Nitrided Austenitic Stainless Steel Research Articles

Effect of Rapid Hollow Cathode Plasma Nitriding Treatment on Corrosion Resistance and Friction Performance of AISI 304 Stainless Steel.

Low-temperature plasma nitriding of austenitic stainless steel can ensure that its corrosion resistance does not deteriorate, improving surface hardness and wear performance. Nevertheless, it requires a longer processing time. The hollow cathode discharge effect helps increase the plasma density quickly while radiatively heating the workpiece. This work is based on the hollow cathode discharge effect to perform a rapid nitriding strengthening treatment on AISI 304 stainless steels. The experiments were conducted at three different temperatures (450, 475, and 500 °C) for 1 h in an ammonia atmosphere. The samples were characterized using various techniques, including SEM, AFM, XPS, XRD, and micro-hardness measurement. Potentiodynamic polarization and electrochemical impedance spectroscopy methods were employed to assess the electrochemical behavior of the different samples in a 3.5% NaCl solution. The finding suggests that rapid hollow cathode plasma nitriding can enhance the hardness, wear resistance, and corrosion properties of AISI 304 stainless steel.

Electrolyte-Plasma Nitriding of Austenitic Stainless Steel under Cathodic and Anodic Polarity

Electrolyte-Plasma Nitriding of Austenitic Stainless Steel under Cathodic and Anodic Polarity

Electrolyte-Plasma Nitriding of Austenitic Stainless Steel with Cathodic and Anodic Polarity

The results of modifying the surface of austenitic stainless steel by plasma electrolytic nitriding at the anode and cathode polarities of the processed product in an electrolyte based on ammonia (5%) and ammonium chloride (5%) are presented. The morphology and surface roughness, phase composition, and microhardness of diffusion layers, tribological and corrosion properties were studied. The influence of physico-chemical processes during anodic and cathodic treatments on the features of the formation of the modified surface and its operational properties is shown. Smoothing of micro-roughness and reduction of surface roughness during anodic nitriding due to electrochemical dissolution, and the course of surface erosion with increased roughness during cathodic nitriding due to electrical discharges were revealed. An increase in the hardness of the nitrided layers to 1150 HV was found with a twice decrease in roughness after anodic treatment, accompanied by an increase in the wear resistance by 166 times, with a decrease in the friction coefficient by 1.6 times. After cathodic treatment, it is possible to increase the hardness only to 580 HV and the wear resistance by 13.6 times. An increase in surface roughness after the cathodic treat-ment always leads to an increase in the friction coeffi-cient. In both cases, a decrease in the pitting formation in the medium of sodium chloride is shown.

Effects of Collisionless and Collisional Plasma Characteristics on the Transport Mechanism of Nitrogen Ions in Nitrided Austenitic Stainless Steel

Effects of Collisionless and Collisional Plasma Characteristics on the Transport Mechanism of Nitrogen Ions in Nitrided Austenitic Stainless Steel

Initial phase formation during nitriding of austenitic stainless steel

Expanded austenite formed by inserting nitrogen into austenitic stainless steel is a puzzling phase since its discovery about 30 years ago: variable nitrogen concentration correlated with a variable lattice constant together with an unusual depth profile normally indicating a compound layer instead of a diffusion layer. While expanded austenite has been described for nitriding experiment durations as low as 5 min and >48 h, the initial phase transition has not been investigated in detail. Here, very low ion current density experiments prolonging the initial phase together with in-situ X-ray diffraction (XRD) experiments during nitriding are presented. Additionally, the formed layers are investigated by depth-resolved in-situ XRD measurements during successive removal of the surface layer. It is shown that two phases, a low expansion phase and a high expansion phase, are formed sequentially over time with the growth rate of the latter phase being much higher, thus obliterating any trace of the initial phase. It is surmised that the driving force is a threshold in the nitrogen concentration.

Effects of surface topography at different scales on the dispersion of the wetting data for sessile water droplets on nitrided austenitic stainless steels

Plasma nitriding is used to increase the durability of steels. The wettability of nitrided surfaces is poorly studied. The ability of nitriding in modifying the wetting and evaporation of sessile droplets on austenitic stainless steel is studied. Transferred plasma for nitriding provides independent substrate biasing giving the opportunity to tailor the surface before and during the nitriding treatments. It was shown that a cleaning treatment (ArH2 plasma) produces topographical modifications of the surface by selective sputtering of the grains giving contact angles between 80 and 90°. Similar results are obtained when the nitriding treatments are carried out with a rather high bias voltage. It is explained using the skewness (Ssk) topographical roughness parameter which reveals the asymmetry of the profile. The more the Ssk is, the higher the contact angle is. During nitriding treatments, the very high internal stresses leads to the formation of spikes at the grain boundaries. Pinning of the three-phase contact line on peaks localized at grain boundaries are responsible for high contact angles. For nitriding treatments carried out with lower bias, surface contamination occurs and a discontinuous oxynitride layer is formed. The nanostructure formed by these oxides gives contact angles up to 120°.

Nanosized Cr-N clustering in expanded austenite layer of low temperature plasma-nitrided Fe-35Ni-10Cr alloy

Expanded austenite (γN) formed by low temperature nitriding of austenitic stainless steel is a N-enriched fcc containing Cr-N short range ordering, but no direct observation of Cr-N clustering has been reported. In the present study, the nanostructure of an Fe-35Ni-10Cr (at%) alloy plasma-nitrided at 673 K was investigated using transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) technique in this study. Nanosized Cr-N clusters were directly observed in the γN accompanied with strong streaks in selected area diffraction, obvious modulated structure in TEM and Cr-N rich regions observed by 3DAP. Thermodynamic calculations suggested that Cr-N clusters were formed by spinodal decomposition due to a strong Cr-N attractive interaction. The hardness of the γN layer was much higher than that of Fe-N austenite steel, which indicates that the hardness is not only due to nitrogen solid-solution hardening but also to a synergetic effect of the coexistence of Cr and N.

Effect of plasma nitriding of austenitic stainless steel produced by direct metal laser sintering

A special additive manufacturing (AM), called as Direct Metal Laser Sintering (DMLS), is a technology that produces 3D workpieces using a wide range of different metals as raw materials. The aim of current research is to investigate the plasma nitriding effect on the DMLS produced samples. The direct current plasma nitriding treatment was achieved at 440 °C for 4 hours with 75%N2 – 25%H2 gas mixture. Before the treatment, the 316L austenitic stainless steels samples were ground with different methods to modify the surface roughness. Scanning electron microscope (SEM), X-ray diffractometer, glow discharge optical electron spectroscopy, Vickers microhardness tester and potentiodynamic corrosion test were used for the characterization of surface properties. The results demonstrated that the surface roughness did not affect the outcome of the plasma nitriding, but the corrosion resistance increases with the decrease of the surface roughness compared to the untreated 3D sample.

Challenges of fitting a Nitrided austenitic stainless steel Mössbauer Spectrum

Challenges of fitting a Nitrided austenitic stainless steel Mössbauer Spectrum

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.

Structure and corrosion behavior of ultra-thick nitrided layer produced by plasma nitriding of austenitic stainless steel

An ultra-thick nitrided layer (>80 μm) was formed by hot wire enhanced plasma-assisted nitriding of AISI 316L austenitic stainless steel under the higher pulsed bias to investigate the structure and the corrosion performance. The phase structure, the cross-sectional morphology, the element distribution, hardness and modulus, as well as the corrosion behavior of the nitrided layer were investigated by XRD, SEM, EDS, nano-indentation tester and electrochemical workstation, respectively. The results showed that the nitrided layer was divided into two sub-layers: the upper thick layer composed of N-expanded austenite and CrN, and the lower thin layer mainly composed of γN with a trace of CrN precipitate. Nitrided at −400 V, CrN and high N-expanded austenite formed in the upper layer, which exhibited a high hardness and an excellent corrosion resistance. As the bias increased to −500 V, more nitrogen atoms precipitated as CrN phase and low N-expanded austenite phase began to form, the thickness of nitrided layer reached a maximum of 84 μm. To −600 V, high N-expanded austenite almost completely transformed to low N-expanded austenite, accompanied by the increase of CrN, which caused the thickness and hardness of the nitrided layer to decrease, and the pitting corrosion occurred at −4.1 mV.

A comparative study of the tribocorrosion behaviour of low temperature nitrided austenitic and duplex stainless steels in NaCl solution

This work studied and compared the tribocorrosion behaviour of low temperature nitridedaustenitic (316L) and duplex (2205) stainless steels. After gaseous nitriding at 450 °C for10 h, the S-phase layer formed on the duplex stainless steel is thicker and harder than that onthe austenitic stainless steel. The formation of S-phase layers reduces both mechanical wearand chemical wear of stainless steels but the duplex type exhibits better tribocorrosion performance. Wearing through of the S-phase layer during tribocorrosion leads to asignificant drop in open circuit potential, friction instability, and total material loss from the nitrided specimens. The work concludes that low temperature gaseous nitriding is effective in improving the tribocorrosion behaviour of these materials to avoiding catastrophic surface failures.

Anisotropy effects on gaseous nitriding of austenitic stainless steel single crystals

Low temperature nitrided austenitic stainless steel exhibits strong orientation dependent case growth kinetics. Anisotropy effects during low temperature gaseous nitriding of single crystals of austenitic stainless steel with surface-normal orientations 〈100〉, 〈111〉 and 〈10 7 2〉 were investigated with thermogravimetry, X-ray diffractometry (XRD), glow discharge optical emission spectroscopy (GD-OES), nano-indentation and electron back-scatter diffraction (EBSD). Case growth kinetics were fastest for 〈100〉 and slowest for 〈111〉. Depth-profiling over the case depth was accomplished by successive layer removal combined with X-ray diffraction and nano-indentation. The results indicate that anisotropic case-growth kinetics is related to early nucleation of long-range ordered (LRO) over-stoichiometric γ’N−Me4N1+x (Me=metal atoms) in 〈100〉, which is inseparable from short range ordered (SRO) expanded austenite, γN. Depth ranges of elasto-plastic and purely plastic accommodation of the nitrogen induced lattice expansion were distinguished. In the elasto-plastic region stress-gradient enhanced diffusion of N is observed.

Correlation between microstructure and micro-hardness of 316L nitrided austenitic stainless steel

This paper presents the effect of duration of solution nitriding towards microstructure and microhardness of AISI 316L stainless steel. The AISI 316L stainless steel was exposed to high temperature gas nitriding process at 1200 °C by using 50% ammonia gas and 50% nitrogen gas for different durations of 8, 15 and 20 hours. The nitrided austenitic stainless steel then underwent several analyses and tests to find the effect of different durations of nitriding towards its microstructures and microhardness. From the experiments conducted to the nitrided samples, it was found that the higher duration of nitriding resulted in the increase of microhardness of the samples due to the increase of nitrogen content in the stainless steel. However, there was no effect found on the microstructure of the nitrided samples. It was noted that the highest hardness was obtained from the nitrided sample of 20 hours that has 245.1 HV which is about 70% higher than the as-received sample.

Magnetic and 57Fe hyperfine structural features of nitrided austenitic stainless steel

Samples of an austenitic stainless steel were plasma-nitrided at 673 K, for 4 h, in 80% H2−20% N2 gas mixture, at different pressures: 4; 6 and 10 Torr. The samples were characterized by four different techniques: X-ray diffraction, Mössbauer spectroscopy, magnetic force microscopy and energy dispersive X-ray. The nitrogen concentration and thickness of the nitrided layer was found to be 7 at.% and 2 μm for the 4 Torr-sample; for those at 6 and 10 Torr, the concentration was 16 and 24 at.%, with nearly the same thickness, namely 7 μm. These values are well correlated with the framework of an expanded austenite, as pointed by the X-ray diffraction data. The magnetic force microscopy data are well correlated with the observed nitrogen concentration, evidencing the absence of any magnetic domain for the 4 Torr-sample; the magnetic patterns for the 6 Torr and 10 Torr-samples are quite similar, despite of the great difference in their nitrogen concentrations. The 57Fe hyperfine parameters at two different sample depths were assessed by Mössbauer spectroscopy, by collecting data with two distinct backscattering setup geometries. Fitting the corresponding spectra with model-independent hyperfine field distributions, starting from very low hyperfine field values, allowed correlating the magnetic force microscopy data to find out that the transition from paramagnetic to magnetic for the expanded austenite occurs at smaller hyperfine magnetic fields than it is usually reported in the scientific literature. Such correlations point to the formation of a submicrometer layer occurring on the outermost surface of the sample. Despite the complexity and difficulties in differentiating the nitrides and the martensitic structure on the surface of the samples obtained at 6 Torr and 10 Torr pressures, the results obtained in this work strongly indicate the existence of them in the surface layer.

Thermo-chemical-mechanical simulation of low temperature nitriding of austenitic stainless steel; inverse modelling of surface reaction rates

A multi-physics thermo-chemical-mechanical 1-dimensional implicit finite difference model is presented to simulate the evolution of composition and residual stress profiles over the expanded austenite case developing during low temperature nitriding of austenitic stainless steels. The model combines 1-dimensional diffusion of nitrogen in the depth direction with a concentration-dependent diffusivity, elasto-plastic accommodation of the lattice expansion, stress gradient-induced diffusion of nitrogen, solid solution-strengthening by nitrogen and trapping of nitrogen by chromium atoms. The rate of the surface reaction governing the transfer of nitrogen from the gas to the solid is unknown and was evaluated by inverse modelling. The modelling was applied adopting the surface reaction rate as the only fitting parameter and taking mass-uptake curves (thermogravimetry) as the constraint, while all other data were taken from established literature values. Very good agreement is achieved between the predicted and experimental composition-depth profiles. Further, the applicability of the present model to plasma nitriding was verified by simulating (not fitting) the evolution of composition-depth profiles obtained after plasma nitriding of stainless steel. The good to very good agreement of the present model's predictions with experimental data for gaseous and plasma nitriding, indicates that the essential multi-physics influences and parameters are taken into account, with a minimum of adjustable parameters.

Microstructure and micro-abrasive wear of sintered yttria-containing 316L stainless steel treated by plasma nitriding

Plasma nitriding of austenitic stainless steel may result in the formation of a nitrided layer, which is responsible for the increase in hardness, as well as wear and corrosion resistance of this material. In this work, the effect of yttria on the micro-abrasive wear behavior of plasma nitrided stainless steel was investigated. Powder metallurgy (P/M) 316L austenitic stainless steel samples, without and with 1 wt.% Y2O3, were plasma nitrided at 450 °C for different nitriding times (from 2 h up to 10 h). The nitrided samples were characterized by X-ray diffraction (XRD), scanning electron microscopy, instrumented indentation and micro-abrasive wear tests. The XRD analysis confirmed the presence of nitrogen expanded austenite (γN) phase for all nitrided layers and the formation of iron and chromium nitrides. The nitride content in the nitrided layer increased with the nitriding time. However, alloying the sintered 316L stainless steel with yttria particles increased the nitrogen content in the nitrided layer, which consequently increased the lattice distortion and the amount of nitrogen expanded austenite and decreased the content of iron and chromium nitrides in the nitrided layer. Micro-abrasive wear tests showed that the addition of yttria to the bulk of the sintered material promotes a more wear-resistant material for all nitriding times. This behavior was related to the hardness, thickness and amount of the nitrogen expanded austenite of the nitrided layer.

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.

Kinetic model of anisotropic stress assisted diffusion of nitrogen in nitrided austenitic stainless steel

The influence of elastic anisotropy of austenite on the transport of nitrogen in austenitic stainless steel (ASS) during the expanded austenite formation is analyzed by the presented kinetic model. The model is based on the thermodynamics involving the anisotropic lattice expansion due to nitrogen insertion. Thus, a diffusion behavior of nitrogen is affected by not only nitrogen concentration but also the anisotropic stress induced by nitrogen atom expanding the lattice of the steel. Developed model describes well known experimental results for polycrystalline and single crystal ASSs. Using presented model, we studied the effect of crystallographic orientations of single crystal on nitrogen penetration depth and surface concentration during formation of the expanded austenite. It was shown, that the thickness of the nitrided layer depends on the crystalline orientation and the diffusion rate is found to be highest for (100) orientation. Furthermore, anisotropic surface energies of different oriented crystals lead to anisotropy of nitrogen adsorption process. As a result, the (100) crystalline orientation is most favorable for nitrogen diffusion.

Influence of the Pre-Treatments and Process Temperature on the Adhesion of TiN Films Deposited by PBII&D Over Nitrided Austenitic Stainless Steel

In this work the influence of pre-treatment cleaning processes in the adhesion of TiN films deposited by the plasma based ion implantation and deposition (PBII&D) process over plasma nitrided AISI 316L stainless steel was investigated, varying the gas composition, the duration of the sputtering process and the temperature in the same deposition chamber. The TiN film morphology was scrutinized with scanning electron microscopy on the surface complemented with focus ion beam for the cross section. The film microstructure was characterized with X ray diffraction. The adhesion of the coatings was studied using the Scratch Test with constant load at different values and the Rockwell C indentation method as well. It was found that only with the sputtering carried out in a H2 - Ar gas mixing at 300 oC temperature used as pre-treatment was successful to prepare the nitrided surface for the subsequent TiN deposition by the PBII&D process.