Gas Nitriding Research Articles

Properties of new-generation hybrid layers combining hardfacing and nitriding dedicated to improvement in forging tools\u2019 durability

This article deals with the subject of layers produced in a hybrid process combining hardfacing and nitriding. These layers are characterized by improved resistance to wear, which arises from the synergy between favorable materials during hardfacing and nitriding treatment. Tests were conducted on hot-work tool steel (H11) samples. Hardfaced layers consisting of three layers were applied to these samples, where layers were made from two materials—Robotool 46 and Hardface VMolc powder wires. Next, samples of both materials were nitrided using ZeroFlow gas nitriding technology, with control of potential aimed at obtaining a diffusion layer without a white zone of nitrides on the surface. The next step was to investigate the properties of hardfaced layers and of layers hardfaced after nitriding treatment. Conducted tests covered observations of the microstructure, microhardness measurements as a function of distance from the surface, measurement of stresses in the surface layer by means of X-ray diffraction, and tribological tests. Obtained test results show that hybrid layers combining hardfacing and nitriding may be suitable for improving the lifetime of tools applied in hot forging processes.

The Effect of the Transformation of ε-Fe2-3N into γ′-Fe4N Phase on the Fatigue Strength of Gas-Nitrided Pure Iron

The effect of the iron nitride phases, ε-Fe2-3N and γ′-Fe4N, on the fatigue strength was investigated. Pure iron was used to observe only the effect of nitride, excluding the effects of factors, such as residual stress, depending on the alloy composition and microstructural change according to working on the fatigue strength. In this work, ε and γ′ phases were respectively grown at a time on the surface of the pure iron specimens using the appropriate nitriding potential KN, the mixture rates of ammonia and hydrogen gases, at same temperature of 570 °C according to the Fe-N Lehrer diagram. Another γ′ phase was prepared by first growing the ε phase and then transformed from ε phase into γ′ phase by changing the KN at the same temperature of 570 °C in the 2-stage gas nitriding. The fatigue strengths of the iron nitride consisted of ε and γ′ phases, γ′ phase, and γ′ phase grown by the 2-stage gas nitriding were evaluated, respectively. As a result, first, it can be seen that the diffusion layer of ε phase was deeper than γ′ phase, but fatigue strength was lower. On the other hand, fatigue strength of both the γ′ phases are higher than that of the ε, and the fatigue strength of γ′ phase nitride grown by 2-stage gas nitriding was almost similar to that of γ′ phase nitride grown at a time, i.e., fatigue strength was not significantly related to diffusion depth and depended on nitride phases in this study. Secondly, we cannot clearly conclude that there was the difference in fatigue strength according to the thickness of nitride layer consisted of γ′ phase. However, it is clear that when ε phase was transformed to γ′ phase, fatigue strength had the same level as γ′ phase formed at one time.

Crystallography of γ'-Fe4N formation in single-crystalline α-Fe whiskers.

Gaseous nitriding of steel and iron can significantly improve their properties, for example corrosion resistance, fatigue endurance and tribological properties. In order to obtain a better understanding of the early stages of formation of the initial cubic primitive γ'-Fe4N, the mechanism and crystallography of the α-γ' phase transformation was investigated under simplified conditions. Single-crystal α-Fe whiskers were nitrided at 823 K and a nitriding potential of 0.7 atm-1/2 for 20 min. The resulting microstructure and phases, as well as the crystallographic orientation of crystallites belonging to a particular phase, were characterized by scanning electron microscopy coupled with electron backscatter diffraction. The habit planes were investigated by single- and two-surface trace analysis. The α-Fe whiskers partly transform into γ'-Fe4N, where γ' grows mainly in a plate-like morphology. An orientation relationship close to the rational Pitsch orientation relationship and {0.078 0.432 0.898}α and {0.391 0.367 0.844}γ' as habit planes were predicted by the phenomenological theory of martensite crystallography (PTMC), adopting a {101}α〈101〉α shear system for lattice invariant strain, which corresponds to a {1 1 1}γ'〈1 12〉γ' shear system in γ'. The encountered orientation relationship and the habit planes exhibit excellent agreement with predictions from the PTMC, although the transformation definitely requires diffusion. The γ' plates mainly exhibit one single internally untwinned variant. The formation of additional variants due to strain accommodation, as well as the formation of a complex microstructure, was suppressed to a considerable extent by the fewer mechanical constraints imposed on the transforming regions within the iron whiskers as compared to the situation at the surface of bulk samples.

Finite Element (FE) Modeling of the Nitrogen Concentration Profile in 31CrMoV9 Steel After Gas Nitriding

This paper presents a possibility for the numerical calculation of the profile of nitrogen (N) concentration in 31CrMoV9 steel after gas nitriding. Samples subjected to gas nitriding at three different temperatures were investigated with different experimental methods. The concentration profile of nitrogen was determined using a Jeol JXA-8900 RL electron probe microanalyzer (Epma). The distance between two points was 5 μm, while the acceleration voltage was 20 kV and the sample current was 40 nA. The nitrogen concentration profile was then modeled using the finite-element method. The program was written in Ansys Parametric Design Language, and the basic element was the three-dimensional element Solid70, while simulation of the model was done in Ansys. Comparison of the results obtained from Epma and modeling confirmed the accuracy of the theoretical approach.

Low-pressure gas nitriding of AISI 304 austenitic stainless steel: reducing the precipitation of chromium nitrides

In this study, low-pressure gas nitriding (gas pressure of 0.01 MPa) was conducted to produce a thicker nitrided layer with high hardness and anti-corrosive properties on AISI 304 austenitic stainless steel. The effects of nitriding temperature and duration on the microstructure and surface property of nitrided layers were systematically evaluated by using optical microscope, X-ray diffraction, elemental analysis, microhardness test and potentiodynamic polarization tests. The samples were also treated under conventional gas pressure of 0.1 MPa for comparison. The results show that the low-pressure gas nitriding could restrain the precipitation of chromium nitrides effectively, which is beneficial for obtaining a thicker nitrided layer. Although the activation energy of nitrogen diffusion for low-pressure nitriding (220 kJ mol−1) is higher than that for the atmospheric pressure nitriding (196 kJ mol−1), the thickness of nitrided layers for low pressure nitriding could reach to a comparable value as that for the conventional atmospheric pressure nitriding. More importantly, the surface toughness and corrosion resistance of nitrided layers could be improved by low-pressure nitriding, which is mainly attributed to the optimized nitrogen content in nitrided layers and the reduced precipitation of chromium nitrides under low-pressure.

Enhancing the lifetime and corrosion resistance of gears made of carbon steel

AbstractGears for structural machines require high fatigue strength for high performance. Generally, gears made of carbon steel easily corrode, thus, shortening their fatigue life. The aim of this paper is to improve the fatigue strength of carbon steel gears by means of heat treatment method which was nitriding composed of 95 % nitrogen gas as well as 5 % hydrogen gas, and to investigate its properties after nitriding. Therefore, in order to find the optimum nitriding temperature to increase the hardness and corrosion resistance of gears, the gas nitriding process was conducted at two different tube furnace temperatures: low (550 °C) and high (1150 °C), both for four hours. Microstructural and mechanical property evaluation of the low and high temperature nitrided low‐carbon steel BS970‐080A15 were studied and the results were compared to identify which gear had better performance in terms of hardness as well as corrosion resistance. The results from Vickers hardness test and weight loss analysis proved that high temperature nitrided carbon steel is harder and more corrosion resistant than the low temperature one.

Gas-Sensing Properties of the SiC Monolayer and Bilayer: A Density Functional Theory Study.

Using density functional theory calculations, the adsorption of gaseous molecules (NO, NO2, NH3, SO2, CO, HCN, O2, H2, N2, CO2, and H2O) on the graphitic SiC monolayer and bilayer has been investigated to explore the possibilities in gas sensors for NO, NO2, and NH3 detection. The strong adsorption of NO2 and SO2 on the SiC monolayer precludes its applications in nitride gas sensors. The nitride gases (NO, NO2, and NH3) are chemisorbed on the SiC bilayer with moderate adsorption energies and apparent charge transfer, while the other molecules are all physisorbed. Further, the bilayer can effectively weaken the adsorption strength of NO2 and SO2 molecules, that is, NO2 molecules are only weakly chemisorbed on the SiC bilayer with an Eads of −0.62 eV, while SO2 are physisorbed on the bilayer. These results indicate that the SiC bilayer can serve as a gas sensor to detect NO, NO2, and NH3 gases with excellent performance (high sensitivity, high selectivity, and rapid recovery time). Moreover, compared with other molecular adsorptions, the adsorption of NH3 molecules significantly changes the work function of the SiC monolayer and bilayer, indicating that they can be used as optical gas sensors for NH3 detection.

A comparative study of the tribocorrosion behaviour of low temperature nitrided austenitic and duplex stainless steels in NaCl solution

This work studied and compared the tribocorrosion behaviour of low temperature nitridedaustenitic (316L) and duplex (2205) stainless steels. After gaseous nitriding at 450 °C for10 h, the S-phase layer formed on the duplex stainless steel is thicker and harder than that onthe austenitic stainless steel. The formation of S-phase layers reduces both mechanical wearand chemical wear of stainless steels but the duplex type exhibits better tribocorrosion performance. Wearing through of the S-phase layer during tribocorrosion leads to asignificant drop in open circuit potential, friction instability, and total material loss from the nitrided specimens. The work concludes that low temperature gaseous nitriding is effective in improving the tribocorrosion behaviour of these materials to avoiding catastrophic surface failures.

Anisotropy effects on gaseous nitriding of austenitic stainless steel single crystals

Low temperature nitrided austenitic stainless steel exhibits strong orientation dependent case growth kinetics. Anisotropy effects during low temperature gaseous nitriding of single crystals of austenitic stainless steel with surface-normal orientations 〈100〉, 〈111〉 and 〈10 7 2〉 were investigated with thermogravimetry, X-ray diffractometry (XRD), glow discharge optical emission spectroscopy (GD-OES), nano-indentation and electron back-scatter diffraction (EBSD). Case growth kinetics were fastest for 〈100〉 and slowest for 〈111〉. Depth-profiling over the case depth was accomplished by successive layer removal combined with X-ray diffraction and nano-indentation. The results indicate that anisotropic case-growth kinetics is related to early nucleation of long-range ordered (LRO) over-stoichiometric γ’N−Me4N1+x (Me=metal atoms) in 〈100〉, which is inseparable from short range ordered (SRO) expanded austenite, γN. Depth ranges of elasto-plastic and purely plastic accommodation of the nitrogen induced lattice expansion were distinguished. In the elasto-plastic region stress-gradient enhanced diffusion of N is observed.

The application of copper-containing nanoparticles as a gas nitriding catalyst

The results of the study of the possibilities of using copper-containing nanoparticles obtained by pseudomatrix synthesis as a catalyst for cyclic gas nitriding of steel products were presented in this paper. It was shown by the example of steel 08Yu (corresponding to the European standard of steel DC04/DC03 EN 10139) that the preliminary application of nanocomposite-catalyst including copper-containing nanoparticles and macromolecules of poly-(N-vinylpyrrolidone) polymer pseudomatrix to the product significantly intensified the chemical heat treatment process. It was found that under identical treatment process conditions the thickness of the diffusion layer for a sample with a pre-applied nanocomposite was 90 urn, and without such pretreatment — 42 urn It is noted that the achievement of this effect is due to the catalyst synthesis peculiarities, which allows to obtain a copper sol with a narrow size distribution of particles resistant to aggregation.

Correlation between microstructure and micro-hardness of 316L nitrided austenitic stainless steel

This paper presents the effect of duration of solution nitriding towards microstructure and microhardness of AISI 316L stainless steel. The AISI 316L stainless steel was exposed to high temperature gas nitriding process at 1200 °C by using 50% ammonia gas and 50% nitrogen gas for different durations of 8, 15 and 20 hours. The nitrided austenitic stainless steel then underwent several analyses and tests to find the effect of different durations of nitriding towards its microstructures and microhardness. From the experiments conducted to the nitrided samples, it was found that the higher duration of nitriding resulted in the increase of microhardness of the samples due to the increase of nitrogen content in the stainless steel. However, there was no effect found on the microstructure of the nitrided samples. It was noted that the highest hardness was obtained from the nitrided sample of 20 hours that has 245.1 HV which is about 70% higher than the as-received sample.

Avaliação de desgaste por perda de massa de sistema extrator de moldes de injeção de termoplásticos revestidos com filme fino de DLC

O presente trabalho tem por objetivo, a deposição de filmes finos de DLC sobre componentes do sistema extrator de moldes de injeção, verificando a influência deste revestimento superficial no desgaste desses componentes, sob diferentes configurações de lubrificação superficial. Para esta avaliação, pinos extratores, atualmente produzidos em aço AISI H13 e nitretados à gás, foram revestidos com um filme fino de DLC (carbono tipo diamante, do inglês Diamond Like Carbon) e buchas extratoras foram confeccionadas em AISI P20 e AISI H13, e, posteriormente, temperadas e revenidas. O filme fino de DLC foi depositado em uma câmara com sistema PECVD (deposição química a vapor assistida por plasma, do inglês Plasma Enhanced Chemical Vapor Deposition) auxiliado por confinamento eletrostático de plasma. Um dispositivo que simula o sistema de extração de um molde de injeção foi utilizado para testar o comportamento ao desgaste dos conjuntos pino-bucha propostos, em função do aumento do número de ciclos. O desgaste nos pinos foi avaliado quantitativamente por perda de massa, após 100.000 ciclos. Os resultados da perda de massa mostram que para as amostras sem utilização de graxa lubrificante, a taxa de desgaste foi consideravelmente menor. Os pinos que menos apresentaram desgaste foram os dos conjuntos PS e HS, ambos sem utilização de graxa, com perda de 5,400 e 5,467 mg, respectivamente. Com isto, ficou evidenciado a possibilidade de utilização de pinos com revestimento de DLC sem a utilização de graxa lubrificante em sistemas extratores de moldes de injeção e que isto ainda propiciou uma redução considerável na taxa de desgaste.

The Use of the Gas Nitriding Process for the Nitridation of Powder for Laser Powder Bed Fusion

AbstractAdditive manufacturing in the powder bed by means of laser powder bed fusion (L-PBF) combines the fast production of metallic components with complex geometrical structures with a high degree of customization and low raw material consumption. However, there is a high demand for the development of new alloys to achieve the desired properties of additively manufactured parts. Due to a lack of knowledge, only a small group of steel powders are currently available for L-PBF. As an example, Cr and CrMn steel powders with additional alloyed nitrogen could be used for L-PBF products for Ni-free medical devices. The present work investigates the possibility of alloying nitrogen to austenitic steel alloy powder and high alloyed martensitic steel powder via gas nitriding. For this purpose, an X30CrMo7-2 powder and an AISI 316L powder for additive manufacturing were arranged to form powder beds of different heights and nitrided at different temperatures, times and nitriding potentials. The nitrogen content was determined from the samples taken at different heights of the powder beds. Individual powder particles were examined by means of metallography, scanning electron microscopy and electron probe micro analysis. It is shown that the nitrogen concentration in the powder is essentially determined by the nitriding temperature. In the X30CrMo7-2 powder, nitride precipitation also occurs at high temperature and N and C gradients are present in the particles with an average nitrogen content of up to 4 mass%. In the 316L powder, a homogeneously distributed nitrogen content of 0.35 mass% can be achieved after gas nitriding at 420 °C for 24 h. The flow properties of both powders are hardly affected by gas nitriding.

Kinetics of nitrided layers formation on low-alloy steels in the gas nitriding process

The study examined cylindrical samples made of low-alloy structural steel 42CrMo4 (40HM) and 41CrAlMo7 (38HMJ) subjected to the nitriding process at 540oC during 2, 7, 12 and 17 hours. During the first 2 hours of the process, the atmosphere was ammonia, while the rest of the process was carried out in the atmosphere consisting of 50% ammonia and 50% dissociated ammonia. After the process, surface hardness, hardness distribution at the depth of 50 µm from the surface up to the hardness of the core, and microstructure of the nitrided layer produced were examined. It has been shown that along with the extension of the duration of the nitriding process on 40HM steel, the surface thickness of the layer of nitrides increases from 6 µm after 2 hours to 14 µm after 17 hours of the process and this layer has a two-phase structure (+’), while the thickness of the diffusion layer was from 0.15 to 0.44 mm (depending on the process time). In the case of 38HMJ steel, the thickness of the layer of nitrides increased from 1 µm after 2 hours to 9.5 µm after 17 hours. The thickness of the diffusion layer was from 0.08 to 0.35 µm (depending on the process time).

Influence of the different variants of the surface treatment on the durability of forging dies made of Unimax steel

The study performs an analysis as well as makes a comparison of the durability of forging tools used in the die forging process made of high-strength steel Unimax. For tool steel WCL (1.2343 according to DIN), which has been applied so far, the obtained average durability has been at the level of about 6000 forgings. Additionally, in order to increase the durability of the Unimax material, two surface treatment variants were applied: in the form of ion nitriding (for nitrides A) and gas nitriding together with a PVD-Alvin coating, which were compared with the results for an insert without surface treatment. For each variant, three tools were produced, in order to obtain repeatable and verified results. In the first place, an analysis of the working conditions of the tools was performed through thorough observations of the industrial forging process, particularly the tribological conditions, including the manner of lubrication as well as the temperature distributions, by means of, among others, thermovisual examinations. Additionally, numerical modeling of the process was carried out with the purpose of a more accurate analysis of the tool work in contact. Next, a detailed analysis of the exploitation of the worn tools was performed, including a macroscopic and geometrical analysis through 3D scanning, microscopic optical, and SEM tests as well as microhardness measurements. The obtained results demonstrated that only the application of the new material, Unimax, itself caused a durability increase by 2.5 times with regard to the WCL steel used so far. In turn, with the application of additional surface engineering techniques, Unimax tools characterized in better operational properties (high thermal and abrasive wear resistance at elevated temperatures), which made it possible to forge over four times more forgings, i.e., 26,000 items, after nitriding with a PVD-Alvin coating had been applied to the tool.

Fracture Toughness of Ferritic Corrosion-Resistant Steel Subjected to High-Temperature Nitriding

The static fracture toughness of sheet corrosion-resistant steel 08Kh17T (0.5 mm in thickness) subjected to gas nitriding at 1075°C for 4 h and subsequent tempering at 585–700°C is analyzed. An increase in the tempering temperature from 585 to 700°C is shown to lead to a decrease in the stress-intensity factor Kc from 30.4 to 22.5 MPa m1/2 at a reasonable high strength; the cause for the decrease is the precipitation of large chromium nitride plates.

Unusual Iron Nitride Formation Upon Nitriding Fe-Si Alloy

The influence of Si, substitutionally dissolved in ferritic Fe-2 at. pct Si and Fe-4.5 at. pct Si alloys, on the nucleation and growth of γ′ iron nitride upon controlled gaseous nitriding was investigated. The nitrided specimens were characterized by XRD, light microscopy, TEM, EELS, EPMA and EBSD. The combination of difficult and thus delayed precipitation of (1) silicon nitride, because of a large misfit with the ferrite matrix, and (2) γ′ iron nitride, because of the negligible solubility of Si, was shown to lead to a series of unusual, nonequilibrium phenomena: high nitrogen supersaturation, development of Si-containing metastable γ′ phase of peculiar morphology, precipitation of amorphous silicon nitride in γ′ and development of ε nitride at thermodynamic conditions unlikely to allow its formation.

TEM analysis of surface layer of Ti-6Al-4V ELI alloy after slide burnishing and low-temperature gas nitriding

Gas nitriding is one of the more effective ways of improving wear and corrosion resistance of titanium alloys, but this treatment performed at a high temperature degrades their high specific strengths. Therefore, we have subjected a Ti-6Al-4V alloy to a hybrid multi-stage treatment including turning with slide burnishing followed by gas nitriding at temperature reduced down to 540 °C. The mechanical treatment introduces a large number of defects in the form of dislocations and grain boundaries—opening additional paths for nitrogen diffusion. High resolution electron microscopy shows that after such mechanical treatment the surface is covered with an amorphous tribo-film. Below the surface, to a depth of some 100 nm, a layer of elongated dislocation cells is seen. The latter was backed by a 500 nm thick layer of strongly defected laths. Gas nitriding at 540 °C caused at first a crystallization of the tribo-layer (after 8 h), producing a mixture of nitrides and oxides. Next, below a tribo-layer, a dense layer of the δ-TiN phase is formed, with a thickness of 5 nm and 50 nm after 16 h and 24 h, respectively. Application of slide burnishing and low-temperature gas nitriding causes an increase in surface hardness by 5–10 % without compromising the strength of the core material.

Investigation of the Compound Layer Formed by Steel 16MnCr5 After Gas Nitriding

The aim of this research is the investigation of the compound layer formed by steel 16MnCr5 after gas nitriding. The investigation was carried out with metallographic methods – that is, with optica...

Influence of the phase structure of nitrides and properties of nitrided layers on the durability of tools applied in hot forging processes

This article presents beneficial results of surface layer modification by gas nitriding. The main purpose of the work is to demonstrate the influence of nitriding parameters on phase structure of iron nitrides and therefore on the durability of nitrided forging dies.First, the laboratory tests determined the parameters of thermo-chemical treatment - nitriding by the ZeroFlow method to obtain the desired phase structures: α, γ' or ε, nitrides. Then the microstructure and properties of the obtained layers were examined. Next, an industrial test was carried out on real tools in the hot forging process. The effect of nitriding on durability was evaluated by comprehensive analysis of used tools including macroscopic observations, laser surface scanning, SEM observations and microstructure and microhardness testing in the surface layer. The results showed a significant effect of improving durability.There were also clear differences in the wear mechanisms depending on the nitride layer used. Alpha-type layer is less prone to cracking, while gamma prime-type layers crack most, which may be due to their crystallographic structure and higher hardness. The results confirmed the rightness of controlling the nitriding process in order to obtain the proper layer, especially in the case of nitriding of forging tools.