Fe2 3N Research Articles

Surface modification and wear test of carbon steel by plasma electrolytic nitrocarburizing

Plasma electrolytic nitrocarburizing (PEN/C) was applied to the surface of carbon steel under the boiling condition of saturated urea electrolyte. In addition to the general effect of the bath temperature, different applied voltages and processing times were also considered in this new process. Optical and scanning electron microscopy, X‐ray diffraction, microhardness and pin‐on‐disc wear tests were used to characterize the PEN/C‐treated surfaces. A mixture of θ‐(Fe3C) and ε‐(Fe2–3N) was found in the compound layers. At certain conditions, dense surface layers with minimum porosity were observed at the top of the samples. The boiling condition resulted in special character of the compound layers on the surface. The layers consisted of some irregularities grown inward the samples andaffected the characteristics of the surface layers. The microhardness of the PEN/C‐treated layers increased up to 1280 HV0.1, which was 3 to 4 times higher than that for untreated material and higher than that obtained by other investigators (750 HV0.1). PEN/C decreased the wear loss of carbon steel significantly due to the change of the adhesive wear of untreated material to the abrasive mode of treated surfaces. The major advantage of this technique was a higher growth rate of the nitrocarburized layers and a more significant improvement in the tribological performance of the treated samples if compared to similarly oriented surface treatments. Copyright © 2011 John Wiley & Sons, Ltd.

Plasma nitriding of 90CrMoV8 tool steel for the enhancement of hardness and corrosion resistance

The aim of the study is to apply a plasma nitriding process to the 90CrMoV8 steel commonly employed in wood machining, and to determine its efficiency to improve both mechanical and electrochemical properties of the surface. Treatments were performed at a constant N2:H2 gas mixture and by varying the temperature and process duration. The structural and morphological properties of nitrided layers were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with EDS microanalyses. Surface hardening and hardness profiles were evaluated by micro hardness measurements. To simulate the wood machining conditions, electrochemical tests were carried out with an oak wood electrolyte with the purpose of understanding the effects of the nitriding treatment on the corrosion resistance of the tool in operation.X-ray diffraction analyses revealed the presence of both γ′ (Fe4N) and ε (Fe2–3N) nitrides with a predominance of the ε phase. Moreover, α-Fe (110), γ′ and ε diffraction peaks were shifted to lower angles suggesting the development of compressive stresses in the post nitrided steel. As a result, it was shown that nitriding allowed a significant hardening of steel with hardness values higher than 1200HV. The diffusion layers were always composed of an outer compound layer and a hardened bulk layer which thickness was half of the total diffusion layer one. No white layer was observed. Similarly, no traces of chromium nitrides were detected. The temperature seemed to be a parameter more influent than the process duration on the morphological properties of the nitrided layer, while it had no real influence on their crystallinity. Finally, the optimal nitriding conditions to obtain a thick and hard diffusion layer are 500°C for 10h.On the other hand, to verify the effect of these parameters on the corrosion resistance, potentiodynamic polarization tests were carried out in an original “wood juice” electrolyte. After corrosion, surface was then observed at the SEM scale. Electrochemical study indicated that the untreated steel behaved as a passive material. Although the very noble character of steel was somewhat mitigated and the corrosion propensity increased for nitrided steels, the passive-like nature of the modified surface was preserved. For the same optimized parameters as those deduced from the mechanical characterization (500°C, 10h), surface presented, in addition to a huge surface hardening, a high corrosion resistance.

Effect of nature of nitride phases on microabrasion of plasma nitrided sintered iron

In the present investigation, the effect of plasma nitriding on the microabrasion behaviour of sintered unalloyed iron is investigated. Plasma nitriding was carried using two different sets of operative parameters in order to produce microstructures composed of different dominants iron nitrides, e.g.Fe4N (y´) and Fe2–3N(e), phases. Microabrasive wear tests were carried out in a 'free ball' microabrasion tester using SiC slurries. The tribological parameters were kept constant during the tests. The microstructures constituted of e-(Fe2–3N) phase presented a superior abrasion resistance.

Influence of layer microstructure on the corrosion behavior of plasma nitrided cold work tool steel

The influence of layer microstructure on the corrosion behavior of plasma nitrided cold work tool steel, of commercial name “DC53”, in 3.5% NaCl solution is reported. The specimens were nitrided at 520 °C for different treatment times using a constant [N2 + H2] gaseous mixture by a DC-pulsed plasma system. The microstructure of the nitrided layers was investigated by optical microscopy and X-ray diffraction. The corrosion behavior was evaluated by potentiodynamic polarization experiments. The plasma nitriding process considerably improves the corrosion resistance of material in NaCl environment as compared to the unnitrided DC53 steel. The modified surface layer consisting mainly of e-nitride (Fe2–3N) and a small amount of γ′-nitride (Fe4N) confers this outstanding behavior. The corrosion resistance dependence on specific nitriding processes is reported and the role of the e-nitride is discussed. In particular, the correlation of pitting current density, density of pits, and volume fraction of e-nitride with nitriding time is analyzed. The results denote that the most important parameter for controlling the corrosion resistance of the material is the volume fraction of e-nitride and the nitrided layer thickness. It is expected that a nitrided layer would be thicker and rich in e-nitride phase to achieve a high corrosion resistance.

Steam oxidation and surface hardness of power plant valve materials under the ultra super critical steam condition

Current supereritical steam power plants operate at 3,600 psi and 1,000°F. If the steam temperature is raised from 1,000 °F (538 °C) to 1,150 °F (621°C), the efficiency increases by 2%. Therefore, study on the high temperature corrosion of power plant materials under ultra-superciritical conditions (USC) is necessary to protect the plant from corrosion. In this study, valve materials of 17% Cr martensitic steels (17Cr steel), Incoloy 901 (1901) and their surface nitrided specimens were exposed to USC of 621 °C and 3600 psi (255 kg/cm2) steam for 200 °C, 400 °C, and 800 h. The oxidation of both 17Cr steel and 1901 under the USC for 800 h is very small due to the formation of a protective thin oxide layer formation on the surface. The USC oxidation of both nitrided specimens were increased due to the decomposition and formation of active nitrogen from the non protective nitrides such as Fe4N, Fe2–3N, and CrN. The oxidation of nitrided 17Cr steel (n17Cr steel) is about two times higher compared to nitrided 1901 (n1901). The surface hardness is improved by more than two times near the surface by nitriding, and the degradation of hardness by USC oxidation is rapid for n17Cr steel, but slow for n1901.

Transmission Electron Microscopy Investigation on the Electron-stimulated Oxidation of Iron Nitrides by 2-MeV Electron Irradiation

An iron nitride sample was irradiated by 2-MeV electrons intermittently for 2100 s with a dose rate of 6.3 × 1024 e.m.−2 s−1 inside a 3-MV high-voltage transmission electron microscope. The electron-stimulated oxidation of Fe4N and Fe2–3N was investigated in situ and ex situ using conventional transmission electron microscopy and high-resolution electron microscopy. It was found that both Fe4N and Fe2–3N nitrides were oxidized by the residual gas in the vacuum chamber to form Fe3O4 oxides. The orientation relationship between Fe4N (γ′) and Fe3O4 (o) was (110)γ′//(220)o, [001]γ′//[001]o, and that between Fe2–3N (ϵ) and Fe3O4 (o) was (110)ϵ//(−220)o, [1–11]ϵ//[001]o. Crystal lattice deformation from iron nitride to iron oxide took place during the dynamic oxidation process. Structural models were proposed to understand the oxide formation, and the models were confirmed by experimental observations. The irradiation effects of Fe4N and Fe2–3N crystals were compared. The results show that Fe4N is more sensitive than Fe2–3N to electron irradiation. These results are important not only for the fabrication of insulating iron oxide film, but also in the field of the surface modification of iron nitride to improve its mechanical properties.

Characterization of plasma-nitrided iron by XRD, SEM and XPS

The plasma nitriding technique has been used to improve the tribological and mechanical properties of materials, especially iron-based alloys. In this work, the pulsed glow discharge (PGD) technique was used for nitriding pure iron. Three samples were nitrided in a gas mixture of 80 vol.% H2 and 20 vol.% N2 under a pressure of 400 Pa, discharge frequency of 9 kHz, and temperature of 580 °C. Samples A, B and C were nitrided for 30 min, 60 min and 90 min, respectively. The nitrided iron samples were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). SEM micrographs showed differences in the surface morphologies among the samples. XRD identified the γ′-Fe4N and ε-Fe2–3N phases in all samples. The Rietveld analysis was performed in order to investigate the concentration variations of the phases as a function of nitriding time. XPS was employed to obtain chemical-state and semi-quantitative information, and revealed a thin film of Fe2O3 covering a Fe3O4 layer formed on top of the Fe2–3N and Fe4N layers.

Modification of composite nitrided layers by phosphorus compounds

Composite diffusive nitrided layers with precipitates of iron phosphides are considered. They are generated in the process of nitriding in an atmosphere of partly dissociated ammonia with a slight admixture of gaseous phosphorus. The addition of small quantities of gaseous phosphorus to the nitriding atmosphere influences the structure and properties of the nitrided layer. The layers consists of: a zone of phosphides of Fe3P and Fe2P, a zone of nitrides of Fe2–3N and Fe4N, and a zone of precipitates of iron phosphide Fe3P in the zone of nitrides of Fe2–3N and Fe4N. The relations between chemical contents of the atmosphere, as well as the parameters of the process, structure and properties of the layer, have been defined. Iron phosphide reinforcement in the diffusive nitrided layer increases its hardness and resistance to wear.

Phase identification during aging of some nitrided ferrous materials

A characterization study of the phases formed during aging of ferrous materials nitrided in a microwave post-discharge was performed by microhardness, electron microscopy (SEM, TEM and HREM) and X-ray diffraction. Iron and AISI-1010 steel samples were microwave nitrided for 10 min at 840 K and then quenched in Ar at 273 K. The quenched samples were then annealed for treatment times ranging from 0.5 h to 8 h at 673 K. The results from the microstructural characterization of steel show the presence of a compact surface layer composed of epsilon nitride (Fe2–3N). For iron, Fe4N needles appeared underneath the surface after nitriding. During annealing, the amount, size and morphology of the needles change as the treatment time progresses. The presence of Fe16N2 was also detected after aging. The amount of this phase increases with aging time. The microhardness measurements correlate with the microstructural changes.

Study on the structure and wear resistance of two-step salt bath nitrocarburized steel

This paper deals with the investigation of two-step salt bath nitrocarburizing technologies. For comparison, ferritic nitrocarburizing was also conducted. By means of metallographic examination, microhardness test, X-ray diffraction analysis, the microstructure, phase construction, surface hardness and hardness profiles of case hardened layers on steel treated by various nitrocarburizing techniques were investigated. The friction coefficient and ring-on-block apparatuses were employed to investigate the friction coefficient and wear-resistance of case hardened layers. Meanwhile, the surface roughness of various nitrocarburized specimens was also tested by a stylus profilometer. The experimental results indicate that thicker compound layers are achieved on the specimen surfaces treated by two-step nitrocarburizing than by ferritic nitrocarburizing; the phase constructions of two-step salt bath nitrocarburized specimen surfaces are different from that observed on ferritic nitrocarburized specimens and two distinct layers are clearly visible on the cross-sections of two-step nitrocarburized specimens which consists predominately of ε-compound (Fe3N) with a tiny minority of Fe3O4 (outer layer) and carbon/nitrogen martensite with a high percentage of austenite (inner later), while the phase construction of specimen surfaces treated by ferritic nitrocarburizing is composed mainly of ε-compound (Fe3N) and a small amount of Fe4N and Fe2–3N.

Sonochemical synthesis of iron nitride nanoparticles

A method for the preparation of nanoparticles of iron nitride powders is reported. Iron nitride particles have been synthesised by two methods. In the first, Fe(CO)5 was sonicated in a decane solution under a gaseous mixture of NH3 and H2 (3.5:1 molar ratio) at ca. 0 °C. The second method was based on nitriding the sonochemically prepared amorphous iron at ca. 400 °C for 4 h under a mixed stream of NH3–H2(3.5:1 molar ratio). Different products were obtained in the two cases. The product of the sonication of Fe(CO)5 was amorphous Fe2–3N and a small quantity of iron oxide. The X-ray diffraction patterns in the second case showed Fe4N as a main product. The magnetic properties of both products were measured. The coercive force HC of the Fe4N is 190 Oe, and the saturation magnetization sigma;s is 170 emu g–1 .

Iron nitride thin films prepared by facing targets sputtering

The iron nitride thin films have been deposited using a facing targets sputtering (FTS) system and a conventional dc diode magnetron sputtering system, and their crystallographic characteristics and magnetic properties have been investigated. The total gas pressure (PN2+PAr) was set at 2 mTorr. The films deposited by the FTS system without heating the substrate at PN2 below 0.3 mTorr possessed α-Fe-like structure with an increased lattice constant. At PN2 of around 0.5 mTorr, a two-phase mixture of α-Fe and Fe2–3N was formed in the films prepared in the FTS system, while ζ-Fe2N phase was formed in the films prepared by dc magnetron sputtering. The films prepared by FTS at PN2 of 0.1 mTorr possessed α-Fe and γ′-Fe4N structures when a substrate temperature (Ts) was in the range from 80 to 300 °C. The films deposited at Ts of about 150 °C possessed a saturation magnetization (Ms) of about 1700 emu/cm3, which is larger than that of pure iron films and coercivity (Hc) of about 1.5 Oe.

Formation and growth of iron nitrides during ion-nitriding

In order to clarify the formation and growth kinetics of iron nitrides, Fe2N (ζ), Fe2–3N (ɛ) and Fe4N (γ′), on the surface of iron during ion-nitriding, and their contribution to the mechanism of ion-nitriding coarse-grained specimens (2 to 6 mm) were ion-nitrided in pure nitrogen and nitrogen-hydrogen (20%–80%) plasma at temperatures between 500 and 600°C. Reflection electron diffraction (RED) showed immediate formation of the nitrides. Growth of these phases in latter stages of ion-nitriding was studied using optical microscopy and X-ray diffraction. The mechanism of the nitride layer formation is discussed and compared with the existing gas nitriding data. In the case region, Fe4N (γ′) and Fe16N2 (α″) precipitation was observed to occur under all experimental conditions studied. Case depth is seen to be parabolic with time and nitriding rate increases slightly when nitrogen-hydrogen plasma is used. Discussions are given to explain the difference in the nitriding efficiency under two different plasma compositions.