Issue 37

D. Krastev et alii, Frattura ed Integrità Strutturale, 37 (2016) 280-286; DOI: 10.3221/IGF-ESIS.37.37 281 elements content in solid solution. In fact the process has been employed for improving the cavitation erosion and corrosion resistance of a number of ferrous alloys. The laser surface melting can be combined with a simultaneous controlled addition of alloying elements. These alloying elements diffuse rapidly into the melt pool, and the desired depth of alloying can be obtained in a short period of time. By this means, a desired alloy chemistry and microstructure can be generated on the sample surface and the degree of microstructural refinement will depend on the solidification rate. The surface of a low-cost alloy, such as low carbon steels, can be selectively alloyed to enhance properties, such as resistance to wear and corrosion [2]. Electrical discharge machining is a thermoelectric process that erodes workpiece material by series of discrete but controlled electrical sparks between the workpiece and electrode immersed in a dielectric fluid [3]. It has been proven to be especially valuable in the machining of super-tough, electrically conductive materials, such as tool steels, hard metals and space-age alloys. These materials would have been difficult to machine by conventional methods, but EDM has made it relatively simple to machine intricate shapes that would be impossible to produce with conventional cutting tools. In EDM process, the shapes of mold cavities are directly copied from that of the tool electrode, so time-consuming preparation work must be done on the fabrication of the corresponding tool electrode. The electrical discharge machining uses electrical discharges to remove material from the workpiece, with each spark producing temperature of about 8000-20000 ºC. This causes melting and vaporizing of small volumes of the metal surface and after cooling in the dielectric fluid the melted zones are transformed in recast layer with specific structure. The EDM modified surface consists from two distinctive zones [4-6]: ● Recast layer ● Heat affected zone The recast layer is also named white layer and it crystallizes from the liquid metal cooled at high rate in the dielectric fluid. The depth of this top melted zone depends on the pulse energy and duration. Below the top white layer is the heat affected zone with changes in the average chemical composition and possible phase changes. At Plasma Electrolysis the processes are of similar nature as EDM and it can be obtained recast layers with the same characteristics. Significant differences are the replacing of liquid dielectric with electrolytes and in result of that increasing the distance between the electrodes which causes displacement of electrical discharges on boundary electrode-electrolyte. There are developed on technology level processes for plasma-electrolysis oxidation and plasma-electrolysis deposition [7]. M ATERIALS AND EXPERIMENTAL PROCEDURES he objects of study are specimens with diameter 6 mm which are made from HS 6-5-2 steel with structure after the typical heat treatment for tools of this steel. In this most popular high speed steel the tungsten content is reduced to 6 % while it is additionally alloyed with 5% molybdenum and vanadium content is about 2 %. The steel is quenched in oil from 1220 ºC and after that triple-tempered at 550 ºC for 2 hours each. The measured hardness of the specimens is 63-65 HRC. The electrolyte composition and its characteristics are of great importance for the process parameters and for the microstructure and properties of recast layers. By these experiments the electrolyte is on water basis and in it are dissolved glycerol and sodium carbonate. In the electrolyte is suspended fine sized B 4 C. Figure 1: Electrical discharge treatment in electrolyte: 1 – workpiece, 2 – electrode, 3 – electrolyte, 4 – magnetic stirrer. For the electrical discharge treatment in electrolyte is developed a laboratory device, shown in Fig. 1 [8,9], giving opportunities for treatment of cylindrical workpieces with diameter up to 20 mm. The electrolyte 3 is in active movement by mixing from a magnetic stirrer 4 . After passing of electric current with determinate characteristics through the T