Pressure Nitriding Research Articles

Sintering optimization of high nitrogen nickel free austenitic stainless steel prepared by Metal Injection Molding

High nitrogen and nickel free austenitic (HNNFA) stainless steel prepared by powder metallurgy has become a research hotspot because of solving nickel precipitation and saving cost. In this paper, metal injection molding (MIM)method was used to prepare such kinds of steel. The effect of sintering parameters such as sintering temperature, holding time, and nitriding pressure on the density, shrinkage rate, and nitrogen content of austenitic stainless steel was systematically evaluated by an orthogonal experiment to obtain the optimum sintering parameters. When the sintering temperature is 1200°C, the holding time is 6 h, and the nitriding pressure is 1100 mbar, the sample possesses a higher density (7.52g/cm3) and nitrogen content (0.76%). The matrix of stainless steel is austenite after nitriding sintering.

Influence of gas pressure on the low-temperature plasma nitriding of surface-nanocrystallined TC4 titanium alloy

The influence of gas pressure on plasma nitriding of surface nanocrystalline TC4 titanium alloy prepared by shot peening (SP) was investigated. SP treatment under 0.3 MPa-25 min (Almen intensity is 0.32 mm) before plasma nitriding effectively refined the surface grain size of TC4 alloy to about 42 nm, which helped to sufficiently lower the nitriding temperature of TC4 to 500 °C. After nitriding treated under different gas pressures from 50 Pa to 300 Pa, nitriding gas pressure was found to has an optimal value at 200 Pa with which a thickest nitriding layer of 1.34 μm and highest hardness of 1569 HV was achieved at 500 °C for 20 h. Furthermore, the influence of gas pressure on the formation and migration of active nitride particles was revealed to be related with their energy and efficiency. Higher nitriding pressure would produce more active nitride particles but consume more energy during collision which made active nitride particles difficult to transit through the nitriding environment to land on the specimen. Balanced between these two aspects, 200 Pa was found to form more active nitride particles as well as maintain a high level energy. Meanwhile, SPed specimen presented a thicker nitriding layer than coarse-grained specimen. So in this study, we combine the promotion effect of surface nanocrystalline and the optimization of nitriding pressure to comprehensively improve the plasma nitriding efficiency.

Surface Modification of Titanium Implants

The influence of nitriding temperature (900 and 850°C) and partial pressure of nitrogen (105 and 1 Pa) on morphology and phase structural state of modified surface layers of commercially pure titanium (cp-Ti) was determined. It was shown that nitrided titanium provided much better anti-corrosion protection than nitrided Ti-6Al-4V alloy, and its corrosion resistance increased with decreasing nitriding temperature and partial pressure of nitrogen.

Corrosion resistance of AISI 316L plasma nitrided at different temperatures and times

Austenitic stainless steels are widely used as biomaterial and corrosion resistance is one of the most important characteristics in determining the suitability of a material for this purpose. Plasma nitriding is a surface treatment that introduces interstitial nitrogen into these steels, which improves this property as a result of the nitrided layer, whose properties depend strongly on the conditions used in the process. In this paper, the nitriding temperatures (623, 673 and 723 K) and times (3, 4 and 5 h) were investigated. The Mossbauer Spectroscopy was correlated to the corrosion results with the purpose of finding the combination of time and temperature that optimize the corrosion improvement. An alternative method of spectra analysis, which analyzes qualitatively the hyperfine magnetic field distributions, was used and showed that the nitrided layer protects against corrosion, not only because of the expanded austenite, but also, and mainly, because of the nitrides, which are formed therein, located up to a depth of 0.1 μm. Moreover, these results indicate that temperature = 673 K and time = 4 h is the most efficient combination to optimize the corrosion resistance of the samples, nitrided at 6 Torr of the 80% H2–20% N2 gas composition. They also confirm that corrosion protection increases for higher nitriding pressure.

Influence of plasma nitriding pressure on microabrasive wear resistance of a microalloyed steel

Microalloyed steels have been studied not only by the advent of the pre-salt, but also by the continuous search for improvement of their tribological properties. In this work, the influence of the plasma nitriding (PN) pressure on the nitrided layer formed on top of an API 5L X70 microalloyed steel was studied. PN treatments were carried out using a pulsed plasma reactor at temperatures of 410°C and 440°C, in a gas mixture of N2 and H2, for 3h and under different pressures. Results showed that the pressure has a significant effect on the uniformity and width of the plasma sheath and, consequently, on the structure and thickness of the resulting nitrided layer. It has been observed that higher pressures and temperatures have led to the formation of nitrided layers containing a thinner white layer and without nitride needles. The best wear results were offered by the thicker nitrided layers, besides thinner white layer. This type of structure can be produced when PN treatments are performed using low-width plasma sheaths.

Influence of the Gas Pressure of Plasma Nitriding on the Structural, Mechanical and Tribological Surface Properties of AISI 316L

In this paper, the influence of gas nitriding (80% H2-20% N2) pressure (between 3 and 7 Torr) on the modification of surface properties of AISI 316L stainless steel is examined. The structural, mechanical, tribological and electrochemical properties of the layer produced in disk samples were studied. The samples’ characteristics were determined by scanning electron microscopy, X-ray diffraction, electrochemical potential curves, microhardness, roughness and wear tests. When pressure increases, the microhardness, elasticity and thickness also increase. The maximum thickness of the nitrided layer was 6.4 µm, the hardness reached 2200 HV0.0036 and the mass loss decreased until 1% relatively to the unnitrided sample. The X-ray diffractograms showed the presence of the expanded austenite in all the samples. The correlation between these diffractograms and the corrosion results corroborate to the most recent previous conclusions of our systematic study, which evince the interconnection between the improvement of the corrosion resistance and the presence of nitrides on the top of the nitrided layer. More significant presence of such nitrides was observed for the sample nitrided at 6 Torr, whose corrosion performance was better than for the other samples. This pressure seems to be the most indicated to improve the properties under study. Moreover, the results verified not only that the sample geometry is an important factor for enhancing these properties but also that the border effect, which causes the rings on the disk border, depends on the pressure, but not on temperature or time from the plasma nitriding process.

Analysis of Gas Nitriding Characteristics Under Different Cold Hardening and Nitriding Pressure Conditions for Low-Carbon Low-Alloy Steel

A new approach to quick preparation of a nitrided case for low-carbon low-alloy steels was proposed. It is based on cold hardening and pressurized gas nitriding. The microstructure, surface hardness, thickness, and corrosion resistance of the nitrided layer on low-carbon low-alloy steel (20CrMnTi) were investigated after the nitriding at 510°C for 5 h under different cold rolling reduction (0–60% CR) and nitriding pressure (1–5 atm). The results show that this technique can significantly improve the nitriding steel efficiency with the nitrided layer mainly composed of Fe2-3N and Fe4N nitrides. At constant nitriding pressure, the nitrided layer hardness first increases and then decreases with the amount of cold hardening. At the constant hardening, the nitrided layer hardness and corrosion resistance increase with the nitriding pressure. For the sample nitrided at 510°C for 5 h under 30% CR and a pressure of 5 atm, the nitrided layer exhibits optimal properties: hardness of 825 HV and thickness of 310 μm.

Study on pressurized gas nitriding characteristics for steel 38CrMoAlA

The effects of nitriding pressure on the microstructure, surface hardness, thickness and wear resistance of the nitrided layer on 38CrMoAlA steel were investigated. Gas nitriding was conducted in flowing NH3 gas at 510°C for 5h under nitriding pressures ranging from 1 to 6atm. The results show that, compared with conventional gas nitriding (1atm), the increase in nitriding pressure markedly accelerated the gas nitriding kinetic process and promoted the rapid thickening of the nitrided layers. Both the microstructure and the microhardness of the nitrided surface layer were strictly controlled by nitriding pressure. The nitrided layer of the sample nitrided under 6atm for 5h exhibited the best wear resistance, and its thickness (410μm) was almost equal to that (440μm) of the conventional gas nitriding at 510°C for 50h, which would result in higher production efficiency and lower cost in industrial application.

Significant acceleration of nitriding kinetics in pure iron by pressurized gas treatment

Abstract The effect of nitriding pressure on the formation of nitrides and the nitriding kinetics in pure iron were investigated by applying pressurized gas nitriding at 500 °C for 5 h. Increasing the nitriding pressure from 1 atm (conventional nitriding condition) to 5 atm significantly increased the nitrided layer thickness from about 210 μm to 1100 μm. Furthermore, it was found that the constituents of the compound layer can be controlled by changing the nitriding pressure. These improvements are related to the fast reaction rates and high inward diffusive flux of nitrogen during the pressurized gas nitriding.

Effects of gas nitriding pressure on the formation of nanocrystalline AlN in plasma nitrided Fe–9Al–28Mn–1.8C alloy

The effects of gas nitriding pressure on the formation of nanocrystalline nitrided layer and its effect on the performance of the Fe–9Al–28Mn–1.8C alloy (in wt.%) were investigated. Plasma nitriding was conducted at 450°C for 12h under nitriding pressures ranging from 133 to 798Pa. The results evidently demonstrated that, due to the unique as-quenched microstructure of the present alloy, the effect of nitriding and aging could be achieved simultaneously with one-step plasma nitriding scheme. Both the thickness of nitrided layer and the nitrogen concentration at the outmost surface were found to increase with increasing gas nitriding pressure in the range of 133–798Pa. Detailed microstructural analyses indicated that the nitrided layer is composed predominantly of nanocrystalline face-centered-cubic (FCC) B1-AlN and FCC γ′-Fe4N with minor amount of expanded austenite phase, which accounts for the excellent surface microhardness and corrosion resistance in 3.5% NaCl solution. The increase in gas nitriding pressure led to a thicker nitrided layer and smaller AlN particles, which in turn, resulted in higher surface microhardness and better corrosion resistance.

Magnetic properties of Ce–Nd–Fe–Mo alloys and their nitrides

New quaternary alloys of Ce1−xNdxFe12−yMoy with x=0, 0.2, 0.4, 0.6, 0.8, 1 and y=0, 1.5, 2 have been prepared and magnetically hardened by melt spinning. X-ray diffraction indicates that the as-spun materials exhibit the tetragonal ThMn12-type structure. Prior to nitriding, the coercivity Hci is less than 0.6kOe in all alloys and is independent of Nd content, while the magnetization 4πM19 (measured in an applied field of 19kOe) and Curie temperature Tc increase with added Nd content x. The effects of nitriding pressure P, time t, and temperature T on magnetic properties have been carefully evaluated on NdFe10Mo2 in order to identify the optimal nitriding parameters. The optimized nitriding profile was subsequently adopted to nitride the remaining samples. After nitrogenation, Tc and 4πM19 have been substantially enhanced primarily due to the increased Fe–Fe exchange from nitrogen induced lattice dilation. Benefitting from the positive contribution from Nd, Hci has been greatly improved in the Nd containing samples. As a result, Ce0.2Nd0.8Fe10Mo2 nitride features Hci=2.9kOe and (BH)max=1.6MGOe and Ce0.2Nd0.8Fe10.5Mo1.5 nitride demonstrates Hci=2.5kOe, (BH)max=1.5MGOe at room temperature, and Tc=337°C, which are substantial advancements compared to the pure Ce based ThMn12-type materials previously reported.

SELECTED PLASMA NITRIDING METHODS USABLE FOR THE THERMO-CHEMICAL TREATMENT OF AIRCRAFT PARTS

The aim of the work was to review selected methods of plasma nitriding, used in the thermo-chemical treatment of aircraft parts. The introduction explains the nitriding process and presents the requirements on the thermoDirect Current Plasma Nitriding (DCPN), Active Screen Plasma Nitriding (ASPN) and Low Pressure Nitriding in AEGD authors drew attention to known occur in this process such as edge effect or hollow cathode effect. The Keller’s model, the Marchand’s model and the Walkowicz’s model of nitriding literature, transportation of nitrogen during Active Screen Plasma Nitriding process and to show that in this nitriding method, defects typical for Direct Current Plasma Nitriding Low Pressure Nitriding in AEGD plasma, authors also presented the model of nitriding mechanism in this process, which shows four nitrogen diffusion paths (physisorption, chemisorption, adsorption of N2, ion implantation). nitriding a layer without film on the surface.

Vacuum Solution Nitriding of Martensitic Stainless Steel Holds Promise

Abstract Vacuum solution nitriding is a promising technique for increasing the surface hardness, load-bearing capability, and fatigue strength of martensitic stainless steels without sacrificing corrosion resistance. This article examines the effects of nitriding pressure and temperature on surface nitrogen content and case depth in four martensitic stainless steels.

Effect of Plasma Nitriding Parameters on the Wear Behaviour of AISI D6 Tool Steel

Abstract In the present investigation, the effects of plasma nitriding pressure and time on the structural and tribological characterization of AISI D6 grade cold work tool steel were studied. DC glow discharge plasma nitriding process under NH3 gas was performed at temperatures of 500 °C and at pressures of 150 Pa, 450 Pa and 800 Pa for durations of 2 h, 7 h and 10 h. The mass loss and the coefficient of friction were measured using a ball-on-disc wear test with 1.668 N, 4.143 N, and 9.113 N test loads. Cross-section and surface characterizations were evaluated using optical microscopy, SEM, and microhardness test techniques. The surface microhardness was found to increase with the nitriding time and the pressure. The wear resistance of the specimen strongly depends on the nitriding parameters and the applied test load in determining whether a plastic deformation of the surface layer has occurred or not.

Hot friction and wear behaviour of plasma nitrided and low pressure carburized hot work tool steel sliding against 6082 Al alloy

Abstract High temperature friction and wear behaviour of a hot work tool steel sliding against AA6082 alloy were investigated by means of a customary test configuration, which provides the conformal contact between a die block and an AA-disc. Uninterrupted tests in air were carried out. The friction behaviour is characterized by limited fluctuations just after the beginning of test, finally evidencing a transition to a more unstable contact regime after progressive transfer of Al to the die surface. The tendency towards Al pick up was evaluated by measuring the mass increase of die samples and their capability in delaying the formation of hard Al oxide particles, which are very detrimental for the surface finishing of the extrudate. The efficacy of different surface treatments, i.e. plasma nitriding and low pressure carburizing, was studied. Furthermore, samples with different level of roughness, corresponding to conventional machining (0.7–0.8 μm) and mirror polishing (0.02 μm), respectively, were produced. Finally the influence of hardness was also evaluated by considering two different hardness of the base steel (56 HRC and 44 HRC). In general, surface hardening improves the load bearing capability of the die surface, causing an extended permanence in the stable friction regime. Plasma nitrided samples (1125 HV 0.1) evidenced a better behavior than case hardened (670 HV 0.1) and base steel (56 HRC, 620 HV 0.1), respectively. Overtempered samples (44 HRC, 500 HV 0.1) confirm this trend. In view of the experimental error, a lower surface roughness did not improve the tribological properties.

Effect of volumetric alloying in a compressed nitrogen atmosphere on its solubility and alloy steel hardness

Thermodynamic aspects are considered for alloying steels in a molecular nitrogen atmosphere under pressure. Examples of standard steels 25Kh5M and 18Kh2N4MA are used to present nitriding kinetics: the effect of nitriding time, nitrogen pressure, and temperature on nitrided layer thickness. The effect of subsequent heat treatment on nitrided layer microhardness is studied.

A New Nitriding Process of 4Cr5MoSiV1 for Extrusion Die

Nitriding is one of the most efficient methods used to enhance the service life of aluminum extrusion dies. In this paper, a new nitriding process has been introduced that is characterized by that 4Cr5MoSiV1 steel, a typical material used for extrusion dies, was nitrided by using two-stage nitriding process. The microstructures of nitrided layer have been analyzed by means of optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD) and microhardness (MH). The results show that pre-heat treatment on the 4Cr5MoSiV1 steel extrusion die can be carried out under the condition of quenching at 1040 °C, double tempering at 600 °C and 560 °C for 2 hours each. Operating parameters of two-stage nitriding are set as follow: stage one, 520 °C for 6 hours, the flow rate of NH3of 0.6 m3/h; nitriding pressure of 453 Pa and stage two, 580 °C for 8 hours, the flow rate of NH3of 0.35 m3/h; nitriding pressure of 453 Pa.

Study of nitrogen impact on VFB–EOT roll-off by varying interfacial SiO 2 thickness

Flat band voltage ( V FB) roll-off in long channel devices at thin equivalent oxide thickness (EOT) is studied on SiO 2/nitrided-HfSiO stacks. V FB increases when SiO 2 interfacial layer thickness decreases, and charges pumping (CP) frequency sweep analysis shows higher trap density near Si/SiO 2 interface. Based on this observation, an atomic diffusion model is introduced. Higher concentration of nitrogen atom in the HfSiO(N) layer diffuses to the Si/SiO 2 interface through the SiO 2 layer in thinner SiO 2 device, and accumulates near Si/SiO 2 interface which can introduce higher density of interfacial traps. Lifetime extracted from negative bias temperature instability (NBTI), and mobility are also degraded in thinner SiO 2 devices due to the higher interfacial trap density. The V FB roll-off can be improved by lowering nitrogen concentration in the HfSiO(N) layer from optimizing plasma nitridation pressure, decreasing post deposition anneal temperature, or using defect absorbing layer on the high-k oxide.

Influence of gas nitriding pressure on the surface properties of ASTM F138 stainless steel

Abstract The present work investigates the influence of gas nitriding pressure on the modification of surface properties of ASTM F138 stainless steel. Nitrided surfaces were characterized by X-ray diffraction, Conversion Electron Mossbauer Spectroscopy, Scanning Electron Microscopy and Vickers microhardness. Plasma nitriding was carried out in a medium of 80% H2 and 20% N2, at 400 °C, for 4 h, pressures between 3 and 7 torr. A modified layer, whose depth ranging from 5.4 to 6.6 μm, was observed for all the nitrided samples, regardless of the nitriding pressure. The sample treated at 7 torr showed the best result of hardness. Besides γN phase, the hexagonal e phase (Fe2 + xN) was also observed on the Mossbauer spectra up to 0.4 μm depth of the nitrided samples. X-ray diffraction spectra were analyzed by full profile fitting, using the triclinic structure for the γN phase. The results indicated that nitrogen concentration reaches a maximum for a pressure of 4 torr.

The comparison of the growth models of silicon nitride ultrathin films fabricated using atmospheric pressure plasma and radio frequency plasma

The reaction process model during initial nitridation of Si (111) using atmospheric pressure plasma source was constructed and it was compared to that using a radio frequency plasma source. In atmospheric pressure plasma, emission lines from the N2 second positive system were dominantly observed. By exposing the atmospheric pressure plasma to Si substrate at the temperature ranging from 25to500°C, silicon nitride films with a thickness below 1.8nm were formed. In order to study the nitridation process, the changes in the film thickness against the substrate temperature and nitridation time were systematically studied at a pressure ranging from 50to700Torr. The film thickness increases with increasing the nitridation pressure below 400Torr and it saturates above 500Torr. It was completely regardless of the substrate temperature. From the time dependence of the film thickness at various nitridation pressures, it was revealed that these experimental results were well fitted to a Langmuir-type adsorption model. In the case of nitridation using atmospheric pressure (AP) plasma, molecular species play an important role for nitridation without thermal diffusion. The difference of silicon nitride films fabricated using AP plasma and rf plasma originates from the difference in the active species.