Plasma Nitrided Stainless Steel Research Articles

The Effect of Heat Treatment on the Plasma Nitriding of Hot-rolled 17–7 PH Stainless Steel

17–7 PH stainless steel is a highly versatile material with a multitude of applications in a diverse range of fields, including aerospace, chemistry and petrochemistry, and medicine. The material’s exceptional mechanical properties and corrosion resistance render it the optimal selection for numerous components and instruments. Nevertheless, the surface properties of 17–7 PH stainless steel are inadequate for applications requiring high hardness and wear resistance in certain extreme environments. Due to its excellent mechanical properties and corrosion resistance, it can be utilized in the manufacturing of pharmaceutical equipment components. However, certain specialized environments still require surface nitriding treatment. Considering the complex heat treatment process required for this material, this paper reports a detailed study of the surface performance changes of 17–7 PH steel before and after ion nitriding following aging heat treatment. The study employs rolled 17–7 PH stainless steel as the subject material. The impact of heat treatment on plasma nitriding of stainless steel is investigated by comparing and analyzing the influence of martensite content and dislocation density within the martensite of the material prior to and following heat treatment on the hardness, thickness, and corrosion resistance of the nitrided layer on the surface of the steel after nitriding. The results demonstrate that 17–7 PH stainless steel, which does not undergo heat treatment, exhibits a high internal dislocation density, a high nitriding efficiency, and consequently, a high surface hardness. Following the application of a heat treatment, there is an increase in the martensite content of 17–7 PH stainless steel, a decrease in the dislocation content, and an increase in the matrix hardness.

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.

Surface modification of 304 stainless steels to improve corrosion behavior and interfacial contact resistance of bipolar plates

In this study, plasma nitrided stainless steel (SS304) at low and high temperature, treated by Thermo Reactive Deposition (TRD) and then pickled is investigated under PEMFC accelerated condition. Surface characterization is performed by SEM, EDS, XRD, AFM, and GD-OES. Potentiostatic (PS) and electrochemical impedance spectroscopy (EIS) is used to assess corrosion resistance of untreated and treated samples. Interfacial contact resistance (ICR) measurements are carried out to investigate the surface conductivity. The results reveal that TRD treated samples are covered by an inhomogeneous layer that cannot protect the substrate. Corrosion resistance and conductivity of pickled samples improve significantly. The improvements are attributed to a rich chromium layer and the low roughness of the modified passive layer. A bi-layer (outer porous and inner dense layer) structure is proposed as the most appropriate model for the passive film. Contact angle measurements indicate improved hydrophobicity of pickled samples.

Evaluation of nitrogen diffusion in thermo-mechanically nanostructured and plasma nitrided stainless steel

Abstract The effect of grain size on diffusion depth of nitrogen in AISI 321 stainless steel during plasma nitriding was investigated. The repetitive cold rolling and subsequent annealing were conducted to achieve nano/ultrafine grains in AISI 321 stainless steel. The grain size range of 130 nm up to 45 μm was obtained under these conditions. Plasma nitriding was performed at temperatures of 400, 450 and 500 °C for duration of 5 h. Microstructural evolutions were conducted by OM, SEM and TEM. The microstructure and composition of the nitrided layer were characterized by SEM and GDOES. Mechanical properties of the S phase were evaluated by micro-hardness testing. Results indicated that nitrided layer of the nanostructured steel have uniform appearance with no CrN precipitates, while CrN precipitations were formed in nitrided layer of the micro-grain one. The corrosion resistance of the nitride layer was improved in nanostructured condition because of uniform appearance of the nitride layer. Furthermore, the hardness of the S phase improved by decreasing substrate grain size. Increasing austenite grain size from 130 nm up to 45 μm, caused to increase surface nitrogen concentration in nitride layer. Decreasing austenite grain size led to decreases the S phase thickness, while the nitrogen diffusion mechanism is the same.

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.

Monitoring the near-surface pH to probe the role of nitrogen in corrosion behaviour of low-temperature plasma nitrided 316L stainless steel

Low temperature plasma nitriding of stainless steels improves tribological properties and pitting resistance, but it can activate anodic dissolution in acidic solutions. This work aimed at determining how anodic behaviour can be affected by nitrogen present in the steel. The electrochemical behaviour of AISI 316L steel after nitriding at 415°C (with up to 17at.% N) was examined in solutions of 0.1M Na2SO4+0.4M NaCl acidified down to pH 2.4. An antimony microelectrode was used to measure pH close to the sample surface. It was found that the nitrided layer was resistant to pitting corrosion at all pH's, but at pH below about 3.5 anodic currents were higher than those on untreated steel. For nitrided steel the near-surface pH was increasing when anodic current was rising in the active region, evidently due to binding of protons into NH4+. Nitrided steel underwent reactivation during reverse cathodic sweeps which is characteristic of low content of chromium oxide in surface film. Injection of NH4OH into the solution improved the passivation. Thermodynamic data indicated that the accompanying pH rise enabled the formation of Fe3O4, Fe2O3, Cr2O3, FeCr2O4, NiFe2O4, MoO42− and NO2−. It is suggested that the pitting resistance of nitrided steel results from an easy repassivation of incipient pits due to the formation of the above species.

Plasma nitriding on welded joints of AISI 304 stainless steel

Plasma nitriding is widely used for the improvement of the mechanical/tribological properties of various steel components in many applications such as automobile parts, turbine parts, textile machinery parts, etc. However, plasma nitriding on components with welding joints have not been intensively studied so far. In this study, we attempted to study the welded region of AISI 304 stainless steel after carrying out the plasma nitriding process. The plasma nitriding of stainless steel welded joints has been done with a H2:N2 (4:1) gas mixture at 570 and 450°C for different time durations. The parent and the welded regions were characterized using a microhardness tester, a scanning electron microscope (SEM) and an X-ray diffractometer (XRD) for surface hardness, case depth and phase formation respectively. The results show that the nitrided layer formed on the welded region consists of Fe4N and Fe3N, CrN and γ phases. Also, welded joints using the multiple pass technique gave better nitriding properties compared with that of the single pass technique. There is an improvement in the hardness by 3 times which could be correlated with structural changes and process parameters.

Material condition tailored to plasma nitriding process for ensuring corrosion and wear resistance of austenitic stainless steel

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.

A source of broad electron beams with a self-heated hollow cathode for plasma nitriding of stainless steel

The principle of operation and characteristics of a broad electron beam source based on the discharge with a self-heated hollow cathode and widened anode part are described. The source is intended for the ion nitriding of metals in the electron beam plasma. The influence of the current density (1–7 mA/cm2) and ion energy (0.1–0.3 keV) on the nitriding rate of the 12X18H10T austenitic stainless steel is studied. It is shown that the maximal nitriding rate is reached by the combining of the minimal bias voltage across the samples (100 V) and maximal ion current density, which ensures the dynamic oxide layer sputtering on the sample surface. The electron source, in which electrons are extracted through a stabilizing grid in the direction normal to the axis of the hollow cathode, ensures the radially divergent electron beam formation with a 700-cm2 initial cross section, a current of up to 30 A, and initial electron energy of 0.1–0.5 keV. The source stably operates at nitrogen-argon mixture pressures of up to 3 Pa.

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.

X-ray diffraction characterisation of expanded austenite and ferrite in plasma nitrided stainless steels

The S phase, known as expanded austenite, is formed on the surfaces of austenitic stainless steels that are nitrided under low temperature plasma. A similar phase was observed for nitrided ferritic stainless steels and was designed as expanded ferrite or ferritic S phase. The authors treated samples of austenitic AISI 304L and AISI 316L and ferritic AISI 409 stainless steels by plasma nitriding at different temperatures and then studied the structural, morphological, chemical and corrosion characteristics of the modified layers by X-ray diffraction, scanning electron microscopy/energy dispersive spectroscopy and electrochemical tests. For both austenitic AISI 304L and AISI 316L stainless steels, the results showed that a hard S phase layer was formed on the surfaces, promoting an anodic polarisation curve displacement to higher current density values that depend on the plasma nitriding temperature. A layer having a high amount of nitrogen was formed on the ferritic AISI 409 stainless steel. X-ray diffraction measurements indicated high strain states for the modified layers formed on the three stainless steels, being more pronounced for the ferritic S phase.

Sliding wear of plasma nitrided AISI 405 ferritic stainless steel

The scope of this paper is to report the qualitative study of the phases present in the microstructure and of their tribological properties in a plasma nitrided ferritic stainless steel AISI 405. To achieve the proposed objective, two groups of samples were selected, one tested with a cast structure and the other submitted to plasma nitriding process in a reactor type chamber, at 673 K for 20 h, in a 25%N2/75%H2, and cooled in an Ar atmosphere. The wear under dry sliding conditions was evaluated with an Amsler wear machine. The phases were studied by optical microscopy and X-ray diffraction and the mechanical properties were evaluated through Vickers microhardness profiles. The tribological behaviour of the nitrided layers was explained by means of the wear mechanisms acting in the different conditions established during the performed tests. The sliding wear resistance of plasma nitrided AISI 405 was up to 149 times higher than that of the untreated one.

Corrosion and passivation of plasma nitrided stainless steels

Corrosion behaviour of nitrided steels was examined in chloride free and chloride containing sulphate solutions acidified to pH 3·0. Conventional nitriding (550–600°C) strongly deteriorates the corrosion resistance, however, high reactivity of nitrided layers enables the formation of thick conversion coatings to protect against corrosion (oxidation and phosphating) or to improve lubrication (phosphating). Low-temperature nitriding (380–450°C) produces thin layers of a supersaturated solid solution of nitrogen in austenite (S phase) of high resistance to pitting corrosion in chloride containing solutions. After nitriding at 450°C for 30 h, the near-surface parts of layers with >7 wt-%N have a deteriorated resistance to general corrosion in the acidified solution, but deeper parts with lower nitrogen contents have the resistance comparable with that of untreated steels. It is proposed that the improved pitting resistance is due mainly to the accumulation on corroding surface of large amounts of passivating products and of Cr–N species.

Tribological Behavior of Plasma Nitrided 1Cr18Ni9Ti Austenitic Stainless Steel Under the Effect of Lubricant Additives

To expand the engineering application of stainless steel as tribological material, it is important to study the tribological interaction of the nitrided layer with lubricating additives. The friction and wear properties of plasma nitrided 1Cr18Ni9Ti stainless steel were investigated under lubricated conditions on an Optimol Schwinyung Reibung Versch oscillating friction and wear tester. The lubrication oil was 1,1,1-trihydroxymethylpropyl trioctoate containing zinc dibutyl dithiophosphate, bismuth dibutyl dithiophosphate, and bismuth N, N-dibutyldithiocarbamate as the antiwear and extreme pressure additives. The variations in the nitrided stainless steel and the unnitrided one under the tribological action of the additives were contrasted and the tribological chemical interaction between the nitrided layer and the additives was revealed. The results showed that the selected additives had good synergetic effect with the nitrided layer on tribological performance and the bismuth containing additive had better friction-reducing and antiwear abilities than the zinc containing additive. Meanwhile, under the effect of these additives, the N/(Fe+Cr) ratio rose and the Fe/Cr ratio decreased in the nitrided layer, while the Fe/Cr ratio in the unnitrided stainless steel varied little. Three main elements, N, Cr, and Fe, in nitrided layer had different actions with the additives and contributed to tribological performance by different methods.

Single-Point Diamond Turning of Plasma-Nitrided Stainless Steel

Stainless steel and other ferrous metals are normally classified as not diamond turnable for the unacceptable tool wear caused by the thermal-chemical reaction between diamond and ferrous metals. In the present research, stainless steel specimens were plasma nitrided at a relatively low temperature (<450oC) to prevent the depletion of Cr content at the austenite matrix and to give a hardened layer where Fe atoms are bonded to nitrogen atom to form γ’-Fe4N. Diamond turning experiments were subsequently carried out under the following machining conditions: single crystal diamond tool, cutting speed up to 180 m/min, cut depth up to 5μm and light mineral oil as the cutting fluid. The results showed that, given the same machining conditions, while rapid tool wear and poor surface finish were obtained when turning the as-received stainless steel, surfaces with Ra better than 3nm and no obvious tool wear were achieved when turning the plasma nitrided specimens

Influence of different layer microstructures induced by different gas compositions on corrosion behavior of plasma nitrided stainless steel

AISI 316 austenitic stainless steels have been plasma nitrided using a dc glow discharge unit in order to investigate the influence of gas composition on microstructure and corrosion behavior of treated samples. Corrosion properties of untreated and plasma nitrided 316 steels have been evaluated using anodic polarization tests in 1 N H 2SO 4 solution. Qualitative evaluation was carried out using surface analyses such as SEM, EDX, XRD and GDS before and after the corrosion tests. The results showed that the untreated sample suffered localized pitting corrosion under the testing conditions. Nitriding in different gas compositions resulted in different surface structures that affect the corrosion behavior of the modified layer. Increasing of H 2 to gas mixture results in improving nitriding efficiency but only in the case of sample nitrided at 723 K and N 2/H 2 with ratio of 50/50 a precipitate-free single phase nitrided layer of nitrogen expanded austenite (γ N phase) was produced, which considerably improved the corrosion properties of the austenite stainless steel. Nitriding at these conditions due to formation of a surface layer without defects and higher thickness allows obtaining a significant improvement of corrosion resistance even at higher treatment time and temperatures.

Effect of treatment time on low temperature plasma nitriding of stainless steel by saddle field neutral fast atom beam source

Recent research carried out in laboratories showed that Saddle field neutral fast atom beam source is a promising method for nitriding of stainless steel. In the present work, the effect of treatment time on the microstructural and mechanical properties of plasma-nitrided stainless steel sample was investigated by this new method. Plasma nitriding was carried out at 420 °C and at a pressure of 0.1 Pa for a time range of 1 to 12 h. SEM–EDX, microhardness tests, optical microscopy and X-ray diffraction (XRD) were used to evaluate the mechanical and structural properties of the nitrided layer. It was found that nitriding time has a pronounced effect on the structural and mechanical properties of low-temperature plasma-nitrided samples and produced a precipitation-free thin hard nitrided layer within a short processing time.

Tribological properties of plasma nitrided stainless steel against SAE52100 steel under ionic liquid lubrication condition

1Cr18Ni9Ti stainless steel was modified by plasma nitriding. The phase composition of the plasma nitrided layer was examined by means of X-ray diffraction. The friction and wear properties of the modified and unmodified 1Cr18Ni9Ti stainless steel specimens sliding against SAE52100 steel under the lubrication of ionic liquid of 1-ethyl-3-hexylimidazolium hexafluorophosphate (L-P308) and poly α-olefin (PAO) were investigated on an Optimol SRV oscillating friction and wear tester, with the interactions among the modified surface layer and the ionic liquids and PAO to be focused on. The morphologies of the worn surfaces were observed using a scanning electron microscope. The chemical states of several typical elements on the worn surfaces of the modified steel surfaces were examined by means of X-ray photoelectron spectroscopy. Results showed that the modified sample had better anti-wear abilities than the unmodified one, but the modified sample had a slightly higher friction coefficient than the untreated one. This was partly attributed to the change in the hardness and phase composition of the stainless steel surfaces after plasma nitriding and tribochemical reactions between the steel and the lubricant. The resultant surface protective films composed of various tribochemical products together with the adsorbed boundary lubricating film contributed to reduce the friction and wear.