Plasma Nitrided Austenitic Stainless Steel Research Articles

Mechanical Preparation Procedure for Characterizing by EBSD the Crystallographic Orientation in Surface Layers of Austenitic Stainless Steels Plasma Nitrided

Abstract This contribution reports on an experimental polishing procedure, that is comprised of early grinding in Al2O3 slurries and late polishing in colloidal silica, which is used for preparing the nitrided region of a plasma nitrided austenitic stainless steel, for crystallographic analysis via electron backscatter diffraction (EBSD). The suitability of the polished surfaces for conducting EBSD characterization was assessed through an analysis of both the surface roughness (appraised by atomic force microscopy) and the quality of the Kikuchi diffraction patterns. We observed that as-nitrided virgin surfaces were not suitable for EBSD characterization, due to intense surface roughening, which was induced by the nitriding process itself. At the subsurface region, exposed by on-top mechanical polishing, the flatter nature of the polished surfaces allowed the acquisition of EBSD patterns with enough quality for microtexture analysis. A resolution of 100 nm in the total removed layer was attainable via careful control of the polishing parameters. Close parallelism between the polished and original surfaces was verified.

Effect of Cr/CrNx transition layer on mechanical properties of CrN coatings deposited on plasma nitrided austenitic stainless steel

In this work, two types of transition layers (pure Cr and gradient CrNx) were fabricated between plasma-assisted nitrided (PAN) stainless steel and magnetron sputtering CrN coating to investigate the effect of transition layer on mechanical properties of CrN coating on PAN stainless steel substrates. For comparison, samples without transition layer were also synthesized. The phase structure, morphology, abrasive resistance and adhesion of the CrN coatings, as well as the nano-hardness and Young's modulus were investigated by XRD, SEM, ball-on-disk tribometer, scratch tester and nano-indentation tester, respectively. The results showed the transition layers of pure Cr and gradient CrNx absorbed the excess nitrogen diffusing outwards from the nitrided layer, and thus suppressed partly the precipitation. As a result, the adhesion force was improved to 22.5 and 25.7 N, respectively. The nano-hardness and Young's modulus of modified layers were improved to the range of 22.8–25.8 GPa and 331–347.3 GPa, corresponding to the substrate of 7.2 GPa and 230.5 GPa. The modified layer with no transition layer had the best wear resistance for nitriding of 1 h; while the layer with gradient CrNx had the best wear resistance for nitriding of 4 h.

Effect of Gas Content and Treatment Temperature on the Characteristic of Surface Layers of Low Temperature Plasma Nitrided 316L Austenitic Stainless Steel

The effect of gas content and treatment temperature on the surface characteristics of hardened layers of low temperature plasma nitrided 316L Austenitic Stainless Steel was investigated. The process was performed at fixed processing time at 15Hr and changing the N2 content from 10% to 25%, changing the treatment temperature from 370 °C to 430 °C. The surface hardness and the thickness of expanded austenite (S-Phase) layer increase with increasing both temperature and nitrogen content. At 430°C processing temperature and 25% N2 content, the S-phase layer thickness increased up to 13 μm and surface hardness increased about 3.5 times of that of untreated sample which is 880 HV0.1 (before treatment 250HV0.1 hardness). However, the XRD pattern of the this sample shows the presence of γ'-(Fe,Cr)4N precipitation on the surface which is the reason for the increased corrosion current density that results in increased corrosion rate. Thus the corrosion resistance degrades and presents even worst behavior comparing that of before treatment. Therefore, for increasing the corrosion behavior, further research was conducted by fixing the N2 content at 10% with changing the CH4 content from 0% to 5%. The best results were found when treated at 430°C with 5% CH4 content. At this condition the S-phase thickness increase to around 17μm and surface hardness (980 HV0.1) is improved about 3.9 times of the hardness of untreated one. This sample also showed superior corrosion resistance than the other treated samples and the untreated one due to much higher pitting potential.

Tribological properties and wear mechanisms of DC pulse plasma nitrided austenitic stainless steel in dry reciprocating sliding tests

Expanded austenite (γN), or S-phase, is a special phase of low-temperature nitrided austenite containing highly super-saturated nitrogen in the form of heterogeneous Cr-N nano-clusters. A nitrided layer of singe phase γN is known to provide austenitic stainless steel with combined high hardness, good wear resistance and superior corrosion resistance. This paper reports recent experiments on a comparative study of the sliding wear properties and wear mechanisms of nitrided austenite stainless steel AISI 316, with a special attention paid on worn surface structural evolutions induced by frictional heating and sliding deformation. The samples were prepared by DC pulsed plasma nitriding treatments of various time at a fixed power. Knoop micro-indentation has revealed hardening behaviour of the nitrided samples. The reciprocating ball-on-disc sliding wear and friction properties were investigated at ambient environment conditions using an alumina counterpart ball. The worn surfaces have been analysed by XRD, FEG-SEM and EDX to show wear induced changes in the crystalline characteristics and the wear mechanisms of tribo-oxidation, cracking, abrasive wear and ploughing deformation. Moreover, longitudinal cross-sectional foils of the worn samples have been prepared and analysed using TEM, to investigate the wear induced structural changes, including tribofilm formation, plastic deformation and delamination in depths of nano-scale.

Synergetic effect of thicker nanocrystalline structure with high amount of strain induced martensite on surface characteristics of plasma nitrided austenitic stainless steel

Abstract Severe shot peening is a simple and effective method to synthesize nanocrystalline layer on metallic surfaces. A new reactor to synthesize thicker nanocrystalline layers on metallic surfaces by shot peening is presented. In this cylindrical reactor air enters inside the reactor tangentially, propels shots on the specimen to be shot peened and leaves the reactor after multiple rotations. Air vortex and very high turbulence formed inside the reactor dissipate heat generated during shot peening and thus suppress dynamic recovery which promotes formation of refined and thicker nanocrystalline structure on a metallic surface. Experiments were conducted on 5specimens of austenitic stainless steel with different shot velocities in the reactor. Characterization (X-ray diffraction profiles, cross-sectional optical micrographs, TEM and micro-hardness profiles) of shot peened surfaces showed that thicker nanocrystalline/nano-twinned layers were developed during all peening conditions used in the experiments. Microstrain in the lattice, dislocation density and volume fraction of strain induced martensite observed to increase with rise in shot velocity. Specimens with thicker nanocrystalline layers were prepared. These specimens contained high amount of strain induced martensite deep down the surface. Plasma nitriding of these pre-shot peened specimens were done at 425 °C and 500 °C for six hours. Presence of thicker nanocrystalline layer with high amount of martensite at the surface of austenitic stainless steel observed to have synergetic effect and significantly influenced kinetics of nitriding process leading to higher surface hardness and 6-fold thicker nitrided layers compare to untreated nitrided surface. Presence of nanocrystalline structure, however, also effects on precipitation of chromium nitrides during nitriding.

Characterization of thick and soft DLC coatings deposited on plasma nitrided austenitic stainless steel

Thick and soft a-C:H:Si coatings containing more than 45% hydrogen (thickness: 25–27μm, hardness: 6GPa, Young's Modulus 38GPa and low ratio of sp3 bonds) were deposited by PACVD with a DC pulsed discharge on nitrided (duplex sample) and non-nitrided austenitic stainless steel (coated sample). After deposition, the chemical, microstructural and tribological properties were studied. Finally, the adhesion and the atmospheric corrosion resistance of a-C:H:Si coatings were also investigated.In pin-on-disk tests, the friction coefficient using an alumina pin of 6mm in diameter as counterpart, under 0.59GPa Hertzian pressure was 0.05 for the coated samples and 0.076 for the duplex samples. These values were more than one order of magnitude smaller than the friction coefficient of the nitrided sample without coating, which was around 0.65. In the coated samples, the wear loss could not be measured. In ball-on-disk tests under dry sliding conditions, the coatings were tested under different Hertzian pressures (1.29, 1.44 and 1.57GPa) using a steel ball with a diameter of 1.5mm as counterpart. Using a normal load of 9N, the a-C:H:Si coating of the coated samples was broken and detached thus leading to a coefficient of friction of around 0.429. However, in contrast to that, the friction coefficient of the duplex samples remained stable and reached as maximum a value of 0.208.In abrasive tests, mass loss was undetectable in both duplex and coated samples. Furthermore it could be seen that the a-C:H:Si film showed only some smaller grooves and no severe damage or deformation. On the contrary, severe damage was observed in the only nitrided sample. With respect to adhesion, the critical load to break the coating was higher in the duplex sample (27N) than in the only coated sample (16.3N). By chemical analysis using the salt spray fog test, the duplex sample remained clean, but the coated sample failed and presented film delamination as well as general corrosion.

Evaluation of the corrosion resistance of plasma nitrided austenitic stainless steel

Abstract Plasma nitriding at 500°C for 14 h was applied to austenitic 304 stainless steel for surface hardening. The effect of surface treatment on the corrosion resistance of the material was investigated in naturally-aerated 0.5 M NaCl solution for 30 days using linear potentiodynamic polarization and electrochemical impedance spectroscopy methods. Both as-cast and plasma nitrided stainless steel samples underwent spontaneous passivation, though the nitrided sample exhibited more positive zero current potential, higher breakdown potential, and lower anodic current densities than the as-cast material. Impedance spectra were interpreted in terms of a duplex passive film, corrosion resistance mainly arising from a thin inner compact layer, whereas the outer layer was more porous and less sealing. Capacitive behaviour and high corrosion resistance were observed in the low and medium frequency ranges for the nitrided samples.

Surface Characteristics of Silicon Nitride Compounds Deposited on Plasma Nitrided Austenitic Stainless Steels 316L

Silicon nitride compound deposited by plasma enhanced chemical vapor deposition (PECVD) on plasma nitrided stainless steel 316L using tetraethylorthosilicate (TEOS):H2:N2 mixtures. Plasma nitriding is extensively used to improve the hardness and wear performance of steels. It is also known that silicon nitride compounds can have interesting properties, such as low friction coefficient, high hardness and wear resistance. Therefore the deposition processes were carried out at optimized gas compositions and temperatures in order to obtain a combination of the best mechanical and enhanced tribological properties. The composition and structure of the surface layers were characterized using XRD, Glancing angle X-ray diffraction (GAXRD), optical and EDX built in electron microscopy, Microhardness and pin-on-disc wear tests. The compound of α-Si3N4 was found on the top surfaces of PECVD and plasma nitrided austenitic stainless steel 316 L. The nitride compounds were composed of Fe2-3N, Fe4N in addition to chromium nitride. PECVD treatments substantially improved the hardness and wear resistance of plasma nitrided SS 316 L and reduced the friction coefficient and wear rate of the samples. The investigation showed that the combination of PECVD of organosilicon compounds and plasma nitriding results in superior hi gh hardness, low friction and high wear resistance of treated surfaces if compared to those of conventional plasma nitrided surfaces.

Influence of the microstructure on the corrosion resistance of plasma-nitrided austenitic stainless steel 304L and 316L

Austenitic stainless steels are widely used in medical and food industries because of their excellent corrosion resistance. However, they suffer from weak wear resistance due to their low hardness. To improve this, plasma nitriding processes have been successfully applied to austenitic stainless steels, thereby forming a thin and very hard diffusion layer, the so-called S-phase. In the present study, the austenitic stainless steels AISI 304L and AISI 316L with different microstructures and surface modifications were used to examine the influence of the steel microstructure on the plasma nitriding behavior and corrosion properties. In a first step, solution annealed steel plates were cold-rolled with 38% deformation degree. Then, the samples were prepared with three kinds of mechanical surface treatments. The specimens were plasma nitrided for 360 min in a H2–N2 atmosphere at 420 °C. X-ray diffraction measurements confirmed the presence of the S-phase at the sample surface, austenite and body centered cubic (bcc)-iron. The specimens were comprehensively characterized by means of optical microscopy, scanning electron microscopy, glow discharge optical emission spectroscopy, X-ray diffraction, surface roughness and nano-indentation measurements to provide the formulation of dependencies between microstructure and nitriding behavior. The corrosion behavior was examined by potentio-dynamic polarization measurements in 0.05 M and 0.5 M sulfuric acid and by salt spray testing.

Multi‐technique characterization of low‐temperature plasma nitrided austenitic AISI 304L and AISI 904L stainless steel

The current study focuses on the characterization of the nitrided layer, formed in American Iron and Steel Institute (AISI) 304L and AISI 904L austenitic stainless steels by industrial low‐temperature plasma nitriding, using combined analysis techniques. The study highlighted that the evolution of the microstructure of the nitrided layers is influenced by surface finishing prior to nitriding, alloying elements and nitriding conditions, all factors affecting S‐phase formation and nitrogen (N) diffusion mechanisms. The chemical bonding characteristics of Cr 2p3/2 and N 1s as revealed by XPS show a shift in binding energy between expanded austenite (S‐phase) and CrN‐like compounds. S‐phase has proven to be more stable in 904L, whereas residual and/or induced ferrite/martensite in 304L acts as a barrier for its development. © 2014 The Authors. Surface and Interface Analysis published by John Wiley & Sons, Ltd.

Mathematical Modelling of Nitride Layer Growth of Low Temperature Gas and Plasma Nitriding of AISI 316L

This paper present mathematical model which developed to predict the nitrided layer thickness (case depth) of gas nitrided and plasma nitrided austenitic stainless steel according to Fick's first law for pure iron by adapting and manipulating the Hosseini's model to fit the diffusion mechanism where nitrided structure formed by nitrided AISI 316L austenitic stainless steel. The mathematical model later tested against various actual gas nitriding and plasma nitriding experimental results with varying nitriding temperature and nitriding duration to see whether the model managed to successfully predict the nitrided layer thickness. This model predicted the coexistence of e-Fe2-3N and γ΄-Fe4N under the present nitriding process parameters. After the validation process, it is proven that the mathematical model managed to predict the nitrided layer growth of the gas nitrided and plasma nitrided of AISI 316L SS up to high degree of accuracy.

Swelling effect on stress induced and concentration dependent diffusion of nitrogen in plasma nitrided austenitic stainless steel

The kinetics of plasma nitriding of austenitic stainless steel (ASS) below temperatures of nitrides formation and mechanisms of nitrogen penetration are analyzed by proposed kinetic modeling. By fitting of experimental nitrogen depth profiles it is shown that the main processes are nitrogen concentration gradient and internal stresses induced diffusion with concentration dependant diffusion coefficient and also swelling. Internal stress induced diffusion is responsible for plateau formation in nitrogen depth profile which is characteristic for nitriding process. In order to fit experimental curves of different samples (with different nitriding duration time) with fixed diffusion coefficient it is necessary to include concentration dependant diffusion and swelling process. It is shown that diffusion coefficient depends on concentration according to Einstein–Smoluchowski relation. The swelling thickness is the square root function of nitriding time.

Microstructure and dry sliding wear resistance evaluation of plasma nitrided austenitic stainless steel type AISI 316LN against different sliders

In this work, Plasma Nitriding was carried out at a temperature of 570°C on nuclear grade austenitic stainless steel type AISI 316 LN (316LN SS) in a gas mixture of 20% N2–80% H2 to improve the surface hardness and thereby sliding wear resistance. The Plasma Nitride (PN) treated surface has been characterized by Vickers microhardness measurements, Scanning Electron Microscopic (SEM) examination, X-ray Diffraction (XRD) and sliding wear assessment. The average thickness of the PN layer was found to be 70μm. Microhardness measurements showed a significant increase in the hardness from 210 HV25g (unnitrided sample) to 1040 HV25g (Plasma Nitrided sample). The XRD reveals that PN layer consists of CrN, Fe4N and Fe3N phases along with austenite phase. The tribological parameters such as the friction coefficient and wear mechanism have been evaluated at ambient conditions for PN treated ring (PN ring) vs. ASTM A453 grade 660 pin (ASTM pin), PN ring vs. Nickel based alloy hard faced pin (Colmonoy pin), PN ring vs. 316LN SS pin and 316LN SS ring vs. 316LN SS pin. The wear tracks have been analyzed by SEM, Energy Dispersive X-ray Analysis (EDX) and Optical Profilometry. The untreated 316LN SS ring vs. 316LN SS pin produced severe wear and was characterized by a combination of delamination and adhesion wear mechanism, whereas wear mechanism of the PN rings reveals mild abrasion and a transfer layer from pin materials.

Hemocompatibility evaluation of plasma-nitrided austenitic stainless steels at low temperature

In this study, plasma-nitrided austenitic stainless steel was prepared using a microwave system as a function of the nitriding duration. The nitride layers were characterized via scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy, atomic force microscopy and a Vickers micro-hardness tester. The effects of plasma nitriding on the compatibility of blood with the austenitic stainless steel were also investigated. Based on the XRD and TEM observations, it was inferred that only a single γN phase was detected in the whole process of experiments. The γN layer, which was formed on austenitic stainless steel, exhibited lower platelet adhesion and activation than the untreated specimens. The clotting time of the PN specimens was prolonged and increased with the treatment time. This work suggested that the blood-compatible manner of plasma nitriding by controlling surface characteristic on austenitic stainless steel improved the anticoagulant properties, and made austenitic stainless steel suitable candidates in the field of surgical and medical instruments.

Stress induced and concentration dependent diffusion of nitrogen in plasma nitrided austenitic stainless steel

The nitrogen transport mechanism in austenitic stainless steel during plasma nitriding at moderate temperatures (around 400 °C) is considered by stress induced diffusion model. The model involves diffusion of nitrogen in presence of internal stresses gradient induced by penetrating nitrogen as the next driving force of diffusion after concentration gradient. Furthermore, in the present work it was found that nitrogen diffusion coefficient vary with nitrogen concentration according to well-known Einstein–Smoluchowski relation D(CN) = f(1/CN). Nitrogen depth profiles in nitided AISI 316L steel at T = 400 °C for 1, 3 and 8 h calculated on the basis of this model are in good agreement with experimental nitrogen profiles. The dependencies of nitrogen flux and nitriding time on nitrogen concentration, nitrogen surface concentration and penetration depth are analyzed by proposed model. It is shown that, with the increase of nitriding time the compositionally-induced stresses and thickness of stressed steel layer increases.

Modeling of stress induced nitrogen diffusion in nitrided stainless steel

The nitrogen transport mechanism in plasma nitrided austenitic stainless steel at moderate temperatures is explained by influence of internal stresses gradient. The model considers the diffusion of nitrogen in the presence of internal stresses gradient induced by penetrating nitrogen as the next driving force of diffusion after concentration gradient. For mathematical description of stress induced diffusion process the equation of barodiffusion is used which involves concentration dependant barodiffusion coefficient. For calculation of stress gradient it is assumed that stress depth profile linearly relates with nitrogen concentration depth profile. Calculated nitrogen depth profiles in an austenitic stainless steel are in good agreement with experimental nitrogen profiles. The diffusion coefficient D=1.68·10−12cm2/s for nitrogen in plasma source ion nitrided 1Cr18Ni9Ti (18-8 type) austenitic stainless steel at 380°C was found from fitting of experimental data. It is shown that nitrogen penetration depth and nitrogen surface concentration increases with nitriding time and with incoming nitrogen flux nonlinearly.

Effect of post-oxidizing temperature on tribological and corrosion behavior of plasma nitrided austenitic stainless steel

In this study, the effect of temperature of post-oxidation process on tribological and corrosion behavior of AISI 316 plasma nitrided stainless steel has been studied. Plasma nitriding was carried out at 450°C for 5h with gas mixture of N2/H2=1/3. The plasma nitrided samples were post-oxidized for 1h with gas mixture of O2/H2=1/5 at different temperature of 400, 450 and 500°C. The structural, tribological and corrosion properties were analyzed using XRD, SEM, microhardness testing, pin-on-disk tribotesting and electrochemical polarization. The results indicated that the nitride layer was composed of S-phase. The amount of S-phase decreased as the treatment temperature rose from 400°C to 500°C. In addition, it was found that oxidation treatment reduces wear resistance of plasma nitrided sample. It was demonstrated that the corrosion characteristics of the nitrided sample were further improved by post-oxidation treatment. The difference in corrosion resistance is mainly attributed to the thickness of the oxide top layer, which is governed by the post-oxidizing temperature.

Stress induced nitrogen diffusion during nitriding of austenitic stainless steel

The nitrogen distribution in plasma nitrided austenitic stainless steel at moderate temperatures is explained by non-Fickian diffusion model. The model involves diffusion of nitrogen induced by internal stresses created during nitriding process. For mathematical description of stress induced diffusion process the equation of barodiffusion is used which involves concentration dependent barodiffusion coefficient. For calculation of stress gradient it is assumed that stress depth profile linearly relates with nitrogen concentration depth profile. The fitting is done using experimental curves of nitrogen depth profiles for 1Cr18Ni9Ti nitrided austenitic stainless steel at two different nitrogen fluxes. Fitting is in good agreement in both cases. The diffusion coefficient D = 1.68 × 10 −12 cm 2 s −1 at nitriding temperature 380 °C was found from fitting.

Synthesis and Characterization of (C, N)-Alloyed Stainless Steel Coatings by High Energy Ion Assisted Magnetron Sputtering Deposition

In this article, (C, N) alloyed austenitic stainless steel coatings (SSC) (or hybrid S-phase with both carbon and nitrogen) were produced by a magnetron sputtering deposition process combined with ion implantation (CMSII). This technique involves a periodical high energy ion bombardment of the coating during its growth, which has a beneficial effect on the structure and properties of the deposited coating. The influence of the nitrogen and carbon addition to the deposition atmosphere on the structure, chemical composition, and coating morphology was investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), and glow discharge optical spectrometry (GDOS) techniques. Wear tests were carried out and the results were compared with low temperature plasma nitrided austenitic stainless steel. Corrosion behavior in Ringer's solution was also evaluated.

TEM studies of plasma nitrided austenitic stainless steel

Cross-sectional transmission electron microscopy and X-ray phase analysis were used to study the structure of a layer formed during nitriding the AISI 316L stainless steel at temperature 440 degrees C. It was found that the applied treatment led to the formation of 6-microm-thick layer of the S-phase. There is no evidence of CrN precipitation. The X-ray diffraction experiments proved that the occurred austenite lattice expansion - due to nitrogen atoms - depended on the crystallographic direction. The cross-sectional transmission electron microscopy studies showed that the layer consisted of a single cubic phase that contained a lot of defects such as dislocations, stacking faults, slip bands and twins. The high-resolution electron microscopy observations were applied to study the defect formation due to the nitriding process. It was shown that the presence of great number of stacking faults leads to formation of nanotwins. Weak, forbidden {100} reflections were still another characteristic feature of the S-phase. These were not detected in the X-ray spectra of the phase. Basing on the high-resolution electron microscopy studies it can be suggested that the short-range ordering of the nitrogen atoms in the octahedral sites inside the f.c.c. matrix lattice takes place and gives rise to appearance of these spots. It is suggested that the cubic lattice undergoes not only expansion but also slight rombohedral distortion that explains differences in the lattice expansion for different crystallographic directions.