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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.

Due to its favorable temperature and velocity distributions, a lamina plasma jet, generated by a laminar plasma torch (LPT), is superior to a turbulent plasma jet on some applications. However, most of the reported LPTs usually work at a relatively low output power with a relatively low arc voltage and thermal efficiency, limiting its capability in the applications such as the surface hardening, synthesis of nanoparticles, remanufacturing of large components, and so on. Therefore, this article aims at designing an LPT with medium level output power and a relatively low arc current but a high thermal efficiency. The characteristics and applications of the proposed LPT have been obtained by experiments. The experimental results show the following characteristics: 1) the mean arc voltage of the proposed LPT is higher than 190 V with a maximum working power larger than 25 kW; 2) its mean thermal efficiency and jet length arrive at 50% and 450 mm, respectively; 3) the experimental data of the proposed LPT were used to fit its theoretical characteristics by using the least squares method. According to these fitted curves, the theoretical characteristics of the proposed LPT at a specified working conditions can be predicted; and 4) in addition, the surface of 45# steel was hardened by using the laminar plasma jet to obtain a hardening surface. The maximum hardness of the treated substrate arrives at about 600 HV, which is three times of the untreated substrate.

Laminar plasma jet (LPJ) which is generated by laminar plasma torch (LPT) has more advantages than the turbulent plasma jet on applications, such as materials processing and plasma spraying. The jet characteristics of LPT are influenced by many factors, e.g., the working parameters, plasma torch construction, gas injection angle, and arc chamber length. A lot of research has been done in recent years. However, a better understanding of how the arc chamber length influences LPT’s jet characteristics is still required. Thus, this paper aims to study the influences of the arc chamber length on LPT’s jet characteristics. The first part mainly focuses on the introduction of the LPT setup we used in this paper, and this setup can switch the chamber length from 67 to 84 mm or 101 mm by adjusting interelectrodes’ number from 3 to 5. The second part introduces the methods and equipment used in this paper. The last part is results and discussions. The results show that when the arc chamber length increases from 67 to 84 mm and from 84 to 101 mm: 1) the corresponding arc voltage and working power of the experimental LPT increase 20% and 12%, respectively; 2) the corresponding thermal efficiency increases 30% and 8%, respectively; 3) the corresponding mean specific enthalpy of the LPJ increases 50% and 20%, respectively; and 4) on the one hand, the LPT’s jet length increases slightly with increasing the arc chamber length, and on the other hand, it increases first and then decreases with increasing the working gas; additionally, the plasma jet changes from laminar flow state to turbulent flow state.

The aim of this study is to investigate the effects of thermal plasma surface hardening on wear and damage properties of rail steel. Wear tests of the rail steel treated by thermal plasma jet and untreated one were carried out on a block-on-ring wear test machine. Their corresponding wear and damage properties were contrasted. Results indicate that the microstructure of the heat-affected zone of the rail steel treated by thermal plasma surface hardening was transformed. It mainly consists of lath and acicular martensite and also small amount of sorbite, bainite, and retained austenite. As a result, the thermal plasma surface hardening effectively improves the wear resistance of the rail steel. The main wear mechanism of the treated rail steel is mild plowing wear, while that of the untreated rail steel is a combination of severe plowing wear and adhesion wear. Therefore, it is verified that thermal plasma surface hardening is effective in improving the wear resistance, alleviating the surface damage and thus prolonging the service life of the rail steel.

The non-transferred direct current (DC) plasma torch has been widely used in various industrial applications due to its special jet characteristics. The jet characteristics are determined by different factors, including the working parameters, the torch construction, the gas injection angle (GIA) etc. As there is little study on the influence of the GIA on the jet characteristics, experimental study on the GIA’s effects on the jet characteristics has been carried out on a specially designed non-transferred DC plasma torch, whose GIA can be changed by replacing a gas injection component. The arc voltages and thermal efficiencies of the plasma torch, the specific enthalpies and jet lengths of the plasma jets at different working conditions were obtained and analyzed. It has been found that the GIA greatly affects the arc voltage, the thermal efficiency, the specific enthalpy and the jet length. Based on these findings, plasma torch with appropriate GIA could be used to help generating the plasma jet with desired characteristics.

Two thermal plasma torches with different internal structures, 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 and used in the experiments and simulations of plasma surface hardening of 45 steel. Experiments with the processing parameters designed by the Taguchi method were carried out and the heat affected zone (HAZ) properties were measured to determine the optimal processing parameters. The HAZ width and depth of the substrate treated by MEPT are 109% and 163% larger than that by TEPT respectively, while the maximum hardness and the average hardness of the HAZ of the substrate treated by TEPT are 30% and 32% higher than that of MEPT respectively. Simulation results show that the heat flux characteristic of the thermal plasma jet generated by TEPT is more concentrated than that by MEPT, which is the main reason for the differences on their performance on the plasma surface hardening. It is suggested that plasma torch with more concentrated heat flux characteristic should be adopted in surface hardening that emphasizes more on the hardness of the treated surface and the processing efficiency. The plasma torch with less concentrated heat flux characteristic should be utilized in surface hardening that emphasizes more on the HAZ width and depth of the treated surface.

Thermal plasma jets have been widely used in various materials processing techniques. However, the conventional thermal plasma torches usually generate turbulent plasma jets with the disadvantages of high axial temperature gradient, a short jet length and difficulties in the process control relatively, limiting its applications to materials processing. Therefore, this paper proposes a new laminar plasma torch (LPT) working with pure nitrogen to generate laminar plasma jet (LPJ). Its design and structural characteristics, e.g. segmented anode, axial gas injection, parallel water cooling structure, etc., are detailed to ensure the stability, the favorable temperature and velocity distribution of the generated LPJ. Experiments on the characteristics of the LPT showed that the generated LPJ possessed high specific enthalpy (ranging between 10 and 90 kJ/g), long jet length (maximum length: 480 mm) and low axial temperature gradient, and its output power a current and the gas flow rate. In addition, the thermal efficiency of the LPT was experimentally determined to be ranging between 25 and 45 %. Furthermore, experiment and simulation on the application of the LPJ for surface quenching verified the even radial temperature distribution of the plasma jet and high heat flux density brought to the surface.

As laminar plasma jet has long stable plasma jet, low temperature gradient, negligible noise emission, good processing reproducibility and controllability, it is suitable for industrial applications with high precision requirement. However, the conventional laminar plasma torches are large in size and thus can hardly be applied in processing occasions with space limitation. In this paper, a miniature DC non-transferred arc laminar plasma torch with an external size of about 57 × 56 × 42 mm is proposed to meet the processing requirements in these occasions. It can generate long laminar plasma jet with a maximum jet length of about 300 mm at atmospheric pressure with pure nitrogen. Experimental study on the proposed plasma torch has been carried out to obtain its arc voltage characteristics and the jet length characteristics. It has been founded that the plasma torch with spiral gas flow pattern generally has higher arc voltage and longer jet length than that with axial gas flow pattern. In addition, a special jumping arc voltage characteristic and the variation appearances of the plasma jet were founded, which may be used to design miniature laminar plasma torch with high arc voltage and thus low arc current for the same output power as the conventional ones.

Direct current plasma torches have been applied to generate unique sources of thermal energy in many industrial applications. Nevertheless, the successful ignition of a plasma torch is the key process to generate the unique source (plasma jet). However, there has been little study on the underlying mechanism of this key process. A thorough understanding of the ignition process of a plasma torch will be helpful for optimizing the design of the plasma torch structure and selection of the ignition parameters to prolong the service life of the ignition module. Thus, in this paper, the ignition process of a segmented plasma torch (SPT) is theoretically and experimentally modeled and analyzed. Corresponding electrical models of different stages of the ignition process are set up and used to derive the electrical parameters, e.g. the variations of the arc voltage and arc current between the cathode and anode. In addition, the experiments with different ignition parameters on a home-made SPT have been conducted. At the same time, the variations of the arc voltage and arc current have been measured, and used to verify the ones derived in theory and to determine the optimal ignition parameters for a particular SPT.