Comparison of ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel

This paper compares the ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel carried out to improve the surface corrosion resistance. The gas composition for plasma nitriding was 85% N₂–15% H₂ and that for plasma nitrocarburizing was 85% N₂–12% H₂–3% CO₂. Both treatments were performed for 5 h, for different process temperatures of 570 and 620 °C for ferritic and austenitic plasma treatment, respectively. Optical microscopy, X-ray diffraction and potentiodynamic polarization technique in 3.5% NaCl solution, were used to study the treated surfaces. The results of X-ray analysis revealed that with increasing the treatment temperature from 570 to 620 °C for both treatments, the amount of ε phase decreased and γ′ phase increased. Nitrocarburizing treatment resulted in formation of a more amount of ε phase with respect to nitriding treatment. However, the highest amount of ε phase was observed in the ferritic nitrocarburized sample at 570 °C. The sample nitrided at 620 °C exhibited the thickest layer. The potentiodynamic polarization results revealed that after plasma nitriding and nitrocarburizing at 570 °C, corrosion potential increased with respect to the untreated sample due to the noble nitride and carbonitride phases formed on the surface. After increasing the treatment temperature from 570 to 620 °C, corrosion potential decreased due to the less ε phase development in the compound layer and more porous compound layer formed at 620 °C with respect to the treated samples at 570 °C.     

On the fundamental mechanisms of active screen plasma nitriding

The nitriding mechanisms of conventional DC plasma treatments have been extensively studied and discussed, but no general agreement has been reached thus far. The sputtering and redeposition theory is among the most accepted ones but, even though this mechanism is feasible, its contribution to the nitriding effect is under question. Furthermore, the novel active screen plasma nitriding technique has been successful in treating samples left at floating potential, where sputtering can not be considered to play a major role. Therefore, it has been proposed that the material sputtered from the cathodic mesh of the active screen furnace (auxiliary cathode) and deposited onto the treated specimens is involved in the mass transfer of nitrogen. The contribution made by this transferred material is the focus of attention of the present study. The hardening effect on the treated specimens showed considerable correlation with the deposition layer, and the XRD analysis of this deposited material yielded possible FeN and Fe_xN peaks. This finding supports the deposition of iron nitrides and their subsequent decomposition on the treated substrate as a mechanism of significance to the plasma nitriding treatments conducted in active screen experimental settings.     

Plasticity characterization of the modified layer produced by plasma nitrocarburizing of nanocrystallized 18Ni maraging steel

The plasma nitrocarburizing of nanocrystallized 18Ni maraging steel was performed at 460 °C for 4 h. The surface phase composition, cross-sectional microstructure and hardness profile of the nitrocarburized layer were investigated by the X-ray diffractometer (XRD), optical microscope (OM) and microhardness tester. Plasticity of the surface layer of original and nitrocarburized samples was analyzed by Taylor factor obtained by electron backscattering diffraction (EBSD) data and nanoindentation tests. The nitrocarburized surface is composed of α-Fe, Fe₄N and a small fraction of low nitrogen compound FeN_0.049. The surface and core hardness of nitrocarburized samples are 200% and 130% of that of the original one, respectively. The Taylor factors for different slip systems of α-Fe grains are all decreased after nitrocarburizing and Taylor factors for Fe₄N grains are lower than those of basal slip system of α-Fe grains. Plasticity factor η_p, i.e. the ratio of plastic deformation work to total deformation work dissipated during loading-unloading process, of the surface layer is reduced about 20% after nitrocarburizing. This suggests that plasticity and wear resistance of the surface layer could be decreased and improved after nitrocarburizing, respectively. The surface layer of the nitrocarburized sample also possesses certain plasticity because its plasticity factor η_p is more than 60%.     

Plasticity characterization of the modified layer produced by plasma nitrocarburizing of nanocrystallized 18Ni maraging steel

The plasma nitrocarburizing of nanocrystallized 18Ni maraging steel was performed at 460 °C for 4 h. The surface phase composition, cross-sectional microstructure and hardness profile of the nitrocarburized layer were investigated by the X-ray diffractometer (XRD), optical microscope (OM) and microhardness tester. Plasticity of the surface layer of original and nitrocarburized samples was analyzed by Taylor factor obtained by electron backscattering diffraction (EBSD) data and nanoindentation tests. The nitrocarburized surface is composed of α-Fe, Fe₄N and a small fraction of low nitrogen compound FeN_0.049. The surface and core hardness of nitrocarburized samples are 200% and 130% of that of the original one, respectively. The Taylor factors for different slip systems of α-Fe grains are all decreased after nitrocarburizing and Taylor factors for Fe₄N grains are lower than those of basal slip system of α-Fe grains. Plasticity factor η_p, i.e. the ratio of plastic deformation work to total deformation work dissipated during loading-unloading process, of the surface layer is reduced about 20% after nitrocarburizing. This suggests that plasticity and wear resistance of the surface layer could be decreased and improved after nitrocarburizing, respectively. The surface layer of the nitrocarburized sample also possesses certain plasticity because its plasticity factor η_p is more than 60%.     

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.     

Influence of increased voltage on the nitriding zone during plasma nitriding

Plasma nitriding is an established process for increasing the corrosion and wear resistance of steel. However, the possibilities of modern developments in the field of high-voltage plasma power supplies have been hardly investigated. For example, modern plasma generators allow improved arc management, which enable plasma nitriding at higher voltages. In the present work, the influence of increased voltage (up to 800 V) on the nitriding of a ferritic steel X38CrMoV5-1 was investigated. It was found that the thickness of the compound layer increases with increasing voltage. Especially at short process times the increased voltage leads to increased growth. An increase in the nitriding depth was also observed. Furthermore, the increased voltage has an effect on the composition of the compound layer too. A moderate increase in ϵ-nitride in the compound layer was observed. One explanation for the observed behavior is the over proportional increase in power with increasing voltage, indicating an increased ionization rate of the plasma. Due to this, more diffusible species would be available for nitriding. The presented results could be used to reduce process times, particularly where the formation of a compound layer is the aim of the process. An example of such a process is oxy-nitriding.     

Comparison of conventional and active screen plasma nitriding of hard chromium electroplated steel

This paper considers the nitriding behavior of hard chromium electroplated steel by conventional plasma nitriding (CPN) and active screen plasma nitriding (ASPN) methods. Indentation test along the cross-section of the treated samples reveals that duplex treatment performed by two methods exhibits almost the same hardnesses. Furthermore, an increase in the time of plasma nitriding from 5 h to 10 h restores 30% of the hardness decline. Morphological studies show that surface particles formed on active screen plasma nitrided samples have orderly formed geometrical shapes while in conventional plasma nitriding they are in cauliflower shape. The reason for reaction between chromium and nitrogen seems to be the difference between thermal expansion coefficient of chromium oxide, chromium and steel substrate which results in partial breakdown of the oxide film. Moreover, the reducing of chromium oxide by hydrogen promotes the process. It looks as if nitriding treatment changes the corrosion behavior of the chromium coating from severe localized to uniform corrosion. Also active screen plasma nitriding treatment lowers the anodic dissolution 50-100 orders of magnitude which would be the result of full closure of surface microcracks.     

Determination of nucleation and growth mechanisms

A model based on that of Gallon is presented which describes the variations in the Auger electron spectroscopy intensities of the relevant elements for adsorbates growing on a substrate according to various growth mechanisms. The results are compared with experimental data from the literature.Our calculations predict that platinum deposited onto a gold substrate grows according to the Volmer-Weber mechanism, which is supported by experimental data. Exposure of a platinum surface to ethylene at 500 K yields a carbonaceous overlayer which grows according to the monolayer mechanism under the given experimental conditions, indicating that the adsorbate is rather mobile on the surface.It is demonstrated by means of a simple model that the growth mode (formation of crystallites or formation of a monolayer) is determined by the difference in binding energy between an adsorbate (atom) in a monolayer and an adsorbate (atom) in the bulk of a crystallite and not by the difference in surface free energy between the adsorbate and the substrate as has been suggested in the literature. This is in accordance with experimental data in the literature for adsorbates (metals, CO, xenon) on various substrates. It is proposed that a similar parameter may determine whether another monolayer (Frank-van der Merwe mechanism) or crystallites (Stranski-Krastanov mechanism) will form on top of one (or more) monolayer(s). These energy parameters can be correlated with the refined Gallon model.     

Plasma nitriding of AISI 52100 ball bearing steel and effect of heat treatment on nitrided layer

In this paper an effort has been made to plasma nitride the ball bearing steel AISI 52100. The difficulty with this specific steel is that its tempering temperature (~170–200°C) is much lower than the standard processing temperature (~460–580°C) needed for the plasma nitriding treatment. To understand the mechanism, effect of heat treatment on the nitrided layer steel is investigated. Experiments are performed on three different types of ball bearing races i.e. annealed, quenched and quench-tempered samples. Different gas compositions and process temperatures are maintained while nitriding these samples. In the quenched and quench-tempered samples, the surface hardness has decreased after plasma nitriding process. Plasma nitriding of annealed sample with argon and nitrogen gas mixture gives higher hardness in comparison to the hydrogen–nitrogen gas mixture. It is reported that the later heat treatment of the plasma nitrided annealed sample has shown improvement in the hardness of this steel. X-ray diffraction analysis shows that the dominant phases in the plasma nitrided annealed sample are ε (Fe_2 − 3N) and γ (Fe₄N), whereas in the plasma nitrided annealed sample with later heat treatment only α-Fe peak occurs.     

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.     

Influence of oxygen in a plasma nitriding process

The influence of oxygen on the nitride layer formation on sintered steels was studied in a plasma nitriding reactor as a function of the gas mixture and sample composition. The nitride layers were characterized by metallographic and electronic microscopy techniques. The thickness, composition and microstructure of the layer were determined for three gas mixtures (100% N2, 75% N2+25% H2 and 90% N2+10% H2) in plain sintered iron in Fe-1.5% Si sintered alloy. The plasma chemistry was studied by optical spectroscopy and mass spectrometry as a function of the oxygen concentration in the atmosphere in the range 0–4%. It was observed that for low oxygen concentrations (O2≲4%), the layer thickness remain practically unaltered for the mixtures containing hydrogen, whereas the layer obtained when H2 was not used is completely damaged if oxygen is present in the gas mixture. When the gas discharge is in a N2-H2 mixture, a loss of oxygen is detected by mass spectrometry. These results are correlated with the sample analysis, and the depletion of the oxygen is interpreted in terms of oxygen-hydrogen reactions. This revised version was published online in November 2006 with corrections to the Cover Date.     

40Cr钢等离子软氮化的后氧化处理研究

为了进一步提高等离子软氮化后40Cr钢的耐蚀性能,对等离子软氮化的试样进行了后氧化处理.氧化处理是在保温式等离子热处理炉内不同比例的H2和O2气氛中进行,通过X射线衍射仪和恒电位仪对复合渗层的组织结构及其耐蚀性能进行了分析比较.结果表明,在H2:O2=1:1的混合气体中进行氧化处理的试样可以获得单一相的Fe3O4氧化膜,试样的耐蚀性能最好.     

Controlling of the electrical resistivity of GaN layer using AIN nucleation layer

The sheet resistance (Rs) of undoped GaN films on AIN/c-plane sapphire substrate was investigated. The Rs was strongly dependent on the AIN layer thickness and semi-insulating behavior was observed. To clarify the effect of crystalline property on Rs, the crystal structure of the GaN films has been studied using X-ray scattering and transmission electron microscopy. A compressive strain was introduced by the presence of AIN nucleation layer (NL) and was gradually relaxed as increasing AIN NL thickness. This relaxation produced more threading dislocations (TD) of edge-type. Moreover, the surface morphology of the GaN film was changed at thicker AIN layer condition, which was originated by the crossover from planar to island grains of AIN. Thus, rough surface might produce more dislocations. The edge and mixed dislocations propagating from the interface between the GaN film and the AIN buffer layer affected the electric resistance of GaN film.     

The effect of temperature on plasma nitriding behaviour of DIN 1.6959 low alloy steel

A series of experiments have been conducted on DIN 1.6959 low-alloy steel using a 5 kVA DC plasma nitriding apparatus with the aim of elucidating the role of treatment temperature in plasma nitriding process. Treatments were carried out in 75%N₂-25%H₂ atmosphere of 4 mbar for 5 h at temperatures ranging from 350 to 550 °C. Optical microscopy, scanning electron microscopy, X-ray diffraction, along with surface roughness and microhardness measurements were utilized to characterize the treated samples. The depth, microstructure, hardness profile and phase constituents of the nitrided layers as well as the surface roughness of the samples were assessed as a function of treatment temperature. The results suggested that the compound layers were mostly dual phase consisting of gamma prime and epsilon iron nitride phases. Increasing treatment temperature increases compound layer and diffusion layer thicknesses. However, maximum surface hardness and roughness were found on the samples treated at 500 and 550 °C, respectively.     

Kinetics of nucleation and halt-in-growth processes in a thin layer

The methods of determining the kinetic exponents in the equation, dX/dVex = (1 – X)2–, used for nucleation and halt-in-growth processes where X is the transformed fraction, Vex the KJMA extended volume fraction which is related to time t, and is the overlap factor which accounts for the overlap between a crystallite and a phantom crystallite, are presented. The applications of the Kolmogorov–Johnson–Mehl–Avrami plot ( = 1) and the Austin–Rickett plot ( = 0) to this process are inappropriate, because the overlap factor is 0 < < 1. The impingement exponent 2- and the time exponent are determined from the linear relation of In {[(1 – X) – 1 – 1]/(1 – )} versus In t. From the value of , the crystal shape and growth dimension can be estimated by referring to the mathematical value of . The methods of evaluating the activation energy, Q, are presented using the Arrhenius relation. The value of Q is not directly related to the overlap factor however, appears as a constant term in the expression for Q.     

On the mechanisms of fatigue facet nucleation in titanium alloys

A crystal plasticity model for near‐alpha hcp titanium alloys embodying a quasi‐cleavage failure mechanism is presented and employed to investigate the conditions necessary in order for facet nucleation to occur in cold‐dwell fatigue. A model polycrystal is used to investigate the effects of combinations of crystallographic orientations (and in particular, a rogue grain combination), the essential role of (cold) creep during hold periods in the loading cycle and the more damaging effect of a load hold rather than a strain hold in facet nucleation. Direct comparisons of model predictions are made with dwell fatigue test results. More generally, the crystal model for faceting is found to be consistent with a range of experimental observations.