Improving Steel Surface Properties Using the Pulse Plasma Method

Improving Steel Surface Properties Using Pulse Plasma Method

1. Introduction

Technologies used for surface processing of machine parts fail to fully solve existing problems in process efficiency, product performance, material cost, machining and heat treatment and other critical areas. Insufficient surface properties of workpieces have prompted the investigation of new techniques. On one hand, large quality variations and low product performance levels reduce process efficiency, while on the other hand, short service life of workpieces increases production costs. [1]. In industries working with metals, wood, synthetics, leather, rubber and similar materials, cutting and forming tools and machine parts such as friction bearings, eccentric shafts, valves and others operate under conditions requiring them to withstand enormous mechanical and thermal loads. It is crucial that these surfaces possess high levels of hardness, wear resistance and heat resistance. [2] Many of the surface techniques commonly used for this purpose are expensive and inefficient. Furthermore, they contain problems in terms of performance and reliability characteristics. [3] The pulse plasma method has been proposed as a technique to solve many of the problems listed above.

1.1 Pulse Plasma System

The pulse plasma system is a modification method involving elasto-plastic deformation, acoustic and pulsed magnetic field effects, heat, electrical pulse processing, and deformation of metals and alloys during operation. A jet with high energy density is created through the explosion of fuel gas mixtures. When an electromagnetic field is superimposed on an explosion wave, the surface layer of the workpiece undergoes rapid melting and solidification with heating and cooling rates typically in the range of 10⁷-10¹⁰K/s. After pulse plasma processing, the surface layer exhibits high anti-friction properties and wear resistance. Pulse plasma has no size limitations or residual stress problems and is therefore a suitable surface modification technique for processing complex-shaped industrial parts. [1-4,6]. Compared with laser processing, electron beam processing and conventional ion implantation, the pulse plasma method has high energy conversion efficiency, processing efficiency and ease of operation characteristics. [caption id="attachment_131310" align="aligncenter"] Figure 1: Schematic representation of the pulse plasma modification system: 1-explosion chamber, 2-central electrode anode, 3-conical electrode-cathode, 4-electrode gap, 5-consumable electrode, 6-power source, 7-electrode gap, 8-pulse plasma generation, 9-working surface[/caption] A schematic representation of the pulse plasma technique applied for surface modification of samples is shown in Figure 1. The plasmatron consists of an explosion chamber (1) where the fuel gas mixture is formed and explosion combustion is initiated, a central electrode anode (2), a conical electrode-cathode (3), an electrode gap (4), a consumable electrode (5) and a power source (6). At the beginning of the explosion, ionized combustion products from the explosion chamber (1) are fed into the electrode gap (4) to complete the electrical circuit. This has led to the formation of a conductive layer (7) created by combustion products under the influence of gas-dynamic and electromagnetic forces. The consumable metal rod (5) is fixed in a position along the central electrode axis. The tip of the rod is vaporized during heating and alloy elements are introduced into the plasma jet. After samples are withdrawn from the plasmatron, the plasma jet (8) interrupts the circuit between the anode electrode and the cathode workpiece (9) [1,3]. Cyclic thermal effect at 3×10⁵ Hz frequency causes periodic heating of the surface up to the melting point by changing the phase state of the metal layer and accelerating the transfer of alloy elements. Multiple heating and cooling of the surface layer with high temperature gradients result in periodic changes in stresses and strains in this layer, causing significant changes in the structural state of a metal alloy. The pulse plasma explosion process is accompanied by the pulsed mechanical pressure of a high-velocity plasma jet decelerating on the surface. The energy of free gas-dynamic shocks (pulses) (up to 4 kJ) is determined by the mass and velocity of the plasma jet. Mechanical impact is considered an effective tool for accelerating chemical and mass transfer processes occurring in solids. It activates oscillatory processes in a metal alloy, excites long-wavelength acoustic phonons, thereby accelerating cooling and crystallization and intensifying mass transfer of alloy elements in the heated layer [1,3]. The high-velocity plasma jet is accompanied by a strong acoustic wave (up to 150 dB) [1-3]. Acoustic effect is the propagation of elastic or semi-elastic oscillations in the processed medium. Acoustic waves intensify chemical transformations and increase the efficiency of mechanical, thermal and chemical effects [3-7]. The chemical effect on the surface is achieved by initiating or accelerating chemical reactions (catalysts) and by introducing materials into the plasma that chemically interact with the metal alloy on the workpiece surface. The device allows the addition to the plasma jet of a combination of chemical materials differing in mass and composition, which can be active both relative to the workpiece surface and to each other [1-3]. When electron current is suppressed by its own magnetic field, ions are effectively accelerated in the radial electric field between the electrodes [1,9]. The working surfaces of samples were subjected to modification in a plasma atmosphere containing alloy elements dissolved from the metal electrode rod in interaction with propane and nitrogen gas components. Tungsten is used as the consumable electrode. Ionization of the consumable electrode enables alloying of the workpiece surfaces with plasma components and surface hardening. The processing time is approximately 1 minute. Each pulse of concentrated energy flow causes super-rapid thermal cycles and modifies the surface. In this process, the consumed electrode causes ionization of tungsten (W) and nitrogen (N) atoms, which are doped into the surface through a diffusion mechanism. Following the modification process, the microhardness values of surfaces increase 5 to 6 fold. Process parameters also affect the thickness and mechanical properties of the modification layer. As the number of pulses increases, the thickness of the modification zone also increases [8-9].

Conclusion

The pulse plasma technique is used for surface modification. The most important advantages of pulse plasma are that processing time is very short (1 minute) and it is economical. It is capable of producing effective and superior mechanical properties on the surface in a short time. Process parameters have significant effects in determining the final structure and mechanical properties of surfaces. Among these parameters, control of gas diffusion across the nozzle distance, the number of pulses and plasma composition are the most important. An appropriate combination of these parameters will provide the best surface properties and establish processing time duration as an important economic factor. Through this technique, with increasing pulse numbers, the thickness of the modified layer increases and the resulting structure becomes more homogeneous. During surface modification, phase and structural transformations occurred. The new phases and structural transformations significantly improve surface mechanical properties. Hardness values increase 4 to 6 fold and wear resistance improves.

References

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