The result is attributed to the concentrated nitrogen formed in the SPN process reacting with the alloying elements contained in the base material to form fine precipitates, thus producing a synergy effect of the extreme hardening effect; that is, the movement of precipitates and dislocations due to the GP-zone (Guinier-Preston zone).
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 Plasma Nitriding of AISI 304 Stainless Steel
Plasma Science & Technology, 2007
Glow discharge plasma nitriding of AISI 304 austenitic stainless steel has been carried out for different processing time under optimum discharge conditions established by spectroscopic analysis. The treated samples were analysed by X-ray diffraction (XRD) to explore the changes induced in the crystallographic structure. The XRD pattern confirmed the formation of an expanded austenite phase (γN) owing to incorporation of nitrogen as an interstitial solid solution in the iron lattice. A Vickers microhardness tester was used to evaluate the surface hardness as a function of indentation depth (μm). The results showed clear evidence of surface changes with substantial increase in surface hardness.
Surface and Coatings Technology, 2013
Austenitic stainless steel is equipped with an excellent corrosion resistance, whereby plasma nitriding of austenitic stainless steel principally results in a high hardness associated with an excellent wear resistance. Whenever a high wear resistance accompanied by the existing corrosion resistance of austenitic stainless steel is required, the application of plasma nitrided stainless steel is restricted. The research goal is maintenance of the wear resistance with the least possible loss of corrosion resistance. In the present study AISI 304 austenitic stainless steel was plasma nitrided in a range of temperature from 325°C to 550°C. Examinations revealed that in addition to treatment temperature, time, pulse ratio and the ratio of gas mixture affected the corrosion resistance of plasma nitrided austenitic steel. Results of microhardness tests and potentiodynamic polarization tests showed that an enhancement of corrosion resistance in addition to preservation of hardness is possible. A reduction of plasma power as well as the addition of argon to the process gas induced an increase of the corrosions resistance with just a slight drop in hardness. Characterization of the chemical composition was determined by glow discharge optical emission spectroscopy (GDOS). Phases of expanded austenite were detected by XRD analysis.
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.
Characteristics of austenitic stainless steel nitrided in a hybrid glow discharge plasma
Brazilian Journal of Physics, 2009
A nitriding process based on two distinct nitrogen glow discharge modes, with sample temperatures ranging from 380 0 C to 480 0 C, was employed to treat the surface of austenitic stainless steel (SS 304). The temperature is controlled exclusively by switching the operation conditions of the discharges. First mode of operation is the conventional one, named cathodic, which runs at higher pressure values (1 mbar) in comparison to the second mode, named anodic, which runs at the pressure range of 10 −3 − 10 −2 mbar. Cathodic mode is used to quickly heat the sample holder, by the high ion flux. On the other hand, in the anodic mode, due to the lower operation pressure, higher effective ion acceleration takes place, which allows deeper ion implantation into the sample surface. This hybrid process was thoroughly explored regarding the duty cycle and conditions of operation, to achieve optimal performance of the treatments, which led to the attainment of surface hardness for samples of AISI SS 304 as high as 20 GPa and improvements including higher elastic modulus and resistance against corrosion. Detailed comparison among samples treated by this process with others treated by conventional method was done using nanoindentation, Auger Electron Spectroscopy (AES) and corrosion resistance testing.
Study of the S phase formed on plasma-nitrided AISI 316L stainless steel
Materials Science and Engineering: A, 2006
Some tribological and corrosion resistance properties of austenitic stainless steels are enhanced by the formation of the S phase, also called expanded austenite. This phase is formed on the surfaces of austenitic stainless steels nitrided under certain conditions. In this work, AISI 316L steel was plasma-nitrided at 350, 400, 450, and 500 • C, and the samples were characterized by X-ray diffraction (XRD), conversion electron Mössbauer spectroscopy (CEMS), and wavelength dispersive spectroscopy (WDS) in order to investigate the S phase. XRD analysis identified the presence of a distorted cubic structure phase. The modified layer consists of an austenitic phase with different content of nitrogen, ranging from approximately 10 to 40 at%, and also ␥-Fe 4 N and-Fe 2-3 N phases. A diminution in the S phase occurs with the increase in nitriding temperature, and this decrease is related to the transformation of the S phase to the ␥-Fe 4 N phase.