Study on the active screen plasma nitriding and its nitriding mechanism

The active screen plasma and DC plasma nitriding of the low alloy steel 722M24 are investigated. Experimental results showed that the metallurgical characteristics and hardening effect on 722M24 steel nitrided by AS plasma nitriding at both floating potential and grounded potential were similar to those nitrided by DC plasma nitriding. Particles sputtered from the active screen and deposited on the specimen surface play the role of the nitrogen carrier in AS plasma nitriding. XRD and high-resolution SEM analysis indicated that the particles with sizes in sub-micron scale were Fe_xN (x > 2). Based on metallurgical analysis and Optical Emission Spectrometer (OES) experimental results, an AS plasma nitriding model has been proposed considering that AS plasma nitriding is a multi-stage process, involving sputtering, physical adsorption, desorption, diffusion and deposition.     

短应力线轧机拉杆离子渗氮畸变控制

为探明杆状工件离子渗氮畸变的影响因素,对短应力线轧机拉杆的加工残余应力、装卡方式、多次升温和降温等因素进行了研究。结果发现,影响拉杆渗氮畸变的最大因素是拉杆的加工应力,所以对于长杆状工件,在精加工和渗氮之前加入合理的去应力工序可以有效控制渗氮畸变;另外,缓慢升温、降温及垂直悬吊的装卡方式也可以在一定程度上减小渗氮畸变。需多次渗氮才能满足图纸技术要求的长杆状工件,在每渗氮一次之后,把工件旋转180°,可以有效控制渗氮畸变。     

An in-situ study of plasma nitriding

Direct in-situ observation of phase generation and growth during heat treatment cycles gives information independent e.g. of effects resulting from cooling and atmospheric changes of properties. In this investigation time resolved in-situ X-ray diffraction (XRD) analysis of growing nitride layers during plasma nitriding was conducted to gain experimental data of growing compound layers for different plasma nitriding parameters. With two gas mixtures of 5% N₂-95% H₂ and 25% N₂-75% H₂. plasma nitriding of an AISI 1045 steel was performed in the temperature range of 450 °C < T < 560 °C. The in-situ XRD-observation consisted of series of 50 to 60 single runs of phase analysis during a 3-h plasma nitriding treatment. Nitriding with the formation of nitride phases starts at different times, depending on the nitriding temperature and the gas composition in the plasma for the given plasma parameters pressure, voltage and current density. The higher the nitriding temperature and the higher the nitrogen content in the process gas the shorter is the time for the first detection of the γ′-Fe₄N-phase. Single phase γ′nitride layers were detected for the 5% N₂-95% H₂ gas mixture in a temperature range 450 °C < T < 560 °C. For the highest temperatures 540 °C and 560 °C and the gas mixture 25% N₂-75% H₂ the ε-Fe_2-3N phase occurred later in the plasma nitriding process. Assuming that nitride layers in plasma nitriding also grow by nucleation of small γ′ particles up to a complete layer, the experimental data fitted in a reasonable way in plots calculated for the incubation time of the γ′-phase during gas nitriding.     

辉光离子氮化炉的改造

辉光离子氮化是机械加工行业对加工工件进行热处理的重要手段,氮化炉设备运行的好坏,直接影响到工件的热处理质量。介绍了辉光离子氮化炉的工作原理、改造方案及效果等。通过改造,该热处理炉的供电质量稳定,灭弧动作可靠,打弧时间缩短,氮化时间减少,工件氮化质量提高。     

Improvement of corrosion and wear resistances of AISI 420 martensitic stainless steel using plasma nitriding at low temperature

The influence of low temperature plasma nitriding on the wear and corrosion resistance of AISI 420 martensitic stainless steel was investigated. Plasma nitriding experiments were carried out with DC-pulsed plasma in 25% N₂ + 75% H₂ atmosphere at 350 °C, 450 °C and 550 °C for 15 h. The composition, microstructure and hardness of the nitrided samples were examined. The wear resistances of plasma nitrided samples were determined with a ball-on-disc wear tester. The corrosion behaviors of plasma nitrided AISI420 stainless steel were evaluated using anodic polarization tests and salt fog spray tests in the simulated industrial environment.The results show that plasma nitriding produces a relatively thick nitrided layer consisting of a compound layer and an adjacent nitrogen diffusion layer on the AISI 420 stainless steel surface. Plasma nitriding not only increases the surface hardness but also improves the wear resistance of the martensitic stainless steel. Furthermore, the anti-wear property of the steel nitrided at 350 °C is much more excellent than that at 550 °C. In addition, the corrosion resistance of AISI420 martensitic stainless steel is considerably improved by 350 °C low temperature plasma nitriding. The improved corrosion resistance is considered to be related to the combined effect of the solid solution of Cr and the high chemical stable phases of ?-Fe₃N and α_N formed on the martensitic stainless steel surface during 350 °C low temperature plasma nitriding. However, plasma nitriding carried out at 450 °C or 550 °C reduces the corrosion resistance of samples, because of the formation of CrN and leading to the depletion of Cr in the solid solution phase of the nitrided layer.     

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 plasma nitriding of electroplated chromium coatings on the corrosion protection C45 mild steel

Plasma nitriding is a promising posttreatment technique to create a nitride layer on electroplated chromium coatings for improving their corrosion resistance. In the present study, the effects of plasma nitriding on the corrosion properties of electroplated chromium/C45 mild steel were investigated using electrochemical characterization. The chromium plated samples were nitrided using a pulsed direct current glow discharge in an NH₃ atmosphere. The polarization curve measurement results showed that the plasma nitrided samples exhibited more positive corrosion potentials (E_corr), smaller corrosion currents (I_corr), and evident passivation when compared with unnitrided chromium plating/substrate system. The high value of E_corr and low value of I_corr imply an improvement of the corrosion resistance of the coating/substrate system after plasma nitriding.     

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.     

Wear and corrosion properties of plasma-based low-energy nitrogen ion implanted titanium

Plasma-based low-energy nitrogen ion implantation, including plasma source ion nitriding/carburizing and plasma source low-energy ion enhanced deposition, has emerged as a low-temperature surface engineering technique for metal and alloy. In this paper, the pure metal Ti samples have been modified by the plasma source ion nitriding process at a process temperature of 700 °C for a processing time of 4 h. The nitrided Ti surfaces were constructed of a continuous and dense Ti₂N compound layer about 2 μm thick and a 7-8 μm diffused layer. During tribological test on a ball on disk tribometer against the Si₃N₄ ceramic counterface, a low friction coefficient of about 0.3 and the faint wear volume were obtained for the nitrided Ti samples. The cyclic polarization curves of the nitrided Ti samples in 3.5% and 6.0% NaCl solutions showed that the improved pitting corrosion resistance with an increase of corrosion potential and a decrease of passive current, compared with that of the unnitrided Ti sample. The plasma source ion nitriding of the Ti samples provided the engineering surfaces for the functional applications with the combined improvement in wear and corrosion resistance.     

渗氮炉内流速场的数值模拟

为了研究渗氮炉内气体的分布状态,建立了三维湍流数学模型,应用计算流体动力学(CFD)的方法分别得到了渗氮炉空炉和加入工件时的气体速度分布.该渗氮炉空炉时工作区域内气体速度比较均匀,工件加入后渗氮炉内流速场变化很大,因此对于不同形状和炉内不同位置的工件,需要分别模拟计算各自的速度场.同时用数字涡轮风速仪对炉内的气体流速进行了测量,测量结果与模拟计算结果基本吻合,所以该数值模拟方法可以作为热处理工艺和热处理炉型优化设计预测的依据.     

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.     

Tribology and cyclic oxidation behavior of plasma nitrided valve steel

In this study, the tribology and cyclic oxidation behavior of plasma nitrided DIN 1.4871 austenitic valve steel were investigated. For this purpose plasma nitriding treatments were carried out in nitrogen and hydrogen with ratio N₂/H₂: 1/3 at 10 Torr pressure. Nitriding cycles of 400, 450, 500 and 550 °C for 7 h were selected. To remove oxide layer and to enhance diffusion, an effective sputter cleaning procedure was applied in argon and hydrogen gases. The pin-on-disc sliding wear experiments were performed at a load of 6 N and sliding velocity of 0.1 m/s in normal atmosphere under dry condition. Cyclic oxidation tests used to evaluate the oxidation characteristics of the samples consisted of 50 cycles each 30 min at 750 °C. The structure and properties of the samples were examined by optical and scanning electron microscopy (SEM), microhardness measurements and X-ray diffraction. The results indicated plasma nitriding at all temperatures increased the wear resistance of valve steel when sliding against bearing steel. The 550 °C nitrided layer, with CrN, Fe₄N and Fe_2-3N on the surface, was most effective in improving wear resistance. In the case of cyclic oxidation, the results showed that oxidation resistance depends strongly on nitriding temperature. Nitriding at 450 °C produced a layer of predominantly “S” phase which was more effective in improving the oxidation resistance of valve steel.     

Response to thermal cycling of plasma nitrided hot work tool steel at elevated temperatures

Thermal fatigue performance of plasma nitrided hot work tool steel was investigated under conditions encountered by thixoforging dies in semi-solid processing of steels. Plasma nitriding does not offer any improvement in the thermal fatigue performance of hot work tool steels at elevated temperatures, due essentially to poor resistance to oxidation and to temper softening. Fe₃O₄ and Fe₂O₃ scales produced on the nitrided surface fail to sustain the thermal stresses produced by thermal cycling. They spall off, generating fresh surfaces for further oxidation. This sequence leads to substantial material loss and impairs the integrity of the surface beyond a quality level that would be tolerated in steel thixoforming. The surface hardening provided by plasma nitriding is also completely erased. The tempered martensitic structure is replaced by fine, equiaxed ferritic grains implying a dynamic recrystallization process during thermal cycling.     

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.     

Surface modification of austenitic stainless steel with plasma nitriding for biomedical applications

In the present study, plasma nitriding of AISI type 303 austenitic stainless steel (SS) specimens was performed using a microwave system. The nitrided layers were characterized by performing scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and a Vickers microhardness test. The antibacterial activities of the nitrided layers were evaluated. XRD and TEM showed that a single γ_N phase was formed by plasma nitriding at the plasma power of 700 W and 450 °C. The analytical results demonstrated that the hardness of type 303 specimens could be enhanced by plasma nitriding because of the formation of the γ_N phase. A bacterial test also demonstrated that the nitrided layer exhibited excellent antibacterial properties.     

Delayed cracking in plasma nitriding of AISI 420 stainless steel

The phenomenon of delayed cracking in nitrided layers after DC-plasma nitriding of AISI 420 steel has been observed by optical microscopy. Prior to the plasma treatment, the samples were austenitized at 1303 K for 30 min and then oil quenched. Two tempering conditions were assessed: one group was tempered at 673 K, while another group was tempered at 943 K.All samples were subjected to sputtering, in the plasma chamber, to remove the passive oxide layer, under a 1:1 Ar/H₂ atmosphere. Finally, specimens were plasma nitrided at 673 K for 20 h, with a 1/3: N₂/H₂ relation, at a pressure of 6.5 hPa and 700 V bias in the nitriding chamber.The nitrided layers were analyzed initially by X-ray diffraction (XRD). Detailed observations were conducted at frequent and regular intervals under optical microscopy (OM) and scanning electron microscopy (SEM) with secondary and back-scattered electrons detectors. The results revealed that after an incubation time, even without any external disturbance, cracks are formed and propagate in the nitrided layers. Both groups of samples were equally affected. The presence of precipitated particles and local residual stresses are possible causes of such a phenomenon.     

Surface engineering with light alloys—Hard coatings, thin films, and plasma nitriding

Light alloys have been attracting increasing attention over the past decade, since they can be used to reduce weight and save energy. For many years, light metals such as titanium and aluminum have also been used to synthesize hard compound coatings such as physically vapor deposited (PVD) TiN, (Ti,Al)N, and chemically vapor deposited (CVD) Al₂O₃. The coatings field is developing rapidly. Combining plasma-aided coating and diffusion processes has led to the development of so-called “duplex treatment,” consisting of plasma nitriding and subsequent hard coating. Another interesting development is TiN coating of aluminum vacuum parts, such as pumps, to reduce degassing and make the cleaning of the surfaces easier. Despite the many advantageous properties of light alloys, their surface properties sometimes cause problems. For example, galling may be a severe problem with titanium parts, and plasma nitriding has been applied successfully to combat it. However, due to adherent oxide scale, plasma nitriding of aluminum has proven to be more difficult. In this paper, we discuss some recent trends in the application of plasma-aided coating, thin film deposition, and diffusion processes, and give practical examples of industrial applications.     

离子渗氮技术及其应用

离子渗氮技术的理论基础是气体辉光放电特性,具有渗氮质量稳定,节能减排等优点。多种材料及不同条件下服役的零件经离子渗氮后,在表面硬度、渗层深度、硬度梯度、畸变量等方面均能达到相关的技术要求。介绍了一些典型工件进行离子渗氮的实例,并指出了生产中离子渗氮方面尚需解决的问题。     

Investigation of nitrogen mass transfer within an industrial plasma nitriding system I: The role of surface deposits

It has been proposed that in plasma nitriding, sputtering of material from biased components within the chamber assists in the nitrogen mass transfer process. Here, we investigate the effects of this sputter deposition process on the nitriding response of biased and unbiased AISI P20 steel samples mounted in a large-scale plasma nitriding system operated at 520 °C. Films with nanostructured morphologies resulting from Volmer-Weber film growth were observed on Si substrates placed adjacent to AISI P20 steel substrates after nitriding experiments. Auger depth profiling revealed that the films on the Si substrates had a stoichiometric ratio of 4:1 Fe:N. This suggested that the particles consisted largely of Fe₄N and it was concluded that they were formed from atoms and small clusters sputtered from biased components in the chamber. Despite the deposition of these films, no significant improvement in surface hardness was observed in the steel samples unless bias was applied to them. Furthermore, the maximum hardness achieved in biased P20 samples after the nitriding process occurred in the samples positioned adjacent to the Si samples supporting the thinnest deposited films. These findings do not support the proposition that in plasma nitriding, nitrogen mass transfer occurs predominantly by sputter deposition of iron nitride.