Nitride Compound Layer Research Articles

Effect of Temperature on Plasma Nitriding of AISI410 Martensitic Stainless Steel

In this study, the AISI410 martensitic stainless steel was treated by plasma nitriding. Three different temperature were employed (420°C ， 460°C， 500°C) for 6h. The microstructure, hardness and corrosion properties of the nitrided samples were characterised by a variety of analytical techniques. The result shows that the hardness of the nitrided samples is four times the size of the untreated samples and the thickness of the compound layer will decline when the nitriding temperature increases. In the electrochemical corrosion tests, the samples nitrided at 420°C and 460°C showed lower current density and corrosion rate than the original sample because of the formation of nitrided compound layer on the martensitic stainless steel. In particular, the corrosion properties of nitriding samples(420°C) are five times larger than original samples. But it reducesnitriding at high temperature(500°C) because of the precipitation of chromium.

First results of a new duplex surface treatment for cast iron: Electron beam remelting and plasma nitriding

Cast irons exhibit very good machinability but very low resistance against wear and corrosion. Furthermore, nitriding of cast irons does not produce satisfactory results. This paper suggests a new duplex surface technology for elimination of these disadvantages. In an initial processing step, the surface of cast iron material was remelted by electron beam (EB) up to a certain depth (0.3 to 1.0mm). The graphite in this layer is dissolved and, as a result of the rapid solidification process, a very fine ledeburite and interdendritic martensite or perlite forms. This microstructure is nitrided in the second step, and a dense, closed nitride compound layer is produced. The paper will present current results concerning electron beam remelting of different cast irons (GJL-250, GJS-600, GJV-300) followed by plasma nitriding. The experimental results were characterised by visual methods, microstructural investigations, and hardness measurements. As a result of the EB surface remelting, graphite can be completely eliminated from the surface area, and nitrideability was also significantly improved. Unlike the untreated base material, the remelted surface is fully covered by the compound layer. According to processing parameters and chemical composition of the base materials, compound layer thicknesses varied from 8μm to 14μm. Surface hardness improved from~200 HV 0.3 (as-cast) to~850 HV0.3 after duplex treatment without preheating, and to 650 HV0.3 with preheating. A considerable improvement in wear and corrosion resistance is expected.

Investigation of the effects of time and temperature of oxidation on corrosion behavior of plasma nitrided AISI 4140 steel

In this paper, the effect of the time and temperature of the post oxidation process on corrosion behavior of AISI 4140 plasma nitrided steel has been studied. Plasma nitriding was carried out at 550°C for 5h with a gas mixture of N2/H2:3/1. The plasma nitrided samples were post oxidized for 30 and 60min with a gas mixture of O2/H2: 1/5 at temperatures of 400 and 450°C. The treated samples were characterized by optical microscopy, SEM, XRD, roughness testing, electrochemical polarization and salt spray testing. The X-ray analysis revealed the formation of the iron-nitride phases ε and γ′ during plasma nitriding and the iron oxide phases of magnetite and hematite through the post-oxidizing treatment. It was also demonstrated that the corrosion characteristics of the nitrided compound layer were further improved by post-oxidation treatment and that the least corrosion rate and optimum condition relative to pitting corrosion belongs to the oxidized sample at 450°C for 30min.

Improvement of corrosion resistance of nitrided low alloy steel by plasma post-oxidation

Post-oxidizing treatments can be performed to improve the corrosion resistance of nitrided steel samples. In this paper, plasma nitriding treatments were performed at 540 °C for 4 h using ammonia as the working gas, and plasma post-oxidizing treatments were carried out at temperatures ranging from 350 °C to 500 °C for 2 h in oxygen gas. The treated samples were characterized by using optical microscopy, SEM, XRD, and electrochemical polarization. The X-ray analysis revealed the formation of iron-nitride phases of ɛ-Fe 2–3N and γ′-Fe 4N during plasma nitriding and iron oxide phases of hematite (Fe 2O 3) and magnetite (Fe 3O 4) through the post-oxidizing treatment. In particular, it was found that the very thin magnetite layer 0.8–1.5 μm in thickness on top of the compound layer was obtained by plasma post-oxidized at 400 °C and 450 °C. It was also demonstrated that the corrosion characteristics of the nitrided compound layer were further improved by post-oxidation treatment.

Feasibility study of active screen plasma nitriding of titanium alloy

A feasibility study of active screen plasma nitriding (ASPN) of Ti6Al4V titanium alloy was conducted using a CP-Ti cage as the active screen under various process conditions. The process variables included active screen setup parameters (two different potentials of 0%-bias and 100%-bias), treatment temperature (750 and 850°C), gas mixture (10/90, 25/75, 50/50 and 100/0 of N2/H2) and treatment time (10 h) under 1·5 mbar pressure. The samples were characterised by X-ray diffraction, optical microscopy, scanning electron microscopy, microhardness and surface roughness tests. The experimental results have demonstrated that Ti–6Al–4V alloy can be treated by ASPN using a Ti screen. A Ti nitride compound layer consisting of TiN followed by Ti2N was formed on top of the surface and an Al rich layer was formed between the Ti nitride layer and the α-Ti case. The surface hardness of the samples nitrided at 750 and 850°C was 800 and 1400 HV respectively.

Enhanced corrosion resistance properties of radiofrequency cold plasma nitrided carbon steel: Gravimetric and electrochemical results

Cold plasma nitriding treatment was performed to improve the corrosion resistance of C38 carbon steel. Nitriding process was conducted using a radiofrequency nitrogen plasma discharge for different times of treatment on non-heated substrates. The modification of the corrosion resistance characteristic of the C38 steel due to the treatment in acid medium (1 M HCl) were investigated by gravimetric and electrochemical tests such as potentiodynamic polarisation curves and electrochemical impedance spectroscopy (EIS). It was shown that the plasma nitriding treatment improves the corrosion resistance. Indeed, in the gravimetric tests, nitrided samples showed lower weight loss and lower corrosion rate in comparison to untreated one. In the Tafel polarisation tests, the nitrided samples showed greatly reduced corrosion current densities, anodic dissolution and also retarded the hydrogen evolution reaction. Using EIS method, an adequate structural model of the interface was used and the values of the corresponding parameters were calculated and discussed. The results obtained from weight loss and electrochemical studies were in reasonable agreement. X-ray photoelectron spectroscopy (XPS) was carried out to establish the mechanism of corrosion inhibition of nitrided C38 steel in 1 M HCl medium. The enhancement of the corrosion resistance is believed to be related to the iron nitride compound layer formed on the C38 steel surface during plasma nitriding, which protected the underlying metal from corrosive attack in the aggressive solutions.

Surface modification of steel surfaces by electron beam excited plasma processing

The nitriding performance of low power electron beam excited plasma is investigated by characterizing the surface of nitrided low alloy Cr–Mo steels. In this research, a particular attention was given to the effect of the acceleration voltage and processing time on the composition and hardness of the processed samples. In an attempt to maximize the dissociation of N 2, the acceleration voltages applied in our experiments were set within a range that corresponds to the maximum dissociation cross-section of N 2. The results show that the peak intensity of the alpha Fe observed for the unprocessed sample decreases as the acceleration voltage increases. Moreover, the peak intensities depicting the formation of the nitride compound layers, Fe 4N and Fe 3N phases, increases with the acceleration voltage of the electron beam. Consequently, the surface hardness of the treated low alloy steel was increased by more than two times that of the unprocessed specimen.

X-ray diffraction line-profile analysis of hexagonal ϵ -iron nitride compound layers: composition–and stress–depth profiles

Two hexagonal ε-Fe3N1+ x layers grown on α-Fe substrates by nitriding in NH3/H2 gas atmospheres were investigated by high-resolution X-ray powder diffraction using synchrotron radiation employing systematic tilting of the diffraction vector with respect to the specimen surface. Considering all recorded reflections simultaneously, the complicated diffraction profiles obtained were analyzed using a model incorporating hkl-dependent (anisotropic) and tilt angle (ψ)-dependent diffraction-line broadening and diffraction-line shifting. The diffraction-line broadening is mainly ascribed to the nitrogen concentration–depth profile within the layers causing depth-dependent strain-free lattice parameters, whereas the line shifts are predominantly caused by the stress–depth profile originating from the concentration-dependence of the coefficients of thermal expansion of the ε phase, with stress parallel to the surface, which is of tensile nature at the surface and of compressive nature at the ε/γ′ interface. This stress gradient additionally leads to a ψ dependence of the line broadening. Fitting of the microstructure and diffraction model led to determination of microstructure parameters, which can be related to the different sets of treatment conditions applied for the ε-iron nitride layer growth.

Surface microstructure of 316L austenitic stainless steel by the salt bath nitrocarburizing and post-oxidation process known as QPQ

Systematic materials characterization of the Quench-Polish-Quench complex salt bath heat-treatment process (QPQ) surface modified 316L steel was investigated. The results reveal that the nitro-carburized sample surfaces consist of Cr 2N/γ´-Fe 4N and CrN/γ-Fe, while the post-oxidized sample surfaces are comprised of CrO 3, (Fe 3O 4)/ε-Fe 2(N,C), γ′-Fe 4N and S(γ N)/CrN/α-Fe. Nuclei ε-Fe 2N 1 − x accumulates at the interface between oxide layer and nitride compound layer by the help of post-oxidation. The diffraction peak lines of S-phase (γ N) move gradually towards higher diffraction angles as increasing depth of 15 μm to 35 μm. An increased content of oxygen is recorded in the post-oxidized surface layer down to the depth of approximately 15 μm, a small peak concentration of carbon occurs at the front of the nitrided layer. Micro-hardness of the post-oxidized samples reaches about 1300 HV 0.1 near the surface region and then reduces sharply across the case/substrate interface.

Phase transformations in low-temperature chromized 0.45 wt.% C plain carbon steel

The phase transformations occurring in a 0.45 wt.% C plain steel subjected to plasma nitriding at 540–560 °C for 5.5 h, followed by a salt bath thermoreactive deposition and diffusion (TRD) chromizing process at 500 °C or 550 °C (a process referred to as low-temperature chromizing or duplex chromizing) was investigated by means of optical microscopy(OM), scanning electron microscopy(SEM), X-ray energy dispersive spectroscopy(EDS), and X-ray diffraction. It was found that a CrN compound layer with an average thickness of 7.4 μm and an average micro-hardness of 1476 HV 0.01 was formed in the prior plasma nitrided compound layer by low-temperature chromizing at 550 °C for 6 h. The chromized coating as a whole was found consisting of three sub-layers, namely the outer CrN layer, the intermediate diffusional layer, and the inner residual nitrided compound layer, all formed in the prior nitrided compound layer, and with the inner sub-layer vanishing by prolonging the chromizing time. The intermediate diffusional layer formed at the initial stages of TRD was seen “black” under OM (hence is called “black zone”), and found consisting of α-Fe as a major phase. The self-exhaustion of the “black zone” promoted the chromium atom diffusion deeper into the substrates. The transformation paths involved in the decomposition of the prior nitrided compound layer was likely to be ε-Fe 2–3N → γ′-Fe 4N → α-Fe; and the high hardness of the chromized coating was attributed to a large amount of nano-sized and evenly distributed CrN grains generated in the compound layer.

電子ビーム励起プラズマによるＣｒ‐Ｍｏ鋼の窒化

The nitriding performance of low power electron beam excited plasma was investigated by characterizing the compound layer formed on the surface of nitrided low alloy Cr-Mo steel specimens. In an attempt to maximize the dissociation of N2, the acceleration voltages applied in our experiments were set within a range that corresponds to the maximum dissociation cross section of N2. In this experiment, particular attention was given to the effect of the treatment time on the surface morphology, composition, and Vickers hardness of the treated specimen. The results show that the peak intensity of the alpha Fe observed for the untreated specimen decreased as the treatment time increased. Moreover, the peak intensities depicting the formation of the nitride compound layers, Fe4N and Fe3N phases, increased with the treatment time. Consequently, the surface hardness of the treated low alloy steel specimen was increased to 1010HV and a nitrided depth of up to 120μm was attained in 4.5 hours of treatment time.

Advanced Surface Hardening Techniques Using Gas Process

Air Water Incorporated Japan (AWI) has succeeded in developing and commercializing new surface treatment technologies. One of them is the NV Nitriding process. Through an activation treatment using fluorine-based gas for the surface treatment of metals before the nitriding treatment, it is possible to remove the oxide film, such as the passive film, formed on stainless steels and Ni-based alloys and also to nitride these alloys on a mass-production scale. The activation effect afforded by fluoride enables nitriding at a temperature about 150°C lower than other conventional processes, achieving minimal thermal deformation and strain. Also, with this activation process, the range of selection of nitriding conditions such as temperature, gas composition and heat pattern has been considerably extended to enable the formation of a nitride layer optimum for the required characteristics of the treated metals. That is, this process can control the thickness of the nitride compound layer. NV Pionite treatment also has been developed as the soluble carbon diffusion process by which to ensure high corrosion resistance and high hardness in austenitic stainless steels using the above activation treatment.

Laboratory assessment of wear on nitrided surfaces of dies for hot extrusion of aluminium

A newly designed “block-on-cylinder” wear resistance testing rig, which allows testing at higher contact pressures than conventional testing methods, was used to elucidate the effect of an iron nitride compound layer (white layer) on the wear resistance of nitrided dies used for hot extrusion of aluminium (Al). The tested dies (AISI H13) with various nitrided microstructures were provided by different manufacturers of equipment for plasma and gas nitriding. The wear surfaces were analysed by SEM, BEC, micrography and roughness measurements. It was found that the iron nitride compound layer is chemically more stable against hot Al in comparison to die material. Deterioration of the compound layer begins with cracking, and as a consequence, its spalling from the nitrided surface. A high thickness of the compound layer combined with a low nitriding depth leads to its earlier spalling and vice versa. Due to the increased roughness at the removal sites, accelerated chemical attack by hot Al takes place. Comparative and simultaneous testing of two nitrided samples was carried out.

Hydrogen absorption in plasma-nitrided iron

Hydrogen absorption by membranes prepared from various parts of plasma-nitrided iron specimens was studied using electrochemical techniques. The membranes were charged with hydrogen by galvanostatic cathodic polarisation in 0.1 M NaOH. After cessation of hydrogen charging, the desorption rate of hydrogen was measured at both sides of the membrane. An analysis of the desorption rates enabled to one distinguish the amounts (concentrations) of the diffusible and the reversibly trapped hydrogen in the nitrided compound layer, the diffusion zone and the unnitrided iron substrate. Trapped hydrogen accounted for the majority of the total absorbed hydrogen. Hydrogen trapping was enhanced by nitride precipitate–iron matrix interfaces, and especially by the nitrided compound layer.

Estimation of fracture toughness of nitride compound layers on tool steel by application of the Vickers indentation method

Nitriding and nitrocarburizing treatments are established methods of improving the wear performance of tool and die steels. However, our understanding of the relationship between nitriding process parameters, and microstructure and fracture behaviour of the surface layers is far from complete. This paper describes an investigation of the application of Vickers indentation fracture testing to nitrided and nitrocarburized AISI H11 hot work tool steel. Optical microscopy and microhardness testing were used to characterize structure and mechanical properties of the compound and diffusion layers. The results show that, where a sufficiently thick compound layer has formed, this method has the potential to be applied as a pseudo non-destructive method of monitoring the fracture properties of treated surfaces on actual tool parts. However, the validity of the method appears to be a function of the presence and thickness of the compound layer, and possibly the mechanical properties of the diffusion layer, and further work is proposed to establish the sensitivity of the method for fracture toughness calculations.

Corrosion properties of plasma nitrided AISI 410 martensitic stainless steel in 3.5% NaCl and 1% HCl aqueous solutions

Samples of an AISI 410 martensitic stainless steel were plasma nitrided at a temperature of 420 °C, 460 °C or 500 °C for 20 h. The composition, microstructure and hardness of the nitrided samples were characterised using a variety of analytical techniques. In particular, the corrosion properties of the untreated and plasma nitrided samples were evaluated using anodic polarisation tests in 3.5% NaCl solution and immersion tests in 1% HCl acidic water solution. The results showed that plasma nitriding produced a relatively thick nitrided case consisting of a compound layer and a nitrogen diffusion layer on the 410 stainless steel surface. Plasma nitriding not only increased the surface hardness but also improved the corrosion resistance of the martensitic stainless steel. In the immersion test, nitrided samples showed lower weight loss and lower corrosion rate than untreated one. In the electrochemical corrosion tests, the nitrided samples showed higher corrosion potentials, higher pitting potentials and greatly reduced current densities. The improved corrosion resistance was believed to be related to the iron nitride compound layer formed on the martensitic stainless steel surface during plasma nitriding, which protected the underlying metal from corrosive attack under the testing conditions.

Phase Transformations in ε-/γ'-Iron Nitride Compound Layers in the Temperature Range of 613 K - 693 K

ε/γ'-iron nitride (ε-Fe3N1+x, γ'-Fe4N) compound layers with thicknesses of about 10 µm were grown on pure α-Fe by gas nitriding at 823 K followed by quenching and were annealed at different temperatures in the range of 613 K – 693 K for different periods of time. These heat treatments led to a redistribution of nitrogen within the compound layer as well as between the compound layer and the adjacent ferrite, inducing thickness changes of the ε- and γ'-layers. The changes were analysed by light microscopy, electron probe microanalysis and X-ray diffraction. Models to describe and interpret the phase transformations in the ε/γ'-iron nitride compound layers as a function of time and temperature are discussed.

Ion nitriding of Fe–30Mn–6Si–5Cr shape memory alloyI. Nitriding parameters and microstructure characterization

Abstract The Fe–30Mn–6Si–5Cr shape memory alloy was ion-nitrided to modify the surface conditions. The phases and microstructures of the nitrided surface were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and glow discharge spectrometry (GDS). Experimental results indicate that the nitrided compound layer consists of Fe 0.93 Mn 0.042 N 0.028 and Mn 4 N phases. Higher nitriding temperature and longer nitriding time can have thicker nitriding layer and more quantities of nitrided compounds. [H 2 ]/[N 2 ]=1:99 is found to be an optimal parameter in the ion nitriding of Fe–30Mn–6Si–5Cr alloy in this study.

Ion nitriding of Fe–30Mn–6Si–5Cr shape memory alloy: I. Nitriding parameters and microstructure characterization

The Fe–30Mn–6Si–5Cr shape memory alloy was ion-nitrided to modify the surface conditions. The phases and microstructures of the nitrided surface were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and glow discharge spectrometry (GDS). Experimental results indicate that the nitrided compound layer consists of Fe 0.93Mn 0.042N 0.028 and Mn 4N phases. Higher nitriding temperature and longer nitriding time can have thicker nitriding layer and more quantities of nitrided compounds. [H 2]/[N 2]=1:99 is found to be an optimal parameter in the ion nitriding of Fe–30Mn–6Si–5Cr alloy in this study.

Nitrogen redistribution in ε/γ ′-iron nitride compound layers upon annealing

An ε/γ ′-iron nitride compound layer grown on α-Fe at 823 K by gas nitriding shows a nitrogen redistribution upon annealing at 630 K: an until now not observed “backwards growth” of the γ ′-sublayer occurs, accompanied by a nitrogen enrichment of the remaining ε-sublayer. The observed thickness changes can be explained theoretically.