Fig 2 - uploaded by Xiuquan Cao
Content may be subject to copyright.
Source publication
Contexts in source publication
Context 1
... ring and an ignition ring. This structure provides an arc channel in which the arc root could be confined to a certain circular range on the anode wall. The arc column is long in arc channel and thus energy loss is relatively high. These properties may contribute to the production of a plasma jet with lower heat flux than that of the TEPT. Fig. 2 shows the schematic diagram of the plasma surface hard- ening. It shows that a plasma torch is set at a certain distance (d) from the surface of the substrate and moved across the treated sub- strate. The size of the treated substrate is 200 × 100 × 15 mm and its material is 45 steel with the composition 0.45% C, 0.24% Si, 0.63% Mn, ...
Context 2
... q m the maximum heat flux at the specified section of the plasma jet, r the distance from the center of the plasma jet, k a constant coefficient that determines the shape of the distribution. When the thermal plasma jet is impinged on the substrate with a distance (d) between the exit of the nozzle and the surface of the substrate as shown in Fig. 2, a heated circle with a radius of r H as shown in Fig. 5 is formed and the heat flux distribution within it also obeys the Gaussian distribution. According to Tekriwal and Mazumder (1988) ...
Context 3
... q(r) is the distribution of the heat flux at any section of the plasma jet, q m the maximum heat flux at the specified section of the plasma jet, r the distance from the center of the plasma jet, k a constant coefficient that determines the shape of the distribution. When the thermal plasma jet is impinged on the substrate with a distance (d) between the exit of the nozzle and the surface of the substrate as shown in Fig. 2, a heated circle with a radius of r H as shown in Fig. 5 is formed and the heat flux distribution within it also obeys the Gaussian distribution. According to Tekriwal and Mazumder (1988) ...
Context 4
... non-transferred DC plasma torches as shown in Fig. 1, including a two-electrode plasma torch (TEPT) and a multi- electrode plasma torch (MEPT), were designed to produce thermal plasma jets with different heat flux characteristics. TEPT is a repre- sentation of the traditional DC plasma torch that is characterized by a coaxial geometry constituted by a central cathode and a tapered anode. The arc column is short in the chamber and thus the energy loss is low, which may contribute to the production of a plasma jet with high heat flux. MEPT is mainly constructed with a cathode, an anode and an inter-electrode that is composed of an inter-ring, a constriction ring and an ignition ring. This structure provides an arc channel in which the arc root could be confined to a certain circular range on the anode wall. The arc column is long in arc channel and thus energy loss is relatively high. These properties may contribute to the production of a plasma jet with lower heat flux than that of the TEPT. Fig. 2 shows the schematic diagram of the plasma surface hard- ening. It shows that a plasma torch is set at a certain distance (d) from the surface of the substrate and moved across the treated sub- strate. The size of the treated substrate is 200 × 100 × 15 mm and its material is 45 steel with the composition 0.45% C, 0.24% Si, 0.63% Mn, 0.014% S, 0.019% P, 0.023% Cr, 0.009% Ni and balanced ...
Similar publications
This work reports the development of atmospheric pressure non thermal plasma reactor with dielectric barrier discharge (DBD) for diagnosis, characterisation and decomposition of VOCs compounds. The DBD discharge has been generated in planar geometry reactor powered by AC voltage transformer. The reactor consisting two electrodes, each electrode pla...
Bioelectrodes mediated by metal oxide nanoparticles have facilitated the development of new sensors in medical diagnosis. High-purity TiO2 nanoparticles (NPs) were synthesized through thermal plasma and deposited directly on an interdigitated electrode. The surface of the TiO2-deposited electrode was activated with (3-aminopropyl) triethoxysilane (...
A minority relativistic electron component can arise in both laboratory and naturally-occurring plasmas. In the presence of high-atomic-number ion species, the ion charge state distribution at low bulk electron temperature can be dominated by relativistic electrons, even though their density is orders of magnitude lower. This is due to the relativi...
Synthesis of yttrium oxide nanoparticles in a specially designed radio frequency thermal plasma reactor is reported. Good crystallinity, narrow size distribution, low defect state concentration, high purity, good production rate, single-step synthesis, and simultaneous formation of nanocrystalline monoclinic and cubic phases are some of the interes...
Citations
... 研究。 潘文霞等 [7] 研究了气流量和电流对射流流态和 长度的影响， 揭示了氩气等离子体射流不同流态的工 作区间， 以及不同流态下工作参数对射流长度的影响 规律。孟显 [8] 利用光谱、皮托管、热流探针分别测 量了射流出口的温度、速度以最大热流密度， 研究了 气流量和电流对射流出口速度温度的影响， 结果表明 射流出口最高温度随电流增加而增加， 射流最大速度 随气流量和电流增加而增加， 高温气体向探针表面的 换热系数随气体温度和速度增加而增加。 Tu [9] 用高响应频率的商用水冷热流计测量了冲 击基板的双弧氩等离子体射流的热流密度， 研究了热 流密度的轴向演化特性与其中的非稳定特征， 结果表 明电源波动、 电弧击穿、 与环境气体的耦合都使热流 密度产生不同频率的扰动。Meng [10] 使用稳态和瞬 态的方法分别测量了射流冲击基板的总热流密度和 热流密度分布，并与数值模拟得到的热流密度进行 比较， 结果表明射流半径沿轴向的变化很小， 层流等 离子体射流的总热流密度轴向衰减远小于湍流， 并分 析了不同测量方法计算的总热流的差异。 Xiang [11] 和 Guo [12] 使用了不同参数下、不同 结构下产生的层流等离子体射流，进行工件的表面 淬火实验，分析了热流密度分布和材料处理性能的 差异，以确定最佳的工艺参数，但没有关注射流进 入空气之后的流动特性，当轴向距离改变时，其结 果将很难推广。 Cheng 等 [13] 利用数值模拟的方法比较层流 和湍流等离子体的射流特性，采用了考虑质量、动 量、能量和物种守恒的物理模型，对于扩散过程采 用 Murphy [14] 提出的组合扩散系数法，对于湍流情 形采用了 K−ε 双方程模型和湍流增强组合系数法， 其数值计算结果揭示了自由状态下两种射流流动特 性的差异， 在此基础上， Wang [15,16] 基于相同的建模 方法，比较了层流和湍流等离子体射流冲击基板过 程的流动和传热特性，结果表明基板的存在显著提 高了射流对冷空气的卷吸并影响了近基板区域的等 离子体参数，并且这种提高对于层流等离子体射流 更为显著，但层流等离子体等离子体参数的轴向衰 减率仍明显小于湍流，两者的流场也大不相同，仅 在轴向距离较小时，两者热流密度分布接近。但值 得注意的是，由于炬内的物理现象十分复杂，需要 建立热力学非平衡、化学非平衡的鞘层模型，还要 考虑弧根重击穿、亥姆霍兹现象带来的不稳定以及 非轴对称情形下的三维效应 [17] ，因此炬内数值模 拟的保真度是有限的，文献 [13], [15] 中也没有进行 炬内的数值模拟，而是基于经验公式 [18,19] Table 1 Processing parameters and size of the hardened zone Fig. 6 Variation of the emission intensity of plasma jet along the axial direction 图 7 高温区半径沿轴线方向的变化 Fig. 7 Variation of the radius of jet high temperature zone along the axial direction ...
Laminar plasma jet impinging on the substrate is a common working condition in typical process like plasma spraying, plasma surface hardening, etc. However, its flow and heat transfer characteristics have not been fully revealed, resulting in the difficulty in the determination of the working parameters in these processes. This paper analyzed the arc voltage and power characteristics of a laminar plasma torch with nitrogen-argon mixed gas based on the electrical signal, and studied the flow characteristics of laminar plasma jet with an image processing method, finally studied the heat transfer characteristics of plasma jet impinging upon the substrate with a surface phase transformation hardening method. Results showed that the increase of the arc current resulted in a slight increase of the jet power, outlet velocity and temperature, a slight reduction of the air entrainment and a slight increase of the length of the high temperature zone, but the heat energy transferred to the substrate was almost the same and the overall heat transfer efficiency was reduced. Increasing the air flow rate and the ratio of nitrogen can greatly increase the jet power, the velocity and temperature of the plasma jet, reduce the air entrainment, increase the radius and length of the high temperature zone, and increase the heat flux density and total heat transfer efficiency transferred to the substrate. In addition, the variation of the working parameters has little effect on the radius of the heat-affected zone. Key words laminar plasma jet; jet impingement; heat transfer characteristics; heat flux density 0 引 言 层流等离子体炬因为其射流长度长、对空气的 卷吸少、运行噪声小、热流密度集中等优点，被广 泛应用于喷涂 [1] 、制粉 [2] 、表面淬火 [3,4] 、快速成 型 [5] 等领域。在许多应用场景中，如喷涂和表面处 理，都会遇见高温高速的等离子体射流进入空气然 后冲击基板的工况，根据流动特性的差异，冲击射 流可以分为自由射流区、滞止区和壁面流动区 [6] ， 在不同的区域当中，等离子体射流与基板和环境 气体之间进行着复杂的动量、能量和组分传输，对 于这些过程，学者们已经进行了大量的实验和仿真
... Zhang et al. [14] investigated the influence of LPQ on RCF behavior of high-speed railway wheel steel in watery conditions, and studied the temperature evolution of the material during LPQ by simplifying the laminar plasma heat source to a Gaussian heat source. Guo et al. [13] as well as Xiang et al. [21], similarly simplified the plasma heat source to a Gaussian heat source and studied the temperature evolution during LPQ of plate materials. Bojinović et al. [22] studied the effects of temperature change rate on austenite kinetics and calculated the hardness distribution of laser hardening specimens. ...
Laminar plasma quenching (LPQ) is an effective surface hardening method for improving the wear resistance of rails. Nonetheless, there has been very little systematic theoretical research on the mechanism of how rail materials are strengthened through LPQ. Therefore, based on the fluid-solid-thermal coupling method, a complete theoretical analysis model for LPQ of a U75V rail was established for this paper. The influence of the laminar plasma jet on the spatial distribution of the hardened layer was investigated, and the residual stress distribution on the rail surface during LPQ was reconstructed. The results show that the distribution of the hardened layer will significantly affect residual stress. As plasma flow rate increases, the width-depth ratio of the hardened layer first decreases and then increases. This research is of great significance for understanding the mechanism of rail plasma quenching, and for the application of fluid-solid- thermal coupled methods.
... Among the parameters that determine the change in the speed and temperature of the plasma jet, a significant role is played by the configuration of the internal channel of the plasmatron, which is determined by the shape of the parts that form the channel: swirlers, anode, cathode, and interelectrode insert. The study of the shape of the parts that make up the plasmatron, some features of their possible influence on the process, are considered in the works [5][6][7][8][9]. Despite the fact that the literature contains data on the influence of the geometry of the components of the plasmatron on the parameters of the plasma flow, recommendations for manufacturers and consumers of technological equipment have not been sufficiently developed so far. ...
A computer model of the plasmatron was created, which adequately describes the plasma flow. The effect of changing the length of the anode on the speed and temperature at characteristic points of the flow is investigated. To confirm the results of the numerical experiment, a field experiment was performed. The results of the comparative analysis of the data of the computer and field experiments are in agreement. The presented results are of practical value for manufacturers and users of technological plasma installation for improving the equipment used in the processes of obtaining metal powders and applying protective coatings and can be used to predict the parameters of a plasma flow. Recommendations for choosing the length of the anode are presented.
... The concentrated force was chosen such that the maximum contact stress is the same as that used in the contact wear tests. Visually, the morphology of the PSQ spot can be regarded as a "crescent" shape, which was due to Gaussian distributed heat flux of the laminar plasma jet [36]. The width and depth of the PSQ spot were around 3.37 mm and 0.34 mm respectively. ...
Plasma selective quenching (PSQ) is a promising technique for creating discrete hardened spots (PSQ spots) in the surface layer of the rail to increase its service life. However, cracks occur at one side of the PSQ spots after long-term service, hindering the large-scale application of PSQ treated rails. To reveal the effect of PSQ on the wear behavior of rail steel and damage mechanism of PSQ spots, rail rollers with a thickness of 10 mm were firstly PSQ treated by a laminar plasma jet. Then rolling-sliding contact wear tests were conducted to explore the wear behavior and damage characteristics of the PSQ treated rail rollers. In addition, contact simulation of the contact wear tests was carried out. Results showed that the wear resistance of the PSQ treated rail rollers was improved by 1.26–1.75 times. Such improvement is a result of the increased hardness of the PSQ spots and the reduced plastic deformation of the substrate between two adjacent PSQ spots. Severe cracks and breaks were observed at the entry side of the PSQ spots while no obvious damage appeared at the exit side. The mechanism of such a special pattern of damage was that there was a strain difference at the boundary between the entry side of hardened PSQ spot and the soft substrate while no different strains were produced at the exit side. This paper clarified the damage mechanism of the PSQ spots under the rolling-sliding contact conditions, providing the basis for further improvement on the performance of the PSQ treated rail steel.
... One of the methods for restoring dies is welding of defects [13,14]. However, this methods attracts additional difficulties connected with crack formation during welding and occurrence of problems of forming the required level of mechanical properties within a deposited layer. ...
... This improves tool quality significantly in contrast to other coating application methods. An example of the plasma cladding process for a transition layer [14] does not provide "transition layer" property stability that considerably reduces coating quality with dynamic forming processes [15]. ...
Currently restoration and strengthening of a component working surface for structural purposes (objects of stamping equipment, highly loaded assemblies of cars and units, and components operating in corrosive media) is conducted mainly by means of welding operations that do not provide a functional coating and as a result do not provide the life required for a restored or strengthened object. This work concerns a study of laser cladding for restoration of working surface for stamping equipment. Work on laser cladding is conducted on the basis of DMD (Direct Metal Deposition) technology performed in a FL-Clad–R-4 robotic unit that includes a KUKA KR-120 robot. Cobalt-based material is used to create a coating and the substrate (stamp) material is AISI L6 steel. This provides solution for the production problem with selection of regimes for a coating with the required physical and mechanical properties for stamp surface hardening, as well as specific compositions of powder materials used to develop technology for object mass production. Attention is drawn to the fact that alloy used for powder production must have the required physical and mechanical properties and during cladding (after cooling) a coating must not experience thermal stresses and also has the required adhesion.
... These characteristics increase the dwell time of particles in the plasma jet allowing them to be heated and melted completely even when the power is low. With laminar plasma jet also there is a high level of controllability of material processes such as surface quenching [9][10][11]. The latest research shows that during the laminar plasma spraying process, the average velocity of NiCrBSi particles is about 60 m/s and the average temperature is above 1800°C [12]. ...
... Therefore, uniformly distributed oxides in the short distance sprayed coatings are few and therefore cannot significantly improve the hardness. In addition, coating A, obtained at 200 mm off the nozzle exit, experiences an enhancement from the plasma jet [10,11,32], which promotes interlamellar diffusion and improves cohesion. This might make coating A a little harder than coating B. ...
A stable laminar plasma jet with a jet length of above 600 mm was adopted to deposit molybdenum coatings to investigate the behavior of high-melting point metal, molybdenum, and particles during laminar plasma spraying process. The temperature, velocity, and species distribution of the plasma jet were calculated by numerical simulation. In the experiment, the temperature and velocity distribution of the in-flight particles were measured, and the microstructure and properties of the coatings were analyzed. Combining the results of the simulation and experimental results, the deposition behavior and oxidation mechanism of the coatings prepared at the spray distance from 200 mm to 400 mm were discussed. The results indicate that both the laminar plasma jet and injected particles exhibit high temperature and low velocity, and low temperature and velocity gradients. The microstructure of the coatings shows a high similarity and hardly changes when the spray distance exceeds 250 mm. When the spray distance is 200 mm, post-deposition oxidation occurs due to the heating effect of the long plasma jet on the substrate. In contrast, when the spray distance is longer, the particles undergo severe in-flight oxidation, and the oxide formed is uniformly distributed in the coating improving the hardness from 500 HV0.3 to 700 HV0.3.
... Compared to the turbulent plasma jet used in traditional atmospheric plasma spray, the laminar plasma jet shows different characteristics because less ambient air is entrained into the plume. The laminar plasma jet is of greater length, lower velocity, and temperature gradient [1][2][3][4][5][6][7][8][9][10][11][12][13]. These characteristics provide a great potential in the domain of thermal spray. ...
The behavior of low-velocity NiCrBSi particles with a high surface temperature of 1800 °C during the long laminar plasma spraying process was studied. During the experiment, the maximum length of the laminar plasma jet exceeded 600 mm, and coatings were obtained using a laminar plasma spray at a wide spray distance ranging from 250 to 500 mm. Interestingly, the coatings obtained at a short spray distance of 250 mm showed special alternate porous and compact lamellar structure with island-like bulges that were evenly distributed on the coating surface. This type of structure was possibly formed by a low-velocity splashes deposition process intensified by the furry-like structure, which can be attributed to vapor phase deposition. The coatings obtained at an extremely long spray distance of 500 mm showed massive cracks. Moreover, the medium-distance sprayed coatings of 350–450 mm showed similar lamellar-like microstructure and mechanical performance because of the similarity in the particle states in the low-energy-gradient laminar plasma jet. Thus, a laminar plasma spray shows great reproducibility and potential for improving the spraying efficiency on an uneven plane.
... According to the flow status, it could be classified as a laminar plasma jet and a turbulent plasma jet. In the previous study [2]- [4], compared with the turbulent plasma jet, the laminar plasma jet is more suitable for materials processing because of its more favorable temperature and velocity distributions, lower noise, and more stable flow [5]. Therefore, different types of laminar plasma torches have been proposed and studied to generate laminar plasma jet in recent years [6]- [8]. ...
Because of the complex generation process, the jet characteristics of laminar plasma torch were always studied by experiments at present. In order to reduce the experimental costs and the workloads, the similarity theory is a good method to study the characteristics of the laminar plasma jet. Thus, with some experimental results under specified conditions, the relationships between the characteristics and the working parameters have been obtained to predict the characteristics with other specified working parameters. Compared with the experimental results, the theoretical arc voltage characteristics, thermal efficiency characteristics, and jet lengths characteristics coincided with the experimental ones with a difference lower than 2%, 2.5%, and 8%, respectively.
... In addition, as a relatively new high energy beam, LPJ's energy density and the heat affected zone can be easily adjusted by changing the working parameters, e.g. the anode nozzle diameter, the arc current, the gas type, the gas flow rate etc., to obtain the desired heating temperature and cooling rate. Therefore, it has presented many advantages in controlling the geometrical dimension, microstructure and hardness distribution of the hardened layer [8,[21][22][23][24]. In addition, there were signs that the plasma surface hardening can produce a hardened layer with hardness gradient for rail steel [6]. ...
... To reveal the relationship between the formation of the hardened layer and the variation of the temperature field, numerical simulations of the LPJ surface hardening for the U75V steel workpiece with a size of 50 × 50 × 20 mm were carried out by ANSYS finite element software. First, the heat flux density distribution of the LPJ impinging up on the workpiece surface was determined by the indirect method proposed by Xiang et al. [10,23]. It obeys the Gaussian distribution as described by ...
... Then the determined heat flux density distribution and natural convection heat transfer were set as the boundary conditions to obtain the variation of the temperature field within the heat affected zone. Details on the numerical simulation can be referred to reference [10,23]. ...
Insight into the mechanism of laminar plasma jet (LPJ) surface hardening is critical for the controllable preparation of ideal hardened layer to improve the wear and fatigue resistance of U75V rail steel. To reveal the surface hardening mechanism, a series of surface hardening experiments on the U75V rail steel surface were performed under different working parameters using a self-designed laminar plasma generator. Results showed that the geometrical dimension, microstructure and hardness distribution of the hardened layer can be adjusted by changing the arc current, the scanning velocity and the anode nozzle diameter. Further characterization and numerical simulation suggested that the geometrical dimension of the hardened layer is determined by the heat affected zone over the critical austenitizing temperature (730 °C), while its microstructure and hardness distribution depend on the local cooling rate below the critical austenitizing temperature. By changing the working parameters, the heat flux density applied on the workpiece surface and the heating time can be controlled to obtain the desired temperature field within the heat affected zone. Not only a full hardened layer but also a transition layer consisting of martensite, pearlite, ferrite and carbides can be achieved. This configuration of LPJ surface hardening process for U75V rail steel indicates that the LPJ surface hardening with the ability of producing hardness gradient is promising for reducing the risk of crack generation on the rail.
... In contrast to traditional plasma quenching, the use of laminar plasma quenching (LPQ) is industrially advantageous since it permits avoiding water cooling, obtaining excellent results with rapid self-cooling. Xiang et al. investigated the hardness of the 45 grade steel [20] and wear properties of the U75V rail steel [21] treated by LPQ, and they concluded that a high hardness martensitic structure was formed in the heat affected zone (HAZ), which significantly improved the wear resistance of rail steel. ...
... Similar as in the original design, in this work, the laminar plasma input power was 14.4 kW, the axial distance from material surface was 10 mm, and the scanning speeds allowed were 700, 900, 1100, 1300 mm/min. The corresponding quenching scanning process schematic as shown in references [20,33]. ...
... A moving heat source was defined on the material surface based on a Gaussian distribution of the heat flux, corresponding to the surface boundary conditions. The heat flux distribution can be described by Eq. (1) according to [20,33]. ...
This study aims at determining the influence of laminar plasma quenching (LPQ) on the rolling contact fatigue (RCF) behaviour under water condition of high-speed railway wheel steel. The wheel and rail test rollers were treated by LPQ with different scanning speeds (700, 900, 1100, 1300 mm/min), which generated different depth of heat affected zones (HAZs) and residual compressive stress in the surface layer. To obtain an insight of the microstructures and their corresponding thermal treatment applied, the temperature evolution was simulated during the LPQ by Finite Element Modelling and the micro-structures of cross-sections below the roller surfaces were characterized. The results showed martensite, retained austenite and undissolved cementite in the HAZ as a consequence of the ultrafast heating and cooling rates. The produced residual compressive stress resulted in an increase of ∼30% of the RCF life indicating that LPQ could effectively improve the RCF resistance. Further investigations showed that the high density dislocation martensitic structure of the HAZ of reduced the plastic deformation, which could delay the RCF crack initiation and decrease the depth of RCF crack growth. During the RCF test, some retained austenite of the LPQ treated wheel roller transformed into twin martensite under the effect of strain induced martensitic transformation. The plastic deformation in the untreated wheel roller resulted in a refined microstructure showing lots of sub-grains generated in both pearlite and ferrite regions. Furthermore, we have observed that the lattice structures of the martensite in the LPQ treated sample and sub-grains of the plastically deformed untreated sample were similar to the BCC structure of the ferrite at the substrate in the high-speed railway wheel steel. We conclude from the obtained results that the LPQ processing enhances the RCF life and that this improvement is influenced by the obtained microstructure in the HAZ.
