Influence of Plasma Nitriding on the Microstructure, Wear, and Corrosion Properties of Quenched 30CrMnSiA Steel

The oil-quenched 30CrMnSiA steel specimens have been pulse plasma-nitrided for 4 h using a constant 25% N₂-75% H₂ gaseous mixture. Different nitriding temperatures varying from 400 to 560 °C have been used to investigate the effects of treatment temperature on the microstructure, microhardness, wear, and corrosion resistances of the surface layers of the nitrided specimens. The results show that significant surface-hardened layer consisting of compound and diffusion layers can be obtained when the oil-quenched steel (α′-Fe) are plasma-nitrided at these experimental conditions, and the compound layer mainly consists of ε-Fe_2-3N and γ′-Fe₄N phases. Lower temperature (400-500 °C) nitriding favors the formation of ε-Fe_2-3N phase in surface layer, while a monophase γ′-Fe₄N layer can be obtained when the nitriding is carried out at a higher temperature (560 °C). With increasing nitriding temperature, the compound layer thickness increases firstly from 2-3 μm (400 °C) to 8 μm (500 °C) and then decreases to 4.5 μm (560 °C). The surface roughness increases remarkably, and both the surface and inner microhardness of the nitrided samples decrease as increasing the temperature. The compact compound layers with more ε-Fe_2-3N phase can be obtained at lower temperature and have much higher wear and corrosion resistances than those compound layers formed employing 500-560 °C plasma nitriding.     

Simulation of the ferritic nitriding process

Abstract

The Lehrer diagram of pure iron is widely used to select the nitriding process of alloy steels. In the present work, the Lehrer diagram of AISI 4140 was determined for the first time by computational thermodynamics. It is demonstrated that the Lehrer diagram of AISI 4140 differs from that of pure iron significantly. Nitriding experiments for AISI 4140 were also carried out to verify the predictions from computational thermodynamics. The scanning electron microscopy results show the existence of two phases in the compound layer and the transmission electron microscopy results at the interface between the compound layer and the diffusion zone verified the coexistence of γ′-Fe₄N and ?-Fe_2–3(C,N) phases. These experimental results agree well to the customised Lehrer diagram of AISI 4140 constructed from computational thermodynamics.     

Effect of the initial microstructure on the plasma nitriding behavior of AISI M2 high speed steel

The nitriding behavior of AISI M2 steel was studied on samples previously submitted to two different heat treatments in order to investigate the effects of the initial microstructure on the thickness and hardness of nitrided layer. Prior to nitriding, one group of samples was fully annealed while the other group was quenched and tempered, thus acquiring the lowest and highest hardness respectively. Plasma nitriding was performed at 450 °C for 8 h with a mixture of N₂ and H₂ in a plasma reactor working under floating potential. Structural and mechanical properties of nitrided layers were characterized using X-ray diffraction (XRD), optical microscopy and microhardness testing. Variations in surface roughness were obtained by 3D surface profilometry analysis. The thicker nitrided layer was obtained for the fully annealed samples, in which the nitrided layer is composed of γ′-Fe₄N and ε-Fe_2-3N phases plus a diffusion zone. For the hardened-tempered samples, the nitrided region mainly consisted of a diffusion zone. Plasma nitriding increased the surface hardness of the fully annealed samples by 330% and that of the quenched-tempered samples by 50%. The nitrided depth was also estimated using cross-sectional microhardness profiles; giving about 140 µm and ∼ 70 µm for the fully annealed and quenched-tempered samples, respectively. Due to the grain to grain nitrogen diffusion, plasma nitriding also increased the surface roughness. The largest roughness was obtained for the fully annealed samples, in accordance with the largest nitrided depth. The difference in the nitriding behavior was explained on the basis of the microstructural aspects of the substrates such as the concentration of the freely dispersed alloying elements and the level of compressive residual stresses.     

Surface microstructure of 316L austenitic stainless steel by the salt bath nitrocarburizing and post-oxidation process known as QPQ

Systematic materials characterization of the Quench-Polish-Quench complex salt bath heat-treatment process (QPQ) surface modified 316L steel was investigated. The results reveal that the nitro-carburized sample surfaces consist of Cr₂N/γ´-Fe₄N and CrN/γ-Fe, while the post-oxidized sample surfaces are comprised of CrO₃, (Fe₃O₄)/ε-Fe₂(N,C), γ′-Fe₄N and S(γ_N)/CrN/α-Fe. Nuclei ε-Fe₂N_1 − x accumulates at the interface between oxide layer and nitride compound layer by the help of post-oxidation. The diffraction peak lines of S-phase (γ_N) move gradually towards higher diffraction angles as increasing depth of 15 μm to 35 μm. An increased content of oxygen is recorded in the post-oxidized surface layer down to the depth of approximately 15 μm, a small peak concentration of carbon occurs at the front of the nitrided layer. Micro-hardness of the post-oxidized samples reaches about 1300 HV_0.1 near the surface region and then reduces sharply across the case/substrate interface.     

Modification of Low-Alloy Steel Surface by High-Temperature Gas Nitriding Plus Tempering

The low-alloy steel was nitrided in a pure NH₃ gas atmosphere at 640 ~ 660 °C for 2 h, i.e., high-temperature gas nitriding (HTGN), followed by tempering at 225 °C, which can produce a high property surface coating without brittle compound (white) layer. The steel was also plasma nitriding for comparison. The composition, microstructure and microhardness of the nitrided and tempered specimens were examined, and their tribological behavior investigated. The results showed that the as-gas-nitrided layer consisted of a white layer composed of FeN_0.095 phase (nitrided austenite) and a diffusional zone underneath the white layer. After tempering, the white layer was decomposed to a nano-sized (α-Fe + γ′-Fe₄N + retained austenite) bainitic microstructure with a high hardness of 1150HV/25 g. Wear test results showed that the wear resistance and wear coefficient yielded by the complex HTGN plus tempering were considerably higher and lower, respectively, than those produced by the conventional plasma nitriding.     

Effect of heating post-treatment on nitrided stainless steel

Although plasma nitriding has been applied successfully to increase the hardness of austenitic stainless steels, the process cycles are long due to the low nitrogen diffusion rate for these steels. An alternative to reduce the nitriding time is to perform a heating treatment after nitriding to prolong the diffusion process. In this work we investigate the properties of plasma nitrided AISI 316 stainless steel after heating post-treatments. The samples were nitrided at 823 K during 3 h. After nitriding, heating post-treatments were performed in a vacuum furnace. The influence of the heating time, ranging from 1 up to 16 h, and heating temperatures, varying from 732 up to 873 K, on the surface properties was investigated. The samples were characterized using microhardness testing, scanning electron microscopy and X-ray diffraction. The nitriding treatment results in a compound layer 44 μm thick with a hardness of 1434 HV_0.1, consisting predominantly of γ'-[Fe₄N] and CrN phases. As expected, an increase of the compound layer thickness and a decrease of the surface hardness with heating time were observed. However, the microhardness profiles show that beneath the surface the layer hardness increases for long treatment times. New phases as Fe₃O₄ and FeCr₂O₄ appear and grow with increasing heating time.     

Effect of ion nitriding on the abrasive wear resistance of ultrahigh-strength steels with different silicon contents

This article studies the effect of silicon (Si) on ultrahigh-strength AISI 4340 steels in connection with the thermal treatment, as well as the influence of this element on nitriding and, consequently, abrasive wear. Four alloys with different Si contents were nitrided at 350 °C (4 and 8 h) and 500 and 550 °C (2 and 4 h) in a gas mixture of 80 vol.% H₂ and 20 vol.% N₂. The nitrided layers were characterized by microhardness and pin-on-disk tests, optical microscopy, scanning electron microscopy with energy-dispersive x-ray spectrometry, and x-ray diffraction (XRD). The increase in Si enhanced the tempering resistance of the steels and also improved considerably the hardness of the nitrided layers. The increase in Si produced thinner compound layers with better hardness quality and high abrasive wear resistance. XRD analysis detected a mixture of nitrides in the layers γ′-Fe₄N, ε-Fe_2–3N, CrN, MoN, and Si₃N₄ with their proportions varying with the nitriding conditions.     

表面形变处理对32Cr3MoVA钢渗氮层组织和性能的影响

测定了32Cr3MoVA钢渗氮层的硬度分布、渗层深度和表面的相组成,对比了表面层形变后渗氮和表面未形变直接渗氮试样的组织和力学性能。结果表明,表面层形变后再渗氮可使渗氮层深度从0．34mm增加到0．61mm,表面层的显微硬度从730HV增加到840HV;X射线衍射分析表明,表面层形变后渗氮层表面的组织主要为体积分数72．6％的Fe3N和27．4％的Fe4N,而表面未形变直接渗氮试样的表面组织为体积分数为17．4％的Fe3N和82．6％的Fe24N10。     

Phase transformations in low-temperature chromized 0.45 wt.% C plain carbon steel

The phase transformations occurring in a 0.45 wt.% C plain steel subjected to plasma nitriding at 540-560 °C for 5.5 h, followed by a salt bath thermoreactive deposition and diffusion (TRD) chromizing process at 500 °C or 550 °C (a process referred to as low-temperature chromizing or duplex chromizing) was investigated by means of optical microscopy(OM), scanning electron microscopy(SEM), X-ray energy dispersive spectroscopy(EDS), and X-ray diffraction. It was found that a CrN compound layer with an average thickness of 7.4 μm and an average micro-hardness of 1476 HV_0.01 was formed in the prior plasma nitrided compound layer by low-temperature chromizing at 550 °C for 6 h. The chromized coating as a whole was found consisting of three sub-layers, namely the outer CrN layer, the intermediate diffusional layer, and the inner residual nitrided compound layer, all formed in the prior nitrided compound layer, and with the inner sub-layer vanishing by prolonging the chromizing time. The intermediate diffusional layer formed at the initial stages of TRD was seen “black” under OM (hence is called “black zone”), and found consisting of α-Fe as a major phase. The self-exhaustion of the “black zone” promoted the chromium atom diffusion deeper into the substrates. The transformation paths involved in the decomposition of the prior nitrided compound layer was likely to be ε-Fe_2-3N → γ′-Fe₄N → α-Fe; and the high hardness of the chromized coating was attributed to a large amount of nano-sized and evenly distributed CrN grains generated in the compound layer.     

Numerical Prediction of Compound Layer Growth of Steel En40B during Plasma Nitriding

PLASMA NITRIDING has developed rapidly over thepast a few years to improve surface properties such aswear,fatigue and corrosion resistance[1'2].Thecombined diffusion/precipitation/compound layerformation reactions make it rather difficult to model thenitriding process.Several attempts have been made byothers.The internal nitridation model is suitable fordescribing the nitriding processing of the matrixwithout carbon,in which there are no formation ofcarbides in surface layer I3).Sun and …     

Tribological properties of duplex treated TiN/TiCN coatings on plasma nitrided PH15-5 steel

Nitriding was carried out in low pressure plasma excited by single- or dual-frequency discharge modes, at a substrate temperature of 523 K, followed by the deposition of 3 μm thick TiCN or TiN/TiCN coatings at a PH15-5 substrate temperature of 723 K. The nitrided layer was comprised of two distinct sublayers, namely a compound layer and a diffusion layer, with a total thickness of ∼ 60 μm. The compound layer was γ′-Fe₄N and the diffusion layer was a solid solution of nitrogen in iron. The thickness of the compound layer fabricated by a single mode plasma is ∼ 5 μm, while that fabricated by dual-frequency mode plasma is ∼ 35 μm.It was found, using a ball-on-disk test, that the plasma nitrided layer fabricated by dual-frequency mode improved wear resistance by nearly one order of magnitude and improved the erosion resistance by a factor of two, compared with untreated steel. This improvement was common to the two nitriding treatments and both types of hard coatings. In particular, a thicker compound layer did not impair the wear resistance or the erosion resistance of the duplex treatment. The erosion resistance shows a linear dependence on the hardness of the uppermost nitrided or deposited layer.     

Oxidation of nitride layers formed on Ti-6Al-4V alloys by gas nitriding

Ti-6Al-4V alloys were nitrided through gas nitriding at 950 °C for 3 h in deoxygenated, atmospheric nitrogen gas. During nitriding, nitrogen reacted and diffused into the alloys to form Ti₂N and a meager amount of TiN in a Ti-N compound layer with a thickness of 20 μm to 25 μm. An α-Ti(N) diffusion layer with a thickness of 40 μm to 80 μm formed below this layer. A small amount of Al was dissolved at the top of the Ti-N compound layer because of the strong interaction of nitrogen with Ti and Al. Nitriding resulted in the dissolution of interstitial nitrogen and the formation of nitrides. Oxidation of the nitrided Ti-6Al-4V alloys initially resulted in the formation of a Ti-N-O layer, which later oxidized to TiO₂. Above 800 °C, the nitrided alloys oxidized rapidly, accompanied by microcracking of the TiO₂ surface layer.     

Low temperature nitriding,nitrocarburising and carburising of AISI 316L austenitic stainless steel

Abstract

AISI 316L grade ASTM F138 austenitic stainless steel specimens were low temperature plasma nitrided (LTPN), nitrocarburised (LTPNC) and carburised (LTPC) using different gas mixtures. Different expanded austenite layers formed after each thermochemical treatment. LTPN and LTPCN led to formation of nitrogen supersaturated expanded austenite (γ_N). After LTPN, a second carbon expanded austenite (γ_C) layer was formed beneath the nitrogen expanded austenite layer (γ_N). LTPC led to formation of a carbon supersaturated expanded austenite (γ_C). Scanning electron microscopy, XRD and microhardness were used to characterise the expanded austenite layers formed on the surface of the specimens. Different mechanisms of formation and growth of the layers are pointed out. XRD results show that the lattice parameter of nitrogen expanded austenite (γ_N) is higher than that calculated for carbon expanded austenite γ_C. As a consequence, the lattice expansion Δa/a for the nitrogen rich (γ_N) phase is higher than the one observed for the (γ_C) phase and the nitrogen rich expanded austenite layer displays higher hardness than the carbon rich expanded austenite layer. The LTPNC bilayer displays a less steep hardness gradient, indicating that the carbon rich expanded austenite layer can grant mechanical support to the harder nitrogen rich layer.     

Intermediate temperature decomposition of supercooled high nitrogen austenite

Abstract

The mechanism of decomposition transformation of Fe–N the austenite system has been investigated. An improved process of austenitic nitriding, achieved by applying controlled nitrogen potential theory, allowed high nitrogen austenite samples with a uniform nitrogen concentration to be produced. The key point of this gas nitriding process is to keep the atmosphere at very low nitrogen potential. As a result, the nitride layer on the surface of the pure iron foil was reduced and pure iron ferrite was thoroughly nitrided, forming high N austenite (γ-Fe[N]) that is thermally stable at room temperature. The nitrogen concentration of this austenite was determined as 9·32 at.-%, which is almost the maximum value achievable in Fe–N austenite.     

Identification of corrosion products resulting from accelerated oxidation process

Abstract

Scanning electron microscopy analysis, X-ray powder diffraction and room temperature ⁵⁷Fe Mössbauer spectroscopy were used to identify the corrosion products of uncoated and coated low alloy steels (LAS) and low carbon steels (LCS) resulting from an accelerated steam oxidation test for 180 h at 660°C. From the Mössbauer spectral analysis, it was shown that in all cases, a series of iron compounds such as α-Fe₂O₃, Fe₃O₄, γ-Fe₂O₃, δ-FeOOH, α-FeOOH, Fe(OH)₂ and Fe(OH)₃ were formed, while XRD measurements revealed only the α-Fe₂O₃ and/or Fe₃O₄/γ-Fe₂O₃ phases. In the LAS uncoated sample, an amorphous phase with magnetic features is found. In the spectra of the borided samples and of the uncoated LCS, an additional doublet was observed, which reveals the presence of a superparamagnetic phase. From the relative areas of the subspectra, it is concluded that the boron aluminised sample underwent the lowest degradation. The mechanism proposed for corrosion products formation is based on the dissociation process.     

Plasma post-oxidation mechanisms of nitrided ferrous alloys

The mechanisms of plasma post-oxidation of plasma nitrided AISI 1045 plain steel were investigated. The influence of plasma post-oxidation temperature and time on the oxide layer thickness, morphology, and composition were addressed. The oxide thickness grows exponentially with temperature, with activation energy of 68 ± 5 kJ mol ^− 1. The time dependence of the oxide layer thickness, on the other hand, is governed by a diffusion-reaction process. It was verified that temperature plays an important role on the morphology of the oxide. Indeed, at the highest temperature, 550 °C, the oxide layer is not homogeneous and has a lower hardness than oxide layers obtained at 480 to 500 °C. The latter seem to be more favorable temperatures to grow compact, homogeneous, and harder oxide layers. The oxide-nitride bi-layer produced here contains a mixture of γ′-Fe₄N and ε-Fe_2-3N and only one iron oxide, Fe₃O₄ (magnetite). The proportions between these phases vary with the plasma processing temperature and time.     

Nitrogen interstitial diffusion induced decomposition in AISI 304L austenitic stainless steel

The nature of the near-surface γ_N phase produced by low-temperature (～400 °C) plasma-assisted nitriding of an austenitic stainless steel 304L is studied. A combination of global probes (X-ray diffraction, nuclear reaction analysis, glow discharge optical emission spectroscopy) and local probes (field ion microscopy, conversion electron Mössbauer, X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopies) is employed to reveal the morphology, phase structure, atomic ordering and chemical state of the obtained γ_N phase. The results consistently reveal the heterogeneous nature of the nitrided layer consisting of nanometric CrN precipitates embedded in a Fe₄N-like matrix. The size of the precipitates is found to be larger at the surface than at the nitrided layer–steel interface. The precipitates have irregular, sphere-like shapes. Moreover, X-ray spectroscopic investigation revealed three different intermetallic distances and different chemical environments for Fe, Cr and Ni, accompanied by a large static disorder. These findings suggest that the presence of the interstitial N destabilizes the homogeneous element distribution in 304L even at such low temperatures. This leads to the segregation into Cr-rich zones that are coherent with the Fe₄N matrix. Possible atomistic decomposition mechanisms are discussed. Based on the heterogeneous nature of the γ_N phase revealed in 304L, an alternative view of its remarkable combination of properties such as large hardness, induced ferromagnetism and preserved corrosion resistance is considered.     

Silver segregation to θ′ (Al2Cu)–Al interfaces in Al–Cu–Ag alloys

θ′ (Al₂Cu) precipitates in Al–Cu–Ag alloys were examined using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The precipitates nucleated on dislocation loops on which assemblies of γ′ (AlAg₂) precipitates were present. These dislocation loops were enriched in silver prior to θ′ precipitation. Coherent, planar interfaces between the aluminium matrix and θ′ precipitates were decorated by a layer of silver two atomic layers in thickness. It is proposed that this layer lowers the chemical component of the Al–θ′ interfacial energy. The lateral growth of the θ′ precipitates was accompanied by the extension of this silver bilayer, resulting in the loss of silver from neighbouring γ′ precipitates and contributing to the deterioration of the γ′ precipitate assemblies.     

Corrosion resistance properties of plasma nitrided Ti-6Al-4V alloy in nitric acid solutions

The corrosion resistance of plasma nitrided Ti-6Al-4V titanium alloy in nitric acid solutions was investigated using polarisation curves, cyclic voltammetry, XRD analysis and surface microhardness. For comparison, untreated alloy samples were tested under the same conditions. While the untreated alloy shows a passive behaviour, the compound layer obtained by glow-discharge nitriding treatment, which is composed of an outer TiN and an inner Ti₂N layer, shows a high electrochemical inertia; however, if the polarisation potential is higher than a threshold or the period of immersion in high oxidising acid solution is sufficiently long, then the compound surface layer will be attacked and removed with corrosion rate higher than in the case of the untreated Ti-6Al-4V alloy. The corrosion resistance of Ti₂N layer results higher in comparison with that of TiN layer.     

A study of the corrosion properties of plasma nitrocarburized and oxidized AISI 1020 steels

Plasma nitrocarburized AISI 1020 steels were oxidized for 15, 30 and 60 min to evaluate their corrosion and microstructural properties. After plasma nitrocarburizing for 3 h at 570°C in a gas mixture comprising 85 vol.% N₂, 12vol.% H₂ and 3 vol.% CH₄, the compound layer composed of ɛ-Fe_2–3(N,C) and γ’-Fe₄(N,C) phases and the diffusion layer above the matrix were observed. The top oxide layer, consisting mainly of magnetite (Fe₂O₄) and hematite (Fe₂O₃) phases, forms after post-oxidation treatment at 500°C. However, the oxide layer was severely degraded by spallation as a result of increases in post-oxidizing time. The difference in corrosion resistance should be attributed to the thickness of the top oxide layer, which was governed by post-oxidizing time.