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Nitriding is a thermochemical surface hardening process, which can be used to improve the performance properties of gears. In particular, the tooth flank and tooth root load-carrying capacity can be significantly increased by nitriding. Previous work has shown that tribological dominated failure modes and limits, such as the micro-pitting and wear load-carrying capacity of gears, can be significantly increased by a suitable compound layer structure. In this work, the relationship between compound layer design and macro-pitting load-carrying capacity of nitrided gears is explored based on experimental investigations. For this purpose, the materials EN31CrMoV9 and EN42CrMo4 were nitrided under varying nitriding conditions to produce compound layers with different structures and properties. The characteristics of the compound layer such as phase composition, compound layer thickness and pore seam were extensively characterized by means of metallographic techniques, XRD, GDOES, and EBSD. Experimental investigations of the macro-pitting load-carrying capacity were carried out on an FZG back-to-back gear test rig with center distance a = 52 mm. It was shown that the pitting load-carrying capacity depends to a large extent on the characteristics of the compound layer, in particular on the formation of the pore seam. A minimum pore seam thickness of CLTP ≥ 1.8 µm has a positive effect on the pitting load-carrying capacity. The test results for the pitting load-carrying capacity are above (σH lim ≥ 1690 N/mm2) or, in some cases, significantly above (σH lim = 1979 N/mm2) the strength values specified in ISO 6336-5:2016 for nitrided nitriding steels (σH lim = 1450 N/mm2; material quality ME) and case-hardened alloy steels (σH lim = 1650 N/mm2; material quality ME). [ABSTRACT FROM AUTHOR]

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This research focused on investigating the impact of nitrocarburization on the surface characteristics and tribological properties of SKH51 high-speed tool steels. We performed various heat treatment procedures, including single tempering quenching (QT1), double tempering quenching (QT2), triple tempering quenching (QT3), and a combination of double tempering quenching followed by nitrocarburization (QT2N). We evaluated attributes such as 3D surface morphology, tribological attributes, surface roughness, and hardness. The results revealed that the QT2N resulted in samples with a rougher surface and approximately 4.4 times harder than the untreated specimen (AR). Moreover, these samples were approximately 1.4 times harder than those subjected to QT3. Surface roughness was observed to increase after quenching and multiple tempering processes. However, the surface roughness of the QT2N specimen was reduced due to the formation of a compound layer following the nitrocarburizing process. The retained austenite of QT3 and QT2N demonstrated a lower standard deviation than those of AQ, QT1, and QT2, owing to the transformation of retained austenite into tempered martensite and precipitated carbide. The compound layer formed during nitrocarburization enhanced surface hardness, improved surface roughness, and reduced the coefficient of friction. Consequently, the QT2N process demonstrated greater efficiency than QT3, suggesting that nitrocarburization can effectively replace the final tempering process. This adaptation could result in a simplified heat treatment procedure and lower production costs. [ABSTRACT FROM AUTHOR]

The article presents the test results on the technology of surface hardening of castings from unalloyed and low-alloy nodular cast iron using the method of surface heat treatment, i.e., induction surface hardening and methods of thermochemical treatment, i.e. gas nitriding, nitrocarburizing, and nitrocarburizing with oxidation. The scope of research included macro- and microhardness measurements using Rockwell and Vickers methods, respectively, as well as metallographic microscopic examinations using a light microscope. Furthermore, abrasive wear resistance tests were performed using the pin-on-disk method in the friction pair of nodular cast iron – SiC abrasive paper and the reciprocating method in the friction pair of nodular cast iron – unalloyed steel. Analysis of the test results shows that the size and depth of surface layer hardening strongly depend on the chemical composition of the nodular cast iron, determining its hardenability and its ability to create diffusion layers. Medium induction surface hardening made it possible to strengthen the surface layer of the tested nodular cast irons to the level of 700 HV0.5 with a hardening depth of up to approximately 4000μm, while various variants of thermochemical treatment provided surface hardness of up to 750 HV0.5 with a hardening depth of up to approximately 200μm. Furthermore, induction surface hardening increased the resistance to abrasive wear of nodular cast iron castings, depending on the test method, by an average of 70 and 45%, while thermochemical treatment on average by 15 and 60%.

M420 stainless steel has good mechanical properties, but its corrosion resistance is poor, which limits its application in marine conditions. Single-technology processing has been unable to meet the development needs of modern industry. In this work, plasma nitrocarburizing (PNC) and CrN/CrTiAlSiN/WCrTiAlN multi-layer coating were carried out on M420 martensitic stainless steel by hollow cathode-assisted plasma treatment and multi-arc ion plating, respectively. The modified layers were characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. The results show that the nitrocarburized layer effectively improves the hardness gradient between the coating and the substrate. The presence of the composite strengthening layer enables the sample to obtain a thick and dense corrosion-resistant surface, effectively blocking the invasion of the corrosive medium. The corrosion rate of the PNC + C sample decreased by an order of magnitude. Combining hollow cathode-assisted rapid nitrocarburizing technology and multi-arc ion plating technology can significantly improve the mechanical properties and corrosion resistance of M420 stainless steel in a shorter time. The duplex surface-treated M420 stainless steel by plasma nitrocarburizing and WCrAlTiSiN multilayer coating exhibited excellent corrosion resistance compared to the nitrided and coated M420 stainless steel.

This work studied the cyclic spherical contact behavior of plasma nitrided (N) and nitrocarburized (NC) AISI 321 steel. The temperature and exposition time were 400 and 500 °C for 6 h in each treatment. A superficial hardness between ~10 (NC 400 °C) and ~17 GPa (NC 500 °C) was found by instrumented indentations; moreover, plastic and total work of indentation were analyzed. In cyclic spherical contact assessment, a critical load was first determined by applying tests between 100 and 1000 N. Next, cyclic tests were conducted with three subcritical loads (120, 150, and 180 N) and up to 105 cycles. A detrimental effect of treatment temperature was observed, 500 °C treatments presented worse failures than 400 °C, which is explained by the decomposition of expanded austenite. Between 400 °C treatments, nitrocarburizing presented a better performance, its higher plastic work of indentation is associated with a better energy absorption capacity.

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The effect of plasma electrolytic nitrocarburizing on the wear resistance of carbon tool steel in friction couples with hardened steel and lead-tin bronze is considered in order to study the mechanism and type of wear, as well as the influence of structural and morphological characteristics of the surface on them. The microgeometry of friction tracks and its change with an increasing duration of friction tests are analyzed. The equilibrium roughness is determined, which is optimal for the friction couple and ensures minimal wear. The optimal values of the plasma electrolytic nitrocarburizing parameters, which provide the lowest values of the friction coefficient and wear rate, have been determined. The phase and elemental composition of the surface layer was studied using X-ray diffraction analysis and EDX analysis. The relationship of the microstructure of the nitrocarburized layer of tool steel with the friction coefficient and weight wear is established. [ABSTRACT FROM AUTHOR]

The Palmqvist method is usually used as a measure to evaluate the toughness of ceramics and thin coatings, as these cannot be determined with classic test methods such as the tensile test or the Charpy impact test due to their brittleness or the small proportion of the coating in the specimen volume. Nitride compound layers, formed during nitriding and nitrocarburizing, are also brittle and only have a thickness of a few micrometers, so determining their toughness is also challenging. In this work, compound layers with different phase compositions and thicknesses were investigated on EN31CrMoV9 and EN42CrMo4 steels using the Palmqvist method. Barrel finishing and polishing were used for specimen preparation, for which a significant effect on the formation of Palmqvist cracks was found. Barrel finishing reduced the length of the Palmqvist cracks with increasing barrel finishing time until a constant level was reached. The surface condition caused by the porosity near the surface of the compound layer was identified as another influencing factor, thus the pore seam was removed for the further investigations. Various specimen variants were prepared and their phase compositions and compound layer thicknesses evaluated in relation to the crack lengths of the Palmqvist cracks. A correlation was found between the length of the Palmqvist cracks and the extent of the compound layer thickness.

Surface engineering of chromium-oxide-passivated alloys (e.g., stainless steels) by low-temperature infusion of interstitial solutes (carbon, nitrogen) from a gas phase requires "surface activation" by removing or perforating the passivating oxide film. We demonstrate a new approach for surface activation based on pyrolysis of a reagent powder, introduce advanced methodology to study its microstructure, and compare it to an established activation method. Rather than a bare alloy surface, stripped of its oxide, we find that an "activated" surface involves a reaction layer containing high concentrations of Cl, carbon, or nitrogen. We propose a model for the microscopic mechanism of surface activation that will enable future systematic development toward more effective process schemes. [ABSTRACT FROM AUTHOR]

• Nitrocarburizing reduces mass loss by cavitation erosion by up to 90 % in 15 h. • Nitrogen-stabilized expanded austenite improves erosion resistance. • Deformation-induced martensite formation precedes cavitation erosion in AISI 304L. • No deformation martensite forms in expanded austenite. In this study, salt bath nitrocarburizing was applied on austenitic stainless steel AISI 304L to stabilize austenite at the surface exposed to cavitation attack. Samples were treated at 470 °C and 500 °C for 6 h, giving an average thickness of the nitrocarburized layer of 6 µm and 26 µm, respectively. The thickness of the nitrocarburized layer is found to influence the formation of deformation-induced martensite (DIM) in the material and, thereby, the cavitation erosion resistance. The mass loss vs. time plot shows that the nitrocarburized specimens performed significantly better than the base material under ultrasonic vibratory cavitation erosion test (ASTM G32). Cross-sectional micrographs show that the phenomenon responsible for the failure in the base material is the formation of DIM. The specimen, nitrocarburized at 470 °C, showed a reduction of about 90 % in mass loss when compared to the base material (AISI 304L SS), owing to a higher yield strength (/resilience) after the formation of more stable austenite. [ABSTRACT FROM AUTHOR]

The effect of plasma electrolytic nitrocarburizing on the wear resistance of carbon tool steel in friction couples with hardened steel and lead-tin bronze is considered in order to study the mechanism and type of wear, as well as the influence of structural and morphological characteristics of the surface on them. The microgeometry of friction tracks and its change with an increasing duration of friction tests are analyzed. The equilibrium roughness is determined, which is optimal for the friction couple and ensures minimal wear. The optimal values of the plasma electrolytic nitrocarburizing parameters, which provide the lowest values of the friction coefficient and wear rate, have been determined. The phase and elemental composition of the surface layer was studied using X-ray diffraction analysis and EDX analysis. The relationship of the microstructure of the nitrocarburized layer of tool steel with the friction coefficient and weight wear is established.

AISI 316L steel was subjected to nitrocarburizing under glow discharge conditions, which was followed by DLC (diamond-like carbon) coatings deposition using the same device. The coatings were applied under conditions of direct current and pulsed glow discharge. In order to determine the influence of the produced nitrocarbon austenite layer and the type of discharge on the microstructure and mechanical properties of the coatings, the following features were analysed: surface roughness, coating thickness, structure, chemical composition, adhesion and resistance to frictional wear. For comparison purposes, DLC coatings were also deposited on steel without a nitrocarburised layer. The obtained results indicate a significant influence of the type of glow discharge on the roughness, hardness, nitrogen content and of the nitrocarburised layer on the resistance to wear by friction and adhesion of the produced coatings.

A new method of surface modification based on the method of electrospark alloying (ESA) using carburizer containing nitrogen—carbon components for producing coatings is considered. New processes have been proposed that include the step of applying saturating media in the form of paste-like nitrogenous and nitrogenous-carbon components, respectively, onto the surface without waiting for those media to dry, conducting the ESA process with the use of a steel electrode-tool, as well as with a graphite electrode-tool. Before applying the saturating media, an aluminium layer is applied onto the surface with the use of the ESA method at a discharge energy of Wp = 0.13–6.80 J. A saturating medium in the form of a paste was applied to the surfaces of specimens of steel C22 and steel C40. During nitriding, nitrocarburizing and carburization by ESA (CESA) processes, with an increase in the discharge energy (Wp), the thickness, micro hardness and continuity of the "white layer" coatings, as well as the magnitude of the surface roughness, increase due to saturation of the steel surface with nitrogen and/or carbon, high cooling rates, formation of non-equilibrium structures, formation of special phases, etc. In the course of nitriding, nitrocarburizing and CESA processing of steels C22 and C40, preliminary processing with the use of the ESA method by aluminum increases the thickness, microhardness and continuity of the "white layer", while the roughness changes insignificantly. Analysis of the phase composition indicates that the presence of the aluminum sublayer leads to the formation of the aluminum-containing phases, resulting in a significant increase in the hardness and, in addition, in an increase in the thickness and quality of the surface layers. The proposed methods can be used to strengthen the surface layers of the critical parts and their elements for compressor and pumping equipment. [ABSTRACT FROM AUTHOR]

Plasma nitrocarburizing of an austenitic stainless steel AISI 316L was performed in an N 2 -H 2 atmosphere (50 % H 2 and 50 % N 2) with a carbon-fiber reinforced carbon (CFC) active screen. The experimental concept of a plasma-discharged CFC active screen in conjunction with a substrate holder at floating potential ensures that no charged particles are present on the sample surface and no sputtering occurs. As a result, carbon contaminations are formed on the surface which were studied in detail with a combination of scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS). The results indicate that a layer of about 25–50 nm of carbon is formed. At the same time, a strong segregation of Mn, Ni and Cr is observed below the coating within the steel substrate. Both effects progress with treatment time and there exists a clear correlation between the formation of the carbon layer and the segregation effect. • Unusual surface morphology on AISI 316L for afterglow-plasma nitrocarburizing • Closed carbon film consisting of nanoparticles with a size of 50–250 nm on surface • Strong segregation effects of Mn, Cr and Ni in the near surface region • Surface segregation observed with time delay indicating thermodynamically activation [ABSTRACT FROM AUTHOR]

Copyright of Electroplating & Finishing is the property of Electroplating & Finishing Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Nitriding can significantly increase the load carrying properties of gears. While the diffusion layer is primarily responsible for improving the tooth root and flank load carrying capacity, the compound layer mainly determines the tribological properties of the gear surface. In the present work, the influence of the compound layer on the tribological load carrying capacity of nitrided gears in the N/N pairing was investigated. For this purpose, compound layers with different thickness, porosity and phase composition were produced and their micro-pitting and wear behavior were investigated in load stage and speed stage tests. The test results confirm that the properties of the compound layer are decisive for the micro-pitting and wear resistance of nitrided gears. For a high micro-pitting resistance, the presence of pores in the near-surface area of the compound layer is of high importance, since no micro-pitting occurred as long as pores were present. With regard to the wear behavior, no dependence on the compound layer thickness or the porous zone thickness was found while the phase composition of the compound layer shows a decisive influence. [ABSTRACT FROM AUTHOR]

One of the urgent problems in reciprocating machinery is ensuring the functional properties of direct-flow valves. Coatings of these parts should increase energy efficiency, reduce compressed gas temperature, and increase compressor performance. In this article, the effect of nitrocarburizing in pastes on increasing the performance and resource of such valves was studied. The primary research methods were metallographic and electron-metallographic research of the surface layer structure of steel 60Si2Cr. Comparison with similar characteristics obtained for stainless steel 09Cr15Ni8Al was also performed. As a result, it was determined that nitrocarburizing of carbon spring steel 60Si2Cr leads to a greater depth of the diffusion layer than nitrocarburizing of alloyed chromium-nickel steel 09Cr15Ni8Al. Simultaneously, the diffusion layers of both types of steel are almost the same. Simultaneously, the increase in the endurance limit of 60Si2Cr steel during nitrocarburizing is about 1.3 times compared to untreated material. Finally, the predictive maintenance showed that the evaluated lifetime of nitrocarburized valves increases by 5.7 times compared with untreated valves. [ABSTRACT FROM AUTHOR]

AISI 316L steel was subjected to nitrocarburizing under glow discharge conditions, which was followed by Dlc (diamondlike carbon) coatings deposition using the same device. The coatings were applied under conditions of direct current and pulsed glow discharge. in order to determine the influence of the produced nitrocarbon austenite layer and the type of discharge on the microstructure and mechanical properties of the coatings, the following features were analysed: surface roughness, coating thickness, structure, chemical composition, adhesion and resistance to frictional wear. For comparison purposes, Dlc coatings were also deposited on steel without a nitrocarburised layer. The obtained results indicate a significant influence of the type of glow discharge on the roughness, hardness, nitrogen content and of the nitrocarburised layer on the resistance to wear by friction and adhesion of the produced coatings. [ABSTRACT FROM AUTHOR]

Surface engineering of chromium-oxide-passivated alloys (e.g., stainless steels) by low-temperature infusion of interstitial solutes (carbon, nitrogen) from a gas phase requires “surface activation” by removing or perforating the passivating oxide film. We demonstrate a new approach for surface activation based on pyrolysis of a reagent powder, introduce advanced methodology to study its microstructure, and compare it to an established activation method. Rather than a bare alloy surface, stripped of its oxide, we find that an “activated” surface involves a reaction layer containing high concentrations of Cl, carbon, or nitrogen. We propose a model for the microscopic mechanism of surface activation that will enable future systematic development toward more effective process schemes.

Copyright of Erzincan University Journal of Science & Technology is the property of Erzincan Binali Yildirim Universitesi and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Nitriding can significantly increase the load carrying properties of gears. While the diffusion layer is primarily responsible for improving the tooth root and flank load carrying capacity, the compound layer mainly determines the tribological properties of the gear surface. In the present work, the influence of the compound layer on the tribological load carrying capacity of nitrided gears in the N/N pairing was investigated. For this purpose, compound layers with different thickness, porosity and phase composition were produced and their micro-pitting and wear behavior were investigated in load stage and speed stage tests. The test results confirm that the properties of the compound layer are decisive for the micro-pitting and wear resistance of nitrided gears. For a high micro-pitting resistance, the presence of pores in the near-surface area of the compound layer is of high importance, since no micro-pitting occurred as long as pores were present. With regard to the wear behavior, no dependence on the compound layer thickness or the porous zone thickness was found while the phase composition of the compound layer shows a decisive influence.

The surface microstructure and growth behavior of an Fe0.4 mass%C alloy formed by lithium-added salt-bath nitrocarburizing were investigated. The Fe0.4 mass%C alloy was prepared by arc melting and nitrocarburized by a salt-bath containing Li+, Na+, K+, CNO-, and CN- at 823K from 0.1 h up to 10 h. A compound layer forms on the surface at the beginning of nitrocarburizing, and then an oxide layer forms on the compound layer after nitrocarburizing for 1.0 h. Afterwards, the thickness of both layers increase. A grain boundary oxide forms at the interfaces of columnar crystals in the compound layer. The oxide consists of LixFe1-xO with an NaCl-type structure, and the growth of the oxide layer is controlled by the outward diffusion of iron in the oxide layer. Meanwhile, the compound layer consists mainly of an ε-Fe2(N,C)1-y phase and a slight £A-Fe4(N,C)1+z phase near the substrate. [ABSTRACT FROM AUTHOR]

The cathodic plasma electrolytic deposition (CPED) was performed in alkaline electrolyte (glycerol + sodium nitrite) to form a nitrocarburized layer on AISI 1045 steel at constant voltage of 380 V. The results indicated that nitrocarburized layer was mainly comprised of ε–Fe2–3N, γ'–Fe4N, Fe3C and Fe [Fe(CN)6]3 phases. As the treatment time was prolonged from 5 to 15 min, the surface porosity increased from 4.13 to 7.43%, while the thickness of layer was increased about 70% from 13 to 22 μm. On the contrary, the corrosion test revealed that the coatings prepared at 10 min treatment time had the best corrosion resistance with a value of 27 384 Ω.cm2. Moreover, the mechanical behavior of coatings was also significantly increased by treatment time. The surface hardness increased 5 times, while the wear friction coefficient and wear rate of nitrocarburized layer were decreased to about and respectively compared to that of the substrate. [ABSTRACT FROM AUTHOR]

In the present work, gaseous nitrocarburizing followed by water spray oxidation in vacuum was carried out on 4330V steel. The X-ray diffraction analysis showed that the thermochemically modified surface layer mainly consisted of ε-Fe2-3(N,C), γ′-Fe4(N,C), and magnetite (Fe3O4). To quantify the residual stress generated due to surface treatment, the Sin2ψ method was used. Residual stress of compressive nature up to a depth of 1 mm was observed, and its maximum value was 383 MPa. Nitrocarburizing followed by oxidation not only increased the surface hardness but also decreased the wear loss. The synergistic effect of corrosion-resistant phases formed after surface treatment improved surface hardness and compressive residual stress which resulted in the enhancement of corrosion fatigue life. [ABSTRACT FROM AUTHOR]

The influence of substitutionally dissolved V in ferritic Fe-4 at.% V alloy, on the structure and morphology of iron-carbonitrides ε-Fe 3 (N, C) 1+ x and γ'- Fe 4 (N , C) 1-y was investigated upon salt bath nitrocarburizing. An unusual plate-type morphology of γʹ and ε carbonitrides development was evidenced during nitrocarburizing of Fe-V alloy as against to the usual layer-type growth of γʹ and ε nitrides reported for nitrided Fe-V alloys. The ε phase developed first along ferrite matrix grain boundaries, followed by its formation as plates on the plates of γʹ within the grains of ferrite matrix and finally as a layer at the surface. In contrast to ε, γʹ has developed only as plates within the ferrite matrix grains either with Kurdjumov-Sachs or Pitsch orientation relationship. Initially the inwardly diffusing N gets consumed for the development of VN precipitates in the ferrite matrix while C gets enriched at the ferrite-matrix grain boundaries leading to the development of ε carbonitride. Once all the V precipitates as VN, the development of γʹ occurs as plates within the grains and upon further increase in the N content lead to the development of ε within grains. This demonstrates that as a consequence of drastically different solubility and diffusivity of C and N in ferrite, ε and γʹ phases can lead complex microstructural evolution upon nitrocarburizing of Iron-based alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

The aim of this study is to exhibit the mutual connection between surface and in depth hardness values in the case of surface-treated metal samples with inhomogeneous hardness distribution in the surface layer. The reason for surface treatments of metal alloys is most commonly to increase the hardness and wear resistance at the surface. Case depth, as a result of surface treatment and the in-depth hardness distribution, can be determined by measuring the hardness of a section perpendicular to the treated surface and by metallographic examination. The result of heat treatment can also be checked rapidly by surface hardness testing. Surface hardness carries only indirect information regarding case depth and hardness distribution. Surface and cross-sectional hardness can be related to the mathematical modeling of the plastic zone developing in the indentation process. The mathematical model applied in this study allows the conversion of the surface hardness function into the in-depth hardness function and vice-versa. The calculation method presented was validated by analyzing the hardness data of nitrocarburized samples of various case depths. The validation result proves that cross-sectional hardness distribution can be adequately estimated by surface hardness data in the case of a surface layer with monotonically decreasing hardness distribution.

Using transmission electron microscopy (TEM), phase composition and fine texture changes in the ferrite-pearlitic steels 0.18C–1Cr–3Ni–1Mo–Fe, 0.3C–1Cr–1Mn–1Si–Fe and 0.34C–1Cr–1Ni–1Mo–Fe due to electrolytic plasma nitrocarburizing has been studied in thin foils. The procedure of electrolytic-plasma enhanced nitrocarburizing has been performed by steel surface saturation with nitrogen and carbon in an aqueous solution at a temperature of 800–860°C for 5 min. All the steels under investigation have been studied before and after the nitrocarburizing procedure. In the initial state, the steels were discovered to be composed of a pearlitic and ferritic grain mixture. The nitrocarburizing procedure leads to the formation of modified layers. Thus, the greater is the amount of pearlite before nitrocarburizing, the thicker is the modified layer. Nitrocarburizing results in significant qualitative changes in the phase state and the steel structure. In the modified layer surface area alongside the matrix, the particles of other phases such as carbides, nitrides and carbonitrides occur. As the distance from the surface of a nitrocarburized sample increases, the phases of set and volume decrease, whereas the only carbide phase—cementite—occurs at the end of modified layer in the case of all the steels. After nitrocarburizing, the matrix of all the steels represents tempered lath and lamellar martensite. In the nitrocarburized layer surface zone, the volume fractions of lath and lamellar martensite depend on the initial steel state: the greater is the amount of pearlite in steel, the less is the amount of lath martensite; then a greater amount of lamellar martensite is formed. Such a dependence is not observed in the nitrocarburized layer central zone, whereas the volume fractions of lath and lamellar martensite at the end of the layer are close to each other. [ABSTRACT FROM AUTHOR]

Abstract Plasma nitrocarburizing based on active screen technology using a carbon-fiber reinforced carbon active screen was applied in an industrial-scale unit for thermochemical surface treatment of austenitic stainless steel. This concept is based on the use of a solid-carbon-source for the generation of highly reactive process gases directly in the active screen plasma. In this work, plasma nitrocarburizing of AISI 316L stainless steel was performed by means of the variation of the precursor gases H 2 and N 2 in the range of 0% N 2 up to 100% N 2 without the use of any additional carbon bearing gas. For a set of H 2 -N 2 gas mixtures, the resulting reaction gas was monitored using infrared laser absorption spectroscopy (IRLAS). The four main stable species HCN, CH 4 , NH 3 and C 2 H 2 were detected. A detailed analysis of the surface microstructure resulting for each specific H 2 -N 2 precursor gas mixture was performed. This included glow discharge optical emission spectroscopy (GDOES), optical microscopy, micro hardness measurements, as well as X-ray diffraction analysis. The concentrations of hydrocarbons CH 4 and C 2 H 2 were most abundant in case of pure hydrogen plasma, and a carbon-expanded austenite layer with hardness values up to 600 HK0.01 and smooth hardness gradient resulted. The admixture of N 2 to the precursor gas significantly increased the concentrations of HCN and NH 3. Due to this, a duplex structure of nitrogen-expanded austenite γ N and carbon-expanded austenite γ C formed. With increasing content of N 2 up to 50% in the precursor gas, the resulting layer thickness increased and the hardness reached values up to 1300 HK0.01. At strong nitrogen excess in the precursor gas, the nitrogen concentration in the expanded austenite significantly increased, the Fe 2–3 (N, C) phase was formed, and simultaneously the layer thickness decreased. Structure and properties of the expanded austenite layer significantly changed by only varying the H 2 -N 2 ratio of the precursor gas mixture. Highlights • Plasma nitrocarburizing with carbon fiber reinforced carbon as solid carbon source. • Interaction of H 2 -N 2 plasma and carbon generates highly reactive species in-situ. • Reactive species provide surface activation, nitrogen and carbon uptake into steel. • H 2 -N 2 ratio variation in feed gas significantly effects N- and C-gradients in layer. • Infrared laser absorption spectroscopy for reaction gas monitoring [ABSTRACT FROM AUTHOR]

Deep nitriding is used to obtain a nitriding hardness depth beyond 0.6 mm. The long nitriding processes, which are necessary to reach the high nitriding hardness depths, mostly have a negative influence on the hardness and strength of the nitrided layer as well as on the bulk material. The compound layer often is considered less, because in most practical cases, it is removed mechanically after nitriding, to avoid spalling in service. However, in former investigations, it was shown, that thick and compact compound layers have the potential for high flank load capacity of gears. The investigations focus on the simultaneous formation of a high nitriding depth and a thick and compact compound layer. Beside the preservation of the strength, a challenge is to control the porosity of the compound layer, which should be as low as possible. The investigations were carried out using the common nitriding and heat treatable mild steel 31CrMoV9, which is often used for gear applications. The article gives an insight on the development of multistage nitriding processes studied by short- and long-term experiments aiming for a specific compound layer build-up with low porosity and high strength of the nitride layer and core material.

In this work, we used mass spectrometry to investigate ammonia (NH3) formed during the nitrocarburizing and nitriding processes of α-Fe sintered and low alloy steel by using d.c. glow discharges in different N2/H2/CH4 gas concentration mixtures. Experiments were conducted using two different configurations: one to collect and analyze species probed primarily in the bulk of the plasma, at a position distant from the heated sample; and the other to collect and analyze species very close to the reactive surface of the heated sample in such a way as to allow the analysis of lower concentration or residence time species, formed at the plasma/sample surface interface. Use of these configurations contributed to the investigation of the physical chemistry involved in the above plasma processes. An exchange the expected stoichiometry in ammonia formation in N2/H2/CH4 mixtures, with a shift towards higher nitrogen consumption, is observed and is interpreted an indication of nitrogen removal from the surface associated with ammonia and carbonitrides/nitrides formation. The present results point to the existence of a correlation between the nitrocarburizing and nitriding processes with respect to the formation of non-stoichiometric ammonia as a product.

Plasma electrolytic saturation (PES) is an atmospheric pressure plasma deposition which is used for fabrication of nanostructural coatings. This process starts with the appearance of some sparks on the surface and finally by the formation of plasma envelope around the sample, particles and free radicals bombard the surface and diffusion takes place into the work-piece. In this paper, the effect of frequency (500, 1000 and 10000 Hz) as pulse parameters on properties of nitrocarburized layers deposited on AISI 1045 steel were investigated. The analytical characteristics of the coating were assessed using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The corrosion behavior of coatings was analyzed by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 3.5% sodium chloride solution. Moreover, the microhardness and wear resistance of coated samples were also studied. The results indicated that applying nitrocarburized layer in all cases, improved the corrosion resistance, microhardness and also wear resistance of the substrate. The coatings treated at the frequency of 10000 Hz, had corrosion resistance about 2.4 and 1.4 times higher than those of the samples prepared at the frequency of 500 Hz and 1000 Hz. The surface hardness of 10000 Hz treated sample was about 1040 Vickers, which was more than those of the two other ones. An increase in frequency also caused wear resistance to increase.

Formation of metastable C–, N–, or even Formation of metastable C–, N–, or even C/N– expanded phases can be observed for typical non-equilibrium conditions attained at the plasma assisted thermochemical treatments when temperatures relatively low are used. In present work, kinetics data are considered in a comparative study comprising low-temperature plasma assisted carburizing, nitriding and nitrocarburizing of AISI 420 martensitic stainless steel samples treated at 350, 400, and 450 °C, aiming to put in evidence the main metallurgical differences of the obtained layers. Microstructural characterization and hardness measurement results for untreated and treated sample surfaces show significant difference for the carburized layer growth in relation to that verified for the nitrided and nitrocarburized layers. While the carburized layer is constituted of a thin outer layer and a deep diffusion layer, just the opposite was observed for the other two treatments, id est., formation of thicker outer layers and thinner diffusion layers. Finally, carbide-/nitride-precipitation-free layers were supposedly obtained for samples carburized, nitrided, and nitrocarburized at 350 °C temperature.

The influence of the carbamide concentration on the nitrocarburizing temperature and changes in the mass of titanium samples, which are related to the anodic dissolution and high-temperature oxidation of titanium in a vapor‒gas shell, is investigated. The modified layer structure is revealed to contain an external oxide layer (rutile) and an intermediate diffusion layer whose microhardness reaches 770 HV. It is demonstrated that an increase in the nitrocarburizing temperature stimulates growth in the oxide-layer thickness, and the diffusion-layer thickness depends on the carbamide concentration in the solution. It is found that, at 850°С, the surface roughness decreases from 1.0 to 0.3-0.5 μm and the wear rate reduces from 6.7 × 10-7 to (1.2-1.7) × 10-7 due to nitrocarburizing in an electrolyte incorporating 10-15 wt % ammonium chloride and 10-12.5 wt % carbamide. [ABSTRACT FROM AUTHOR]

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The methods of optical and scanning electron microscopy are used to study the structure and phase composition of nitrocarburized layer in medium-carbon steels alloyed with manganese, chromium and nickel. Transmission electron microscopy is applied to study the structure of the heterophase zone and to determine its phase composition. The wear resistance and the wear mechanism of steels of different alloying systems are determined. [ABSTRACT FROM AUTHOR]

Despite the fact that ceramics and polymers have found numerous applications in several mechanical systems, metals and metallic alloys still remain the main materials family for manufacturing the bulk of parts and components of engineering assemblies. However, in cases of components that are serving as parts of a tribosystem, the application of surface modification techniques is required to ensure their unhampered function during operation. After a short introduction on fundamental aspects of tribology, this review article delves further into four representative case studies, where the inappropriate application of wear protection techniques has led to acceleration of the degradation of the quasi-protected metallic material. The first deals with the effects of the deficient lubrication of rolling bearings designed to function under oil lubrication conditions; the second is focused on the effects of overloading on sliding bearing surfaces, wear-protected via nitrocarburizing; the third concerns the application of welding techniques for producing hardfacing overlayers intended for the wear protection of heavily loaded, non-lubricated surfaces; the fourth deals with the degradation of thermal-sprayed ceramic coatings, commonly used as wear-resistant layers.

This paper presents the results of microstructural examinations on gas carbonitrided layers using scanning electron microscopy (SEM), wavelength dispersive spectrometry X-ray microanalysis (WDS), magnetic force microscopy (MFM), electron backscattered diffraction (EBSD), and X-ray diffraction. The main objective of the work is to discuss the microstructural and morphological changes occurring in the layers depending on the process parameter such as temperature and atmosphere composition. The carbonitrided layers comprise two sublayers: an outer layer enriched in nitrogen and an inner layer enriched in carbon. It is found that both the microstructure and the thickness depend on the technological parameters of the thermochemical treatment. The treatment temperature is particularly important, increasing both the total and the nitrogen-rich sublayer thickness regardless of the atmosphere type. The nitrogen potential has a positive influence on the thickness of the nitrogen-stabilised S-phase sublayer. Layers treated for 5 h at temperatures of 400 and 450 °C are composed of an internal carbon-enriched S-phase sublayer and an outer nitrogen-enriched S-phase sublayer that can be divided into ferromagnetic and paramagnetic zones. In the outer sublayer, symmetry distortion of the fcc crystal system is observed and stacking faults may have occurred. With layers produced at 500 °C for 5 h, the outer nitrogen-enriched sublayer contains fine precipitates of nitrides and carbides.

Vein-like grain boundary (VLGB) structures commonly appear in the nitrided or nitrocarburized iron-based materials. Such VLGB structures are extremely bad for its mechanical properties. However, the formation mechanisms of the VLGB structures are still unclear. Here we employ transmission electron microscopy and electron probe X-ray microanalysis to reveal the detailed microstructures and the formation mechanism of the VLGB structure in nitrocarburized Cr12MoV cool-work tool steel at 540 °C and 580 °C. The VLGB structures entirely consist of orthorhombic (Cr,Fe) 7 C 3 phase, with high density of micro-twins and stacking faults, and specific crystallography relationship between the (Cr,Fe) 7 C 3 -typed VLGB structure and matrix is: (101) α //( 2 ¯ 4 ¯ 1 ¯ ) C + 7.5° and [ 1 ¯ 11] α //[ 1 ¯ 02] C. The formation of the (Cr,Fe) 7 C 3 -typed VLGB structures are closely associated with the C/Cr concentrations, elastic strain energy, low-energy interface and formation enthalpy of the VLGB structure. Similar interpretation is also carried out to illuminate the formation of the VLGB structures in the other nitrocarburized or nitrided alloying steels. [ABSTRACT FROM AUTHOR]

This paper presents the results of researching of the structure, the phase composition, and the working properties of the samples from (24 - 25) HGT steel after volume quenching and tempering, as well as after usual thermohardening with additional chemical and thermal treating, which is nitrocarburizing using nitrogenous carbonaceous paste carburizers at different temperatures (540 - 760°C). [ABSTRACT FROM AUTHOR]

Forged low-carbon alloy Co - 27% Cr - 6% Mo - 0.06% C is studied after 2-h plasma nitrocarburizing in a gaseous mixture of 10% CO + 20% N + 70% H at 400, 500 and 600°C. The surface layer and the core are studied by diffractometric analysis and scanning electron microscopy. The microhardness and the tribological characteristics of the alloy are determined. The effect of the temperature of nitrocarburizing on the wear resistance, surface roughness and friction coefficient of the alloy is considered. [ABSTRACT FROM AUTHOR]

Porous iron foils were used for synthesizing homogeneous samples of iron carbides and (carbo)nitrides. Homogeneous distributions of interstitial nitrogen and carbon were obtained without long treatment times due to limited required diffusion distances in the porous material. By adjustments of the nitriding and carburizing potentials, tailored nitrogen and carbon contents can be achieved, which allows assessment of a phase stability diagram for the Fe-N-C system, for which available experimental data is limited. Thermal decomposition sequences were established for the various iron carbides and (carbo)nitrides using in situ synchrotron X-ray diffraction. Hägg carbide (χ) and ε-carbonitride, Fe 2 (N,C) 1- z , with high carbon content decompose to cementite (θ) above 850 K, while ferrite (α) forms above 950 K and austenite (γ) above 1025 K. For high nitrogen contents ζ- Fe 2 (N,C) is transformed to ε from 680 to 770 K, which decomposes to γ′-Fe 4 (N,C) 1+x between 795 and 900 K as nitrogen is released as N 2 . Ferrite forms above 850 K while austenite may be briefly formed around 900 K. The two iron carbides, cementite and Hägg carbide, exhibit different coefficients of thermal expansion. Below approximately 480 K, cementite is ferromagnetic and a volumetric thermal expansion coefficient of α V = 1.5 × 10 −5 K −1 is obtained. The average value in the paramagnetic state is α V = 4.3 (3) × 10 −5 K −1 . For Hägg carbide the average value is α V = 3.8 (5) × 10 −5 K −1 and only a minor change in unit cell volume is observed at the magnetic transition temperature. [ABSTRACT FROM AUTHOR]

A barrel is one of the most tribologically and thermally loaded parts of artillery weapons. The influence of the salt bath nitrocarburizing and hard-chrome plating of three steels used for artillery barrels on their abrasion resistance is studied in this paper. The tested steels are 42CrMo4, 30CrNiMo8, and 36CrNiMo4 in the initial quenched and tempered state. The test results from the ''dry sand/rubber wheel'' method, after 700 m of wear path, showed that the hard-chrome plating of 42CrMo4 and 30CrNiMo8 steels improved their abrasion resistance. In addition, reduced abrasive wear of 42CrMo4 steel samples was also achieved by nitrocarburizing. Hard chrome-plated samples exhibited good wear resistance due to their greater hardness and bigger thickness of the compound zone in comparison with that obtained on nitrocarburized samples. [ABSTRACT FROM AUTHOR]