Increase In Nitriding Temperature Research Articles

Structural Characterization of Ion Nitrided 316L Austenitic Stainless Steel: Influence of Treatment Temperature and Time

The ion nitriding behavior of AISI 316L austenite stainless steel was investigated at different nitriding times (2 h, 4 h, and 9 h) and temperatures (450 °C, 500 °C, and 550 °C). The structural characterization has been assessed by several considerations which can be listed: (i) the evaluation of phase distribution through Rietveld analysis of X-ray diffraction patterns and accompanying peak fitting process, (ii) hardness profile and related nitride layer thickness by microhardness and microscopic measurements, and (iii) displacement measurements to assess the residual stress accumulation. The diffusion of nitrogen atomic species into the sample surface caused a transformation of the γ phase matrix into an expanded austenite (γN) phase, which is recognized with its high hardness and wear resistance. Furthermore, depending on the nitriding condition, chromium nitride (Cr1-2N) and iron nitride (ε-Fe2-3N and γ′-Fe4N) phases were detected, which can be detrimental to the corrosion resistance of the 316L austenite stainless steel. The γN phase was observed in all nitriding conditions, resulting in a significant increase in the surface hardness. However, decomposition of the γN phase with an increase in nitriding temperature eventually altered the surface hardness distribution in the nitriding layer. Considering the phase-type and distribution with the consequent hardness characteristics in the nitride layer, to our best knowledge, this is the first report in which an ion-nitriding temperature of 500 °C (higher than 450 °C) and time of 9 h can be proposed as ideal processing parameters leading to optimal phase composition and hardness distribution for 316L austenite stainless steels particularly for the applications requiring a combination of both wear and corrosion resistance.

Application of Active-Screen Plasma Nitriding to an Austenitic Stainless Steel Small-Diameter Thin Pipe

Low-temperature active-screen plasma nitriding (ASPN) was applied in this study to improve the bending rigidity and corrosion resistance of a small-diameter thin pipe composed of austenitic stainless steel (SUS 304). The inner and outer diameters of the pipe were ϕ0.3 and ϕ0.4 mm, respectively, and the pipe length was 50 mm. The jig temperature was measured using a thermocouple and was adopted as the nitriding temperature because measuring the temperature of a small-diameter pipe is difficult. The nitriding temperature was varied from 578 to 638 K to investigate the effect of temperature on the nitriding layer and mechanical property. The nitriding layer thickness increased with an increase in nitriding temperature, reaching 15 μm at 638 K. The existence of expanded austenite (S phase) in this nitriding layer was revealed using the X-ray diffraction pattern. Moreover, the surface hardness increased with the nitriding temperature and took a maximum value of 1100 HV above 598 K. The bending load increased with an increase in the nitriding temperature in relation to the thicker nitriding layer and increased surface hardness. The nitrided samples did not corrode near the center, and corrosion was noted only near the tip at high nitriding temperatures of 618 and 638 K in a salt spray test. These results indicated that the bending rigidity of the small-diameter thin pipe composed of austenitic stainless steel was successfully improved using low-temperature ASPN while ensuring corrosion resistance.

Influence of ceramic phase content and its morphology on mechanical properties of MgO–C refractories under high temperature nitriding

To evaluate the influences of ceramic phase content and its morphology on the mechanical properties of MgO–C refractories, pre-prepared Si powder-phenolic resin loaded with Fe2O3 was introduced to prepare MgO–C refractories via catalytic nitridation. The effects of nitriding temperature and Fe2O3 content on the phase composition, microstructure evolution and properties of MgO–C refractories were studied and compared. The results show that the increase of the nitriding temperature was conducive to the in-situ formation of the ceramic phases, and a new phase of Mg2SiO4 was formed at temperatures ≥1450 °C. Both the increase in nitriding temperature and the addition of catalyst could inhibit the growth of α-Si3N4 to promote the formation of β-Si3N4 and MgSiN2. In addition, the formation of excessive ceramic phases caused samples after nitriding to expand violently and form more porous, thereby reducing the physical properties of MgO–C refractories.

Investigation of the Surface Properties and Wear Properties of AISI H11 Steel Treated by Auxiliary Heating Plasma Nitriding

This paper presents an auxiliary heating method to maintain a uniform specimen temperature and precisely control nitriding temperature during plasma nitriding. The surface properties and wear properties of AISI H11 steel treated by auxiliary heating plasma nitriding are investigated. Firstly, the specimens with different diffusion layers and different hardness levels are fabricated through changing the plasma nitriding temperature. Secondly, the surface properties of the plasma-nitrided H11 steel specimens are characterized by a scanning electron microscope (SEM), X-ray diffractometer, metallographic microscope and microhardness tester. The results show that the surface hardness of the plasma-nitrided specimen is almost twice as high as that of the untreated specimen. The thickness of diffusion layer increases with the increase of nitriding temperature. However, the surface hardness firstly increases and then decreases with the increase of the nitriding temperature. Finally, the wear properties of untreated and plasma-nitrided H11 steel specimens are investigated under different friction conditions. The results show that the plasma-nitriding method can significantly improve the wear resistance of AISI H11 steel. The friction coefficient fluctuations of the plasma-nitrided specimens are all lower than those of the untreated specimens. In addition, the wear rates of the plasma-nitrided specimens rise along with load, and reduce along with the sliding speed and friction temperature.

Characterization and growth kinetics of plasma nitrided layer fabricated on Incoloy 901 superalloy

The phase composition, microstructure, mechanical performance and growth kinetics of nitrided layers on Incoloy 901 (IN-901) at 460– 610 °C were investigated. XRD results indicated that CrN, γ-FeNi, γN and Ni4N were the main phase compositions of the nitrided layer. The formation of Ni4N took longer duration or higher temperature than those of CrN and γN. EDS results suggested that the content of Cr in the nitrided layer was larger than that in the substrate, while fewer Ni and Fe entered the nitrided layer from the matrix. The kinetic studies showed that the weight and thickness of the nitrided layer significantly increased with the increase of nitriding temperature, and the diffusion activation energy of IN-901 was calculated as ~153 kJ/mol (1.6 eV). Micro-hardness and sliding tests proved a high surface hardness (up to ~1892 HV0.025) and a low friction coefficient (~0.22) of the nitrided IN-901.

Nitriding behavior and mechanical properties of carburizing and nitriding duplex treated M50NiL steel

In this study, carburizing and nitriding duplex treatments were carried out for 10 h at various nitriding temperatures in the range of 460–540 °C on M50NiL steel. The results show that an increase in nitriding temperature greatly increases the thickness of the nitrided layer, but a further increase in the nitriding temperature to 540 °C does not obtain a deeper nitrided layer. Moreover, an increase in nitriding temperature promotes the dissolution of grainy carbides and leads to a precipitation of intergranular precipitates. The pre-existing carbides are easier to be transformed into M(N,C) at a low nitriding temperature. The compound layer is a mixed phase of γ′ (Fe4N) and ε (Fe23N). With increasing the nitriding temperature, the magnitude of the residual compressive stresses decreases, the surface roughness increases, and the proportion of γ′ phase increases. The results of the wear tests carried out at different loads demonstrate that the wear mechanism controlling the wear rate transfers from oxidative wear to abrasive wear when the load increases. The sample nitrided at 500 °C shows the best wear resistance to abrasive wear under a high load due to its thicker compound layer, high ε proportion and high surface residual compressive stress.

Effect of QPQ Nitriding Parameters on Properties of Pearlite Ductile Cast Iron

The effects of nitriding temperature and time on properties of pearlite ductile cast iron were studied by wear and corrosion resistance tests. The results show that the main phases of oxide layer are Fe3O4 and Li2Fe3O4, and the main phases of compound layer are Fe3N and Fe4N. With the increase of nitriding temperature, the micro-hardness and wear resistance increase first and then decrease. The micro-hardness and wear resistance increase continuously with the prolongation of nitriding time. The corrosion resistance of the sample is relatively good when nitriding at 640 °C for 120 min.

Fabrication of composite modified layer on aluminium alloy by surface mechanical nano-alloying combined with nitriding

A composite modified layer was fabricated on 6061 aluminium alloy by means of surface mechanical nano-alloying treatment (SMNAT) combined with nitriding treatment. The microstructure, mechanical properties and interfacial bond of composite modified layer were investigated systematically. The experimental results showed that a Fe coating with thickness of 40–65 μm could be fabricated on the surface of 6061 aluminium alloy by means of SMNAT for 1 h. The mean grain size of top surface layer in Fe coating is about 15 nm. After nitriding, a composite modified layer was generated and its microstructure is depended on the nitriding temperature. The phase structure of modified layer on samples after nitriding at 450 °C, 550 °C and 640 °C is Fe2-3N/Fe(N), Fe2-3N/Fe(N) and Fe2-3N/Fe2Al5, respectively. The properties measurements showed that the surface hardness of 6061 aluminium alloy was greatly enhanced by SMNAT combined with nitriding treatment. The hardness increased with increase of nitriding temperature. Additionally, the mechanical properties match and interfacial bond strength of layer-substrate on the N450 and N550 samples is better than those on the N640 sample. This paper offers a novel method to prepare a gradient modified layer on aluminium alloy for improving surface properties.

Effect of Prior Cold Deformation and Nitriding Conditions on Microstructure and Mechanical Properties of Plasma Nitrided IF Steel

The effect of prior cold deformation and nitriding parameters on the kinetics and mechanism of plasma nitriding and the resultant hardness, wear and corrosion properties of automotive-grade interstitial-free (IF) steel has been investigated. Following controlled prior cold deformation (uniaxial rolling), plasma nitriding was carried out in pulsed direct current glow discharge mode with applied voltage, current, temperature and time varied in the range 540 to 710 V, 3 to 6 A, 350 to 480 °C and 1 to 4 hours, respectively. The phases formed after nitriding were found to be mostly γ-Fe4N with a small volume fraction of Fe3N embedded in a ferrite matrix. Prior cold deformation increases the kinetics of the nitride formation. The volume fraction of nitride phases increased with an increase in nitriding temperature and time. Detailed characterization suggested that 80 pct cold deformation followed by plasma nitriding led to significant improvement in hardness and wear resistance of IF steel, particularly when nitrided at 480 °C for 4 hours. Moreover, plasma nitriding also enhanced the corrosion resistance of IF steel, enhancement being directly related to the nitride volume fraction at the surface. Hence, it was concluded that prior cold deformation was effective in enhancing the kinetics of plasma nitriding and in turn surface hardness and resistance to wear and corrosion of IF steel, which otherwise possess a fairly poor bulk strength and does not respond to usual bulk/surface-hardening treatments.

Tribological Behavior of Titanium Alloy Treated by Nitriding and Surface Texturing Composite Technology.

This study experimentally investigated the effect of surface textures on the tribological mechanism of nitrided titanium alloy (Ti–6Al–4V). The titanium alloy samples were nitrided at various temperatures ranging from 750 to 950 °C for 10 h in a plasma nitriding furnace. Then, surface textures were fabricated on the polished titanium alloy and plasma nitrided samples by laser process system. The surface roughness, microhardness, and constitution of samples treated by single nitriding and samples treated by composite technology were characterized. The tribological properties of the samples were investigated on a CSM ball-on-disc tribometer. The results show that plasma nitriding effectively enhances the wear resistance of the substrate. The wear rate decreases first and then increases with the increase of nitriding temperature, and the wear rate reaches the minimum at 900 °C. However, the increase in roughness caused by nitriding treatment leads to an increase in the friction coefficient. It is found that surface textures can obviously reduce the friction coefficient of the nitrided titanium alloy. In addition, it can also reduce the wear rate of titanium alloys after nitriding at 900 and 950 °C. It can be concluded that the nitriding and surface texturing combined treatment can obviously reduce the friction coefficient and wear rate at the nitriding temperatures of 900 and 950 °C. This is attributed to the combined effect of high hardness of nitride layers and the function of micro-trap for wear debris of surface textures.

Effect of Vacuum Ion-Plasma Treatment on Surface Layer Structure, Corrosion and Erosion Resistance of Titanium Alloy with Intermetallic a2-Phase

Results are provided for a study of the effect of vacuum ion-plasma nitriding on the phase composition, structure, microhardness, salt corrosion resistance, and erosion resistance of alloy Ti – 14Al – 3Nb – 3V – 0.5Zr with an original bimodal structure and different surface microgeometry. It is shown that with an increase in nitriding temperature from 550 to 650°C the content of Ti2N nitrides in the surface layer increases and Ti3AlN nitride in formed, which raises the microhardness but reduces the thickness of the hardened diffusion zone; pores appear in the surface at 650°C. Vacuum ion-plasma nitriding is shown to raise substantially the resistance of specimens of alloy Ti – 14Al – 3Nb – 3V – 0.5Zr with a polished surface to salt corrosion. Additional nitriding after deposition of a TiN coating improves ground specimens salt corrosion resistance. Vacuum ion-plasma nitriding with additional deposition of a TiN coating increases the resistance of specimens with polished and ground surfaces to erosive action.

Study on the Performance of Nano-Titanium Nitride-Coated Stainless Steel Electrodes in Electro-Fenton Systems.

The electro-Fenton (EF) process is a type of electrochemical oxidation process; ·OH radicals are generated on the cathode using electricity and decolorize dye wastewaters. Most studies on EF systems in the past have focused on the operating parameters of this process. In recent years, the influence of electrode performance on the EF process has begun to receive more attention. In this study, direct nitridation was used to prepare titanium nitride powders, which were thereafter coated on an SUS304 stainless steel substrate. The performance of this system in the treatment of rhodamine B dye wastewaters via the EF process was investigated. The experimental methods used in this work include: (1) scanning electron microscopy (SEM); (2) X-ray diffraction (XRD); (3) electrochemical Tafel curves; (4) linear sweep voltammetry (LSV); (5) and cyclic voltammetry (CV). It was shown that high-purity TiN can be formed at nitriding temperatures above 900 °C, and the strength of the (111) crystal plane increases with the increase in nitriding temperature; the TiN coating effectively activates the reactive surface of the electrode owing to its porous structure. In terms of corrosion resistance, the corrosion potential and corrosion current of the TiN 1000 °C/SUS304 electrode were 116.94 mV and 205 nA/cm2, respectively, and the coating had a coating porosity of 0.89 × 10−7. As compared with SUS304 stainless steel, the TiN 1000 °C/SUS304 composite electrode had a significantly greater degree of corrosion resistance and exhibited higher redox activity in LSV tests. This composite electrode could achieve a decolorization rate of 49.86% after 30 min, and 94.46% after 120 min. In summary, the TiN 1000 °C/SUS304 composite electrode is very stable and has excellent decolorization efficacy in the EF process. Our findings will serve as a useful reference for future studies on EF electrodes.

Plasma Ion Nitriding of Low Carbon Stainless Maraging Steel

Low carbon stainless maraging steel (0.03%C-12%Cr-10Ni-0.6Mo-0.2Ti) is being used widely for various components of the aerospace engines. To improve the wear resistance of the steel various surface hardening processes are being utilized to improve the surface hardness above 900HV. In this present research, plasma nitriding was carried out at two different temperatures of 450 °C and 475 °C for the holding duration of 10 hrs. Temperature of the nitrding process was ensured below the ageing temperature (500 °C) of the steel to avoid lowering of mechanical properties. Effect of plasma nitriding parameters on the surface hardness, case depth, microstructure and phases present in the nitrided layer were investigated in detail using microhardness analysis across the nitrided layer, X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM). It was observed that increase in nitriding temperature increased the surface hardness and case depth. In addition, the presence of Fe3N and Fe4N phases in the nitrided layer were observed using X-ray diffraction technique.

Effects of nitriding temperature on microstructures and vacuum tribological properties of plasma-nitrided titanium

Facing the outer space application of titanium and its alloys in drilling pipes and drilling robots, it is necessary to find an optimized surface engineering method to improve their vacuum tribological properties. In order to find an optimized nitriding temperature, a series of plasma nitriding experiments were carried out for commercial titanium (UNS R50400/Gr. 2/TA2/CP3) at various temperatures ranging from 650 to 950°C for 8h to investigate the effects of nitriding temperature on microstructures and mechanical properties. Vacuum tribological properties of the nitrided and untreated samples were examined using a ball-on-disc vacuum tribo-meter. The results show that the wear resistance of titanium tested under vacuum condition is effectively enhanced, due to the formation of the hard nitrided layer. The surface roughness, thickness and hardness of the nitrided layer increase with the increase of nitriding temperature. Nevertheless, the load bearing capacities of the samples nitrided at 900 and 950°C are much lower than those of the samples nitrided at 800 and 850°C. Therefore, the wear volume keeps dropping at the nitriding temperature from 650 to 850°C, whereas, it converts to rise up when the nitriding temperature is above 850°C. In general, it is an optimized process to be nitrided at the temperature of 850°C for 8h to improve the vacuum tribological properties of titanium.

Microstructure and mechanical properties of surface layer of M50NiL steel plasma nitrided

The quenched M50NiL steel specimens have been plasma nitrided at temperatures from 460 to 590°C for 4 h under a constant mixture gaseous supply of 0·05N2–0·4H2 L min−1. The effects of the treatment temperature on the microstructure, microhardness and wear resistance of the surface nitrided layers are studied. The results show that the plasma nitrided layer includes only the diffusion layer without conventional compound layer. The surface hardness is significantly enhanced. α′-Fe and γ′-Fe4N phases in surface layer are formed at relatively low nitriding temperatures (460–560°C), while a low nitrogen phase FeN0·076 forms when the nitriding temperature exceeds 575°C. With the increase in nitriding temperature, the nitrided layer thickness increases, whereas both the surface and core microhardness of the nitrided specimens decreases. The wear resistance of specimens can also be improved significantly by plasma nitriding.

α/β Phase Transformation in Porous Reaction-Bonded Silicon Nitride Ceramics

Porous reaction-bonded silicon nitride ceramics with different values of α/β ratio were obtained by setting the nitriding temperature and time. The influence of α/β phase transformation on the dielectric properties of porous silicon nitride ceramics has been investigated. The results show that the α/β transformation occurs when the nitriding temperature is higher than 1400°C . The values of α/β ratio decrease with increase of nitriding temperature and time. The dielectric constant ε′ and the dielectric loss tan δ of the samples increase because of the α/β transformation, and the change of the dielectric loss is more obvious. The increase of concentrations of point defects due to the α/β transformation leads to the significant increase of the dielectric loss.

Microstructure and corrosion resistance of the layers formed on the surface of precipitation hardenable plastic mold steel by plasma-nitriding

Plasma-nitriding is used to improve the wear resistance and corrosion resistance of plastic mold steels by modifying the surface layers of these steels. In this study, a precipitation hardenable plastic mold steel (NAK80) was plasma-nitrided at 470, 500, and 530 °C for 4, 8, and 12 h under 25% N 2 + 75% H 2 atmosphere in an industrial nitriding facility. The microstructures of the base material and nitrided layers as well as the core hardness were examined, and various phases present were determined by X-ray diffraction. The corrosion behaviors were evaluated using anodic polarization tests and salt fog spray tests in 3.5% NaCl solution. The results had shown that plasma-nitriding does not cause the core to soften by overaging. Nitriding and aging could be achieved simultaneously in the same treatment cycle. Plasma-nitriding of NAK80 mold steel produced a nitrided layer composed of an outer compound layer constituting a mixture of ɛ-nitride and γ′-nitride and an adjacent nitrogen diffusion layer on the steel surface. The amount of ɛ-nitride and total nitrides increased with an increase in nitriding temperature and nitriding time. Corrosion study revealed that plasma-nitriding significantly improved the corrosion resistance in terms of corrosion potential, corrosion and pitting current density, and corrosion rate. This improvement was found to be directly related to the increase in the amount of ɛ-nitride at the surface, indicating the amount of ɛ-nitride controlling the corrosion resistance.

Study of the S phase formed on plasma-nitrided AISI 316L stainless steel

Some tribological and corrosion resistance properties of austenitic stainless steels are enhanced by the formation of the S phase, also called expanded austenite. This phase is formed on the surfaces of austenitic stainless steels nitrided under certain conditions. In this work, AISI 316L steel was plasma-nitrided at 350, 400, 450, and 500 °C, and the samples were characterized by X-ray diffraction (XRD), conversion electron Mössbauer spectroscopy (CEMS), and wavelength dispersive spectroscopy (WDS) in order to investigate the S phase. XRD analysis identified the presence of a distorted cubic structure phase. The modified layer consists of an austenitic phase with different content of nitrogen, ranging from approximately 10 to 40 at%, and also γ′-Fe 4N and ɛ-Fe 2–3N phases. A diminution in the S phase occurs with the increase in nitriding temperature, and this decrease is related to the transformation of the S phase to the γ′-Fe 4N phase.

Kinetics of layer growth during plasma nitriding of nickel based alloy Inconel 600

The response of nickel based superalloy, Inconel 600, to plasma nitriding has been investigated in the present work. Plasma nitriding experiments carried out at temperatures between 400 and 600 °C for times between 1 h and 40 h revealed that the growth of the nitrided layer varied with temperature in a way significantly deviating from the conventional diffusion law. For similar nitriding times, the maximum layer thickness was obtained at about 450 °C. A further increase in nitriding temperature resulted in a decrease in layer thickness. The growth of the nitrided layer with the square root of time followed two linear regimes, i.e. an initial fast growth regime and then a slow growth regime. The critical time for the transition from the fast growth regime to the slow growth regime decreased with increasing temperature, such that for prolonged nitriding times a thinner layer was produced at high temperatures than at low temperatures. In conjunction with structural and compositional analysis, the reasons for such an abnormality in nitriding kinetics have been analysed. It was found that the reduction in layer growth rate in the slow growth regime was associated with the formation of an external scale on top of the nitrided layer, which impeded the transfer of nitrogen from the plasma to the nitrided layer.

Effect of nitriding temperature in a glow discharge on the thickness and composition of the nitride zone

1. With an increase in nitriding temperature in a glow discharge the zone of chemical compounds increases, reaching a maximum value at ti=600°C. In this way the amount of e-phase in the nitride zone decreases, but with γ′-phase increases. 2. With nitriding in ammonia or carbon-containing atmospheres with ti>600°C the zone of chemical compounds is almost absent on carbon and low-alloy steels. 3. The dependence of nitride layer composition and thickness on temperature under impregnation conditions in a glow discharge differs markedly from dependences for similar properties of a layer obtained with nitriding by classical methods.