Nitride precipitation in salt-bath nitrided interstitial-free steel

Nitride precipitation and its effect on microstrain in salt-bath nitrided interstitial-free steel were investigated using transmission electron microscopy and neutron diffraction. As the cooling rate after nitriding decreased, two nitrides, γ′-Fe₄N and α"-Fe₁₆N₂, were identified in diffusion zone. Combined analyses using Rietveld whole-profile fitting and size–strain analysis revealed that the microstrain in the nitrided specimen increased due to nitrogen supersaturation and then decreased after nitride precipitation, whereas the effective particle size continuously decreased. It was found that microstrain is the dominant factor in peak broadening of the nitrided specimen.     

The effect of laser surface nitriding with a spinning laser beam on the wear resistance of commercial purity titanium

Laser nitriding of commercial purity titanium using various concentrations of helium and nitrogen has been carried out. The surface appearance and microstructure of a treated layer were found to be dependent on the beam power density, interaction time, velocity and concentration of nitrogen. X-ray diffraction analyses have led to the conclusion that the dendrite layer in the resolidified zone of the nitrided specimens consisted mainly of TiN. The surface roughness of specimens after various laser treatments was investigated by SEM and a surface profilemeter. Using optical microscopy, the dendrite TiN and needle-like structure in the melt zone, and the large grain structure in the heat affected zone, were investigated. The surface wear resistance of nitriding CPTi was significantly improved compared to the untreated or laser glazed material, and the wear data were found to correlate with scanning electron microscopy observations. Two layers, having different microstructures, thickness and abrasive wear resistance were identified. Further, 100% overlapping considerably improved the wear resistance of the nitrided specimens.     

Characteristics of the nitrided layer formed on AISI 304 austenitic stainless steel by high temperature nitriding assisted hollow cathode discharge

A series of experiments have been conducted on AISI 304 stainless steel using a hollow cathode discharge assisted plasma nitriding apparatus. Specimens were nitrided at high temperatures (520–560 °C) in order to produce nitrogen expanded austenite phase within a short time. The nitrided specimen was characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, potentiodynamic polarization and microhardness tester. The corrosion properties of nitrided samples were evaluated using anodic polarization tests in 3.5% NaCl solution. The nitrided layer was shown to consist of nitrogen expanded austenite and possibly a small amount of CrN precipitates and iron nitrides. The results indicated that rapid nitriding assisted hollow cathode discharge not only increased the surface hardness but also improved the corrosion resistance of the untreated substrate.     

Effect of plasma on nitriding of Fe-18Cr-9Ni alloy

The plasma nitriding behaviour of Fe-18Cr-9Ni alloy was compared with gas nitriding. The alloy was nitrided under the following conditions: specimen temperature: 823 K, nitriding time: mainly 108 ks, total pressure: 0.4–0.7 kPa, mixture ratio of N₂ and H₂∶ 0.25, discharge voltage: 350–450 V, current: 0.8–1.1 A. Formation of a surface layer of iron nitrides was not observed. Formation of a homogeneous internal nitriding layer, consisting of small precipitates of CrN and the γ-phase matrix, was, however, noted. The lattice constant at the specimen surface was smaller than that at greater depth. This may have been because the sputtering effect decreased the dissolved nitrogen content at the specimen surface. The sputtering of iron nitrides at the specimen surface by the plasma was experimentally confirmed through γ′-Fe₄N formation on Si beside an alloy specimen. The characteristics of the plasma nitriding mentioned above are discussed in relation to the sputtering.     

Influence of nitriding on corrosion resistance of martensitic X17CrNi16‐2 stainless steel

Nitriding increases surface hardness and improves wear resistance of stainless steels. However, nitriding can sometimes reduce their corrosion resistance. In this paper, the influence of nitriding on the corrosion resistance of martensitic stainless steel was investigated. Plasma nitriding at 440 °C and 525 °C and salt bath nitrocarburizing were carried out on X17CrNi16‐2 stainless steel. Microhardness profiles of the obtained nitrided layers were examined. Phase composition analysis and quantitative depth profile analysis of the nitrided layers were preformed by X‐ray diffraction (XRD) and glow‐discharge optical emission spectrometry (GD‐OES), respectively. Corrosion behaviour was evaluated by immersion test in 1% HCl, salt spray test in 5% NaCl and electrochemical corrosion tests in 3.5% NaCl aqueous solution. Results show that salt bath nitrocarburizing, as well as plasma nitriding at low temperature, increased microhardness without significantly reducing corrosion resistance. Plasma nitriding at a higher temperature increased the corrosion tendency of the X17CrNi16‐2 steel.     

Duplex surface treatment of AISI 1045 steel via plasma nitriding of chromized layer

In this work AISI 1045 steel were duplex treated via plasma nitriding of chromized layer. Samples were pack chromized by using a powder mixture consisting of ferrochromium, ammonium chloride and alumina at 1273 K for 5 h. The samples were then plasma-nitrided for 5 h at 803 K and 823 K, in a gas mixture of 75%N₂ + 25%H₂. The treated specimens were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis and Vickers micro-hardness test. The thickness of chromized layer before nitriding was about 8 μm and it was increased after plasma nitriding. According to XRD analysis, the chromized layer was composed of chromium and iron carbides. Plasma nitriding of chromized layer resulted in the formation of chromium and iron nitrides and carbides. The hardness of the duplex layers was significantly higher than the hardness of the base material or chromized layer. The main cause of the large improvement in surface hardness was due to the formation of Cr_xN and Fe_xN phases in the duplex treated layers. Increasing of nitriding temperature from 803 to 823 K enhanced the formation of CrN in the duplex treated layer and increased the thickness of the nitrided layer.     

Nitriding behaviour of maraging steel: experiments and modelling

The microstructure and the kinetics of growth of the nitrided zone of a Mo-containing maraging steel were investigated by performing gaseous nitriding at temperatures between 713 K (440 °C) and 793 K (520 °C) and at nitriding potentials up to 0.5 atm^?1/2 for both solution-annealed and precipitation-hardened specimens. The microstructure of the nitrided zone was investigated by means of X-ray diffraction (phase constitution; crystal imperfection). Fine, initially largely coherent Mo₂N-type precipitates developed in the nitrided zone. The elemental concentration-depth profiles were determined employing glow discharge optical emission spectroscopy (GDOES). The nitrogen content within the nitrided zone exceeds the nitrogen content expected on the basis of the molybdenum content and the equilibrium solubility of nitrogen in a (stress-free) ferritic matrix: excess nitrogen occurs. A numerical model was applied to predict the nitrogen concentration-depth profile within the nitrided layer. The model describes the dependence on time and temperature of the nitrogen concentration-depth profiles with, as fit parameters, the surface nitrogen concentration, the diffusion coefficient of nitrogen in the matrix, a composition parameter of the formed nitride and the solubility product of the nitride-forming element and dissolved nitrogen in the matrix. Initial values for the surface nitrogen concentration and the composition parameter were determined experimentally with an absorption isotherm and fitted to the measured nitrogen concentration-depth profiles. The results obtained revealed the striking effects of the amount of excess nitrogen and the extent of precipitation hardening on the developing nitrogen concentration-depth profile.     

Chromizing of plasma nitrided AISI 1045 steel

In this study AISI 1045 steel specimens were plasma nitrided at 803 K for 5 h, in a gas mixture of 75% N₂ + 25% H₂. The specimens were then chromized in powder mixtures consisting of ferrochromium, ammonium chloride and alumina at 1273 K for 5 h. Scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis and Vickers micro-hardness test were used as characterizing techniques. The thickness of white nitrided layer was around 5 μm, which was mainly composed of iron nitrides and its hardness was around 740 HV. Chromizing of nitride layer resulted in formation of Cr₂N chromium nitride and Fe₃N iron nitrides. A significant increase was observed in hardness after chromizing of the nitrided layer. Despite its higher hardness, the post chromised specimen showed higher wear rate than single plasma nitrided specimen.     

Increasing the wear resistance of stainless steels

The wear resistance of martensitic, austenitic and duplex stainless steels is increased by carbon and nitrogen in solid solution and in addition by carbides and nitrides, which are formed during conventional ingot metallurgy or by powder metallurgical processing. High temperature and low temperature nitriding provide a hard surface zone. Characteristic alloys and applications are discussed, which try to optimize the resistance to wear and corrosion.     

Corrosion behavior of ion nitrided AISI 316L stainless steel

In the present work the corrosion susceptibility of ion nitrided AISI 316L stainless steel was investigated for two different nitriding times and compared with the corrosion susceptibility of the untreated material. Plasma nitriding for short times (30 min) produced the “S” phase or expanded austenite (γ_N), with a thickness of ∼ 5 μm and a micro-hardness of 1300-1400 HV_0.025 (6.5 times higher than the untreated material). Plasma nitriding for long times (6 h) resulted in the precipitation of iron and chromium nitrides.To evaluate the corrosion resistance of both untreated and nitrided samples, anodic potentiodynamic polarization curves and immersion tests were performed in 1 M NaCl at room temperature. It was found that the corrosion resistance depends on the nitriding time. Samples nitrided for half an hour developed a much better corrosion resistance - close to that observed in the untreated samples - than those nitrided for 6 h. Samples nitrided for half an hour showed high roughness probably due to the presence of sliding bands developed in the expanded austenite phase. These sliding bands provide appropriate sites for the developing of the corrosion process. This would explain the results obtained in the corrosion tests. Samples ion nitrided for 6 h showed a severe and massive surface damage due to corrosion.Ion nitriding of AISI 316L stainless steel for short periods of time (30 min in the present case) may be an interesting surface treatment process that efficiently improves the surface hardness of the steel with some reduction in its corrosion resistance.     

Effects of nitriding on fatigue strength of quenched and tempered steel: role of interstitial nitrogen

Abstract

This paper describes an investigation into the effects of nitriding, denitriding, and renitriding, on the rotating bending fatigue strength of notched specimens of the quenched and tempered steels 24CrMo13 and 42CrMo4. The effects of the different treatments were characterised by microstructural investigation, hardness measurements, determination of the nitrogen and carbon contents in the diffusion layer, and determination of surface macrostresses. Nitriding induced a doubling of the fatigue limit. Denitriding only partly removed the effect of the nitriding treatment, because the strength increase caused by precipitation of alloying element nitrides remained unchanged, and only the effect of interstitially solved nitrogen disappeared. For nitrided and renitrided specimens an approximately linear relationship was found between the surface hardness and the fatigue strength. Denitriding caused a larger decrease in the fatigue strength than could be explained by the loss of surface hardness. The change in macrostresses from compressive to tensile also played a part. The role of interstitially dissolved nitrogen in determining the fatigue strength was found to be larger than assumed thus far.     

Effects of conventional and active screen plasma nitriding on properties of electroless Ni–B coating

Abstract

In the present study, the properties of nitrided electroless Ni–B coatings prepared by conventional plasma nitriding and active screen plasma nitriding were investigated. For this purpose, electroless Ni–B coatings were deposited from an alkaline bath on AISI 4140 substrates. Then, some of the prepared coatings were plasma nitrided by conventional method and the other ones by active screen method under the same conditions. Microstructure, morphology, microhardness and wear resistance of the coatings were evaluated. Based on the results, post-treatments change the amorphous as deposited coating structure to a crystalline one, which increases microhardness and wear resistance. Employing plasma nitriding treatment on the coatings results in higher microhardness and superior wear resistance than conventional heat treatment. The sputtering of iron atoms during plasma nitriding process can be the main reason for these results. In addition, active screen plasma nitriding demonstrates less surface roughness and superior wear resistance than conventional plasma nitriding.     

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

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.     

The wear and corrosion resistance of shot peened–nitrided 316L austenitic stainless steel

316L austenitic stainless steel was gas nitrided at 570 °C with pre-shot peening. Shot peening and nitriding are surface treatments that enhance the mechanical properties of surface layers by inducing compressive residual stresses and formation of hard phases, respectively. The structural phases, micro-hardness, wear behavior and corrosion resistance of specimens were investigated by X-ray diffraction, Vickers micro-hardness, wear testing, scanning electron microscopy and cyclic polarization tests. The effects of shot peening on the nitride layer formation and corrosion resistance of specimens were studied. The results showed that shot peening enhanced the nitride layer formation. The shot peened–nitrided specimens had higher wear resistance and hardness than other specimens. On the other hand, although nitriding deteriorated the corrosion resistance of the specimens, cyclic polarization tests showed that shot peening before the nitriding treatment could alleviate this adverse effect.     

Comparison of compound layer nucleation mechanisms in plasma nitriding and nitrocarburizing: the effect of CHn species

This article presents a new study of the mechanisms of plasma nitriding and nitrocarburizing processes, with particular emphasis on plasma diagnostics and light element detection at the treated sample surface. The results indicate that in the nitriding process neutrals, such as Fe, N and ions (such as N⁺), are abundantly present near the cathode region, suggesting that the formation of iron nitrides occurs through the combination of these species. Light element detection by elastic recoil analysis (ERA) shows that hydrogen is not present on the nitrided surface, but it is abundant on the nitrocarburized surface. The latter result is discussed in view of existing nitriding models. The role of CH_n radicals in plasma nitrocarburizing is also discussed.     

Microstructure and mechanical properties of the modified layer obtained by low temperature plasma nitriding of nanocrystallized 18Ni maraging steel

In the present study, low temperature plasma nitriding of nanocrystallized 18Ni maraging steel has been carried out at 360 °C from 1 to 24 h in a mixed gas of 25%N₂ + 75%H₂. The surface phase constitutions and microstructures of the nitrided layer have been investigated by X-ray diffraction analysis, transmission electron microscopy and optical microscopy. Nanoindentation and microhardness tests have been performed to determine the surface hardness and the hardness profile in the nitrided layer. The plasticity of the nitrided surface has been analyzed based on the nanoindentation results. The results show that at the initial stage of nitriding, the surface phase consists of a solid solution of nitrogen in α-Fe, and nanoscale nitrides and aging phase are formed with increasing of treatment time. The surfaces nitrided for 8 and 16 h possess the highest hardness. The plasticity factor calculations suggest that the nitrided surfaces have a good wear resistance and possess excellent plasticity.     

Laser Nitriding of Ti-5.0Al-2.i5Sn

Laser surface treatment provides excellent wear resistance with good oxidation and corrosion resistance. Laser surface nitriding is one such technique resulting in high surface hardness to a depth of a few microns. This can be carried out in pure nitrogen and dilute nitrogen environments. This paper investigates the effect of laser nitriding on Space Shuttle Main Engine (SSME) Ti-5.0Al-2.5Sn alloy under pure nitrogen environment. The nitriding was carried out using 3 kW CW CO2 laser at different laser powers 900 W, 1.0 kW, and 1.2 kW with scan speeds 0.5 m min^-1, 1.0 m min^-1 and 1.5 m min^-1 respectively. Optical microscopic and Vickers hardness tests were conducted on the test specimen to reveal the effect of laser nitriding in melt zone of laser nitrided trail. The extra high surface hardness of 3785 VHN at 25-50 (m depth was observed using the laser variable 1.0 kW laser power, 1.0 m min^-1 speed and 3 mm beam dia. This may be attributed to the TiN dendrite formation. The melt zone of laser nitrided trail at other processing parameters shows fine needlelike structure of alpha prime with larger grain size and alpha in the heat affected zone with smaller grain size, with an average hardness 450 VHN. This present investigation shows that the surface of the nitrided trail is free from any cracks, even under the pure nitrogen atmosphere for all laser processing conditions.     

Mathematical Models and Computer Simulation of Nitrogen Concentration Profiles in Pulse Plasma Nitrided Layers

Two materials, pure Fe and pure Al, were nitrided in a pulse plasma nitriding facility. The nitrogen profiles in surface layers and the surface phase structures of specimens nitrided at 500℃ for 8 h for Fe and for 6 h for Al were measured using the glow     

Enhancing the mechanical and anticorrosion properties of 316L stainless steel via a cathodic plasma electrolytic nitriding treatment with added PEG

A cathodic plasma electrolytic nitriding(CPEN) treatment with a urea aqueous solution was performed on 316L stainless steel to rapidly improve its surface properties in this work.Test results show that the PEG2000 macromolecules increased the nitriding energy via enhancing the ability to bond the produced gas film to the metal/electrolyte interface.The cross-sectional morphologies indicate that a thick nitrided layer was obtained when the urea concentration was 543 g I~(-1),corresponding to a Vickers hardness 450 HV_(0.1),which was 3.5 times larger than that of the substrate.The nitrided layer mainly contained expanded austenite(γ_N),oxides and iron nitrides(e.g.,Fe_3O_4 and FeN_(0.076)).In terms of its performance,coefficient of friction(COF) of the nitride layer decreased to nearly two-thirds that of the untreated layer,and the passivation current densities of the nitrided sample in a 3.5% NaCl solution decreased by an order of magnitude compared to that of the substrate.Therefore,the approach presented herein provides an attractive way to modify the effect of CPEN in a urea aqueous solution.