Structure of duplex CrN/NbN coatings and their performance against corrosion and wear

In tribological applications the coating-substrate combination can be considered as a system, since both greatly influence the properties of that affect the tribological performance. Further, it is often desirable that both high wear resistance and corrosion resistance can be achieved even when low cost and easily machineable substrate materials are considered. Duplex surface treatment combining pulse plasma nitriding and PVD coating can provide solution for excellent wear and corrosion resistance for low alloy and constructional steels.In this work three different pulse plasma nitriding processes were carried out prior to the CrN/NbN PVD coating to attain high surface hardness and enhanced load bearing behaviour for S154 high strength construction steel. The phase composition of the compound layer, formed in the nitriding process, was found to greatly affect the tribological properties of the duplex system. The compound layer with high amount of ?-phase contributed to superior corrosion and wear resistance, whereas the ductile γ'-phase compound layer provided better impact resistance and enhanced. The best duplex treated S154 samples had wear resistance comparable to that of similarly coated HSS. The corrosion resistance was also improved by duplex process. If anodic current at + 500 mV vs. SCE is considered as criteria, the best system has almost 3 orders of magnitude lower corrosion current than with the PVD coating alone.     

Differences in nitrided layer between classic active screen plasma nitriding and active screen plasma nitriding with hemispherical cathodic cage

Abstract

Active screen plasma nitriding (ASPN) is commonly used when regular surface hardening is necessary. The ASPN technique produces a more homogeneous surface coating than direct current plasma nitriding (DCPN) due to different process principles. The term active screen in plasma nitriding refers to a cathodic cage with a defined geometry. The purpose of this work was to study the differences between ASPN using a hemispherical cathodic cage and ASPN using a normal cylindrical cathodic cage. Following some trials using similar parameters, the tests were carried out with three conditions: with DCPN, with a cylindrical cathodic cage in ASPN and with a hemispherical cathodic cage in ASPN. X-ray diffraction and scanning electron microscopy analysis together with energy dispersive spectroscopy were applied to characterise the nitrided layers. The nitrided layers are not the same for each of the conditions used. The ASPN with a hemispherical cathodic cage produced a layer of almost Fe₃N alone, while the other processes gave significant amounts of Fe₄N in the nitrided layer. Scanning electron microscopy analysis showed different surface morphology for each condition.     

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.     

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.     

Progress in control of nitriding potential in ASPN process

Abstract

A novel active screen plasma nitriding (ASPN) process provided excellent temperature homogeneity in the load and showed further progress in the control of nitriding potential. In addition to a variation of the nitrogen partial pressure in the process gas commonly used in the conventional plasma nitriding, the applied bias power strongly impacted the nitriding results. In the present work, an application of both methods for the control of nitriding potential in the ASPN process was systematically investigated for a wide range of process parameters to meet the treatment requirements for different types of engineering steel. A two-stage technique based on proper choice of process temperature and required nitriding potential in each stage has been applied in the ASPN process to avoid unnecessary compromises between sufficient thickness of the compound layer, the maximum case hardness and the acceptable nitriding hardness depth.     

Modeling the Gas Nitriding Process of Low Alloy Steels

The effort to simulate the nitriding process has been ongoing for the last 20 years. Most of the work has been done to simulate the nitriding process of pure iron. In the present work a series of experiments have been done to understand the effects of the nitriding process parameters such as the nitriding potential, temperature, and time as well as surface condition on the gas nitriding process for the steels. The compound layer growth model has been developed to simulate the nitriding process of AISI 4140 steel. In this paper the fundamentals of the model are presented and discussed including the kinetics of compound layer growth and the determination of the nitrogen diffusivity in the diffusion zone. The excellent agreements have been achieved for both as-washed and pre-oxided nitrided AISI 4140 between the experimental data and simulation results. The nitrogen diffusivity in the diffusion zone is determined to be constant and only depends on the nitriding temperature, which is ~5 × 10^?9 cm²/s at 548 °C. It proves the concept of utilizing the compound layer growth model in other steels. The nitriding process of various steels can thus be modeled and predicted in the future.     

Plasma nitriding of plastic mold steel to increase wear- and corrosion properties

In this study, the wear- and corrosion resistance of the layers formed on the surface of a precipitation hardenable plastic mold steel (NAK55) by plasma nitriding were investigated. Plasma nitriding experiments were carried out at an industrial nitriding facility in an atmosphere of 25% N₂ + 75% H₂ at 475 °C, 500 °C, and 525 °C for 10 h. The microstructures of the nitrided layers were examined, and various phases present were determined by X-ray diffraction. Wear tests were carried out on a block-on-ring wear tester under unlubricated conditions. The corrosion behaviors were evaluated using anodic polarization tests in 3.5% NaCl solution.The findings had shown that plasma nitriding does not cause the core to soften by overaging. Nitriding and aging could be achieved simultaneously in the same treatment cycle. Plasma nitriding of NAK55 mold steel produced a nitrided layer consisted of a compound layer rich in ε-nitride and an adjacent nitrogen diffusion layer on the steel surface. Increasing the nitriding temperature could bring about increase in the thickness of the nitrided layer and the nitride volume fraction. Plasma nitriding improved not only surface hardness but also wear resistance. The anti-wear property of the steel was found to relate to the increase in the thickness of the diffusion layer. Corrosion study revealed that plasma nitriding significantly improved corrosion resistance in terms of corrosion potential and corrosion rate. Improvement in corrosion resistance was found to be directly related to the increase in the nitride volume fraction at the steel surface.     

Plasma nitriding and nitrocarburising of a supermartensitic stainless steel

Abstract

Supermartensitic stainless steels (SMSSs) are a new generation of the classic 13%Cr martensitic steels, lower in carbon and with additional alloying of nickel and molybdenum offering better weldabilty and low temperature toughness. Several works have shown that plasma nitriding and nitrocarburising of stainless steels at low temperatures produces a hard surface layer which results in increased wear resistance. In this work, SMSS samples were plasma nitrided and nitrocarburised at 400, 450 and 500°C. The plasma treated SMSS samples were characterised by means of optical microscopy, microhardness, X-ray diffraction and dry wear tests. The thickness of the layers produced increases as temperature is raised, for both plasma nitriding and nitrocarburising. X-ray diffraction demonstrates that the chromium nitride content grows with temperature for nitriding and nitrocarburising, which also showed increasing content of iron and chromium carbides with temperature. After plasma treating, it was found that the wear volume decreases for all temperatures and the wear resistance increased as the treatment temperature was raised. The main wear mechanism observed for both treated and untreated samples was grooving abrasion.     

Corrosion behaviour after anodising of pre-nitrided CP-Ti

Abstract

This study presents corrosion characterisation of CP-Ti after nitriding, anodising and duplex surface treatment. The structural properties and corrosion characteristics were investigated by using scanning electron microscopy, X-ray diffraction, Raman spectroscopy and electrochemical polarisation unit. After anodisation and duplex treatment, the porous oxide layer including anatase phase and unshaped or circular pores formed on CP-Ti surface. Size and shape of the pores were changed according to the anodising parameters. It was observed a double layered structure including porous layer on the top and a dense columnar layer beneath that section formed on duplex treated CP-Ti surface at low temperature of anodisation process. Pitting corrosion was observed on all of the samples after the electrochemical polarisation tests. Dense columnar microstructure provided good corrosion resistance via acting as a barrier. As a result, double oxide layer provided an important improvement in corrosion resistance in contrast with nitriding and anodisation processes.     

High frequency and low voltage plasma immersion ion implantation of nitrogen on industrial pure iron at different Rf power

Traditional plasma ion immersion implantation (PIII) can effectively improve material mechanical property and corrosion resistance. But the modified layer by PIII is too thin for many industrial applications. High frequency and low voltage plasma immersion ion implantation (HLPIII) has advantages of PIII and nitriding. Comparing with traditional ion nitriding, HLPIII can obtain higher implantation energy and create a thick modified surface layer. In the present paper nitriding layers were synthesized on industrial pure iron using high frequency and low voltage plasma immersion ion implantation with different RF power (400 W, 600 W, and 800 W). The microstructure of the nitriding layers was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mechanical properties such as microhardness and wear resistance were analyzed using HXD1000 microhardness and CSEM pin-on-disk wear testing machine. The anodic polarization characteristics were measured in a 0.9% NaCl solution at room temperature to examine the corrosion resistance of the nitriding layer. The results reveal that Fe₂N, Fe₃N and Fe₄N coexist in the nitriding layer. The nitriding layer is a corrosion protective coating on industrial pure iron in 0.9% NaCl solution. The hardness, wear resistance and corrosion resistance of the nitrided layers on industrial pure iron increase with RF power.     

贫铀表面激光氮化层的结构和氧化动力学研究

为了研究激光氮化对金属铀表面结构和性质的影响,利用X射线衍射结合原位高温反应器,研究了不同激光氮化工艺后的贫铀表面结构,并对不同温度下UN的氧化过程进行了原位XRD实验分析,获得了相应条件下的氧化动力学曲线。结果显示,激光扫描速率对贫铀表面氮化层的相结构有显著影响,较快的扫描速率能够获得单一的UN结构,较慢的扫描速率将获得UN和U_2N_3的双相混合层。氧化动力学实验表明,表面激光氮化制备的UN层对基材有很好的保护作用,能够显著降低金属铀的氧化腐蚀速率。较低的温度下UN层的氧化非常缓慢,温度升至180℃以上时氧化速率明显加快,其氧化动力学曲线与贫铀有明显差异,文中对此现象和氮化层的氧化机制进行了分析和讨论。     

The edge force components in oblique cutting

It is shown that in an oblique cutting operation two of the edge force components can be derived on the basis of the hypotheses: (a) that there exists an equivalent orthogonal cutting operation which is such that the depth of the layer of workpiece material which is extruded below the cutting edge in oblique cutting, h, is equal to that which would be extruded below the cutting edge in this equivalent orthogonal cutting operation, and (b) the area of contact between the flank face and the extruded layer in oblique cutting is that which would exist in the equivalent orthogonal cutting operation.By applying these hypotheses, expression for the edge force component acting normal to the generated surface, F_v′, and that acting along the cutting speed direction, F_c′, are derived and are verified using empirical cutting data. It has not proved possible to formulate a model from which the edge force component acting along the cutting edge direction, F₂′, can be deduced. However, on the basis of conjecture, a simple-minded argument is advanced which leads to an expression for F₂′. This expression is verified using empirical force data.     

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.     

High temperature wear and friction properties of duplex surface treated bearing steels

Duplex treatments by thermo reactive diffusion (TRD) chromizing and puls plasma nitriding were carried out on AISI 52100 and 8620 bearing steels. Tribological behaviors of TRD chromized and duplex treated bearing steels were investigated against Al₂O₃ ball in ball-on-disc system at room temperature and 500 °C. The samples were pack chromized in a furnace at temperature of 1000 °C for 5 h. After chromizing, the samples were puls plasma nitrided for 5 h at 500 °C. The coated steels were characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), scratch and microhardness testing. Plasma nitriding of chromized steels increased the total thickness of the compound layer. The subsequent plasma nitriding increased the surface hardness to 2135 HK_0.025 due to the formation of CrN and Cr₂N. The surface hardness and scratch resistance of coating can be increased with duplex treatment of chromizing followed by plasma nitriding, resulting in high wear resistance. Tribological tests indicated that puls plasma nitriding process decreased the coefficient of friction values and wear rate of the chromized steels at room temperature and 500 °C. Also, examination of the worn surface of the samples showed that particularly at high temperature, the oxidized compact layer occurs and tribo-oxidation played an important role in oxidation behaviour of the steels after the duplex treatment.     

渗氮温度对40Cr钢QPQ组织与性能的影响

采用光学显微镜(OM)对不同渗氮温度QPQ处理的40Cr钢表面渗氮层显微组织进行观察分析,同时进行维氏硬度试验与销盘摩擦磨损试验,获得渗氮层硬度梯度与磨损失重,并观察分析了磨损表面SEM形貌。结果表明,经不同渗氮温度的QPQ处理后,40Cr钢表面均形成了由氧化膜层、化合物层与扩散层构成的表面渗氮层。但随渗氮温度的升高,其渗氮层厚度呈先增加后减小的变化趋势,4种渗氮温度(580、600、620、640 ℃)下样品有效渗层深度分别为0.14、0.20、0.29、0.26 mm。随渗氮温度的升高,磨损量呈先减小后增大的趋势,在620 ℃下达到最低值。4种渗氮温度下磨损形式均以磨粒磨损与粘着磨损为主,但随着渗氮温度升高带来渗氮层厚度与硬度的变化,磨损程度呈现逐渐减轻的趋势。     

Effect of Plasma Nitriding on the Performance of WC-Co Cutting Tools

This paper presents the effect of nitriding process parameters on the cutting performance of WC-Co tools. The cutting performance was measured by CNC machining of GG25 cast iron parts. The hardness and phase composition of nitrided layer were determined for different plasma nitriding temperatures and times. The hardness of the nitrided layer increased at all plasma nitrided conditions investigated. However, the machining performance of the cutting inserts varied in the range between a 60% increase and a 40% decrease after plasma nitriding. The maximum number of machined parts was seen when the insert was nitrided at 600 °C-4 h and at 500 °C-4 h.     

Microstructural behavior of nitriding compound layer for Nb-carbonitride coating grown by thermo-reactive diffusion process

This study aims to understand the microstructural behavior of nitriding compound layer and its effect on Nb-carbonitride growth produced by the thermo-reactive diffusion (TRD) process. Gas nitriding was performed at 550 °C for 3 and 6 h, followed by TRD at 900 °C for 6 h. The nitriding compound layers had thicknesses of 10 and 16 μm for nitriding time of 3 and 6 h, respectively. The corresponding Nb-carbonitride layers produced by TRD were 7.2 and 11.2 μm thick, respectively. Reheating at 900 °C transformed the microstructure of the nitriding compounds to Fe₃O₄ and FeN_0.0939. As reheating proceeded to 30 min, high concentration of nitrogen, initially existing in the nitride layer diffused to 80–90 μm into the substrate. Therefore, the TRD process produced NbN layer at the interfacial area due to intensively dissolved nitrogen from FeN_0.0939. As the TRD proceeded, supply of C atoms from the base metal became competitive with the N diffusion. Thus, the TRD coating layer was grown to above the interface. Reheating at 900 °C for the 16-μm-thick nitride layer resulted in a nitrogen content ～0.4 at% higher than that for the 10-μm-thick nitride layer, thereby producing a thicker Nb-carbonitride layer.     

Optimization of the ASPN Process to Bright Nitriding of Woodworking Tools Using the Taguchi Approach

The subject of the research is optimization of the parameters of the Active Screen Plasma Nitriding (ASPN) process of high speed steel planing knives used in woodworking. The Taguchi approach was applied for development of the plan of experiments and elaboration of obtained experimental results. The optimized ASPN parameters were: process duration, composition and pressure of the gaseous atmosphere, the substrate BIAS voltage and the substrate temperature. The results of the optimization procedure were verified by the tools’ behavior in the sharpening operation performed in normal industrial conditions. The ASPN technology proved to be extremely suitable for nitriding the woodworking planing tools, which because of their specific geometry, in particular extremely sharp wedge angles, could not be successfully nitrided using conventional direct current plasma nitriding method. The carried out research proved that the values of fracture toughness coefficient K _Ic are in correlation with maximum spalling depths of the cutting edge measured after sharpening, and therefore may be used as a measure of the nitrided planing knives quality. Based on this criterion the optimum parameters of the ASPN process for nitriding high speed planing knives were determined.     

Intermittent metal cutting at small cutting depths—1. Dead zone phenomena and surface finish

Chip formation in intermittent metal cutting at small cutting depths was investigated by single edge experiments. Single cutting strokes were performed in a modified Charpy pendulum tester which offers force measurement, accurate selection of cutting speed and feed in the ranges typical of many intermittent high speed steel (HSS) tool operations. The pendulum is also provided with an excellent quick-stop mechanism.

The cutting performance of HSS tools in three widely used steel grades (including one plain carbon, one quenched and tempered and one austenitic stainless steel) was studied. A number of double rake micro geometries, with primary rake angles ranging from +20° (parrot bill) to −60°, all with a prepared 0.1 mm wear land were tested. The performance of the different edge geometries was investigated with respect to class of dead zone developed on the cutting edge, and its relation to chip curl and finish of the cut surface. The results are visualized in a dead zone map. The influence of cutting length, cutting speed, cutting depth and TiN-coating was treated specifically.

Among the most important observations were:

• the micro geometry of the edge influences the dead zone formation mechanism and hence the class of dead zone,

• the surface finish is strongly dead zone class dependent,

• the chip curl is determined by edge micro geometry and dead zone class.

The relationships between the varied parameters, generated dead zones and resulting cutting forces are presented in part 2 of this paper.     

Low-temperature nitriding of 38CrMoAl steel with a nanostructured surface layer induced by surface mechanical attrition treatment

A nanocrystalline surface layer was fabricated on a 38CrMoAl steel plate by means of a surface mechanical attrition treatment (SMAT). The average grain size in the top surface layer (10 μm thick) is about 10 nm, and the grain size stability can be maintained up to 450 °C. The effect of the surface nanocrystalline layer on the gas nitriding process at a lower temperature was investigated by using structural analysis and wear property measurements. The surface nanocrystallization evidently enhances nitriding kinetics and promotes the formation of an ultrafine polycrystalline compound layer. The results of the investigation showed that this new gas nitriding technique can effectively increase the hardness and wear resistance of the resulting surface layer in comparison with conventional nitriding, demonstrating a significant advancement for materials processing.