Issue 37

D. Krastev et alii, Frattura ed Integrità Strutturale, 37 (2016) 280-286; DOI: 10.3221/IGF-ESIS.37.37 282 suspended electrolyte between the workpiece 1 which is on the cathode and electrode 2 starts an active sparking on the workpiece surface. The sparking characteristics depend on different factors such as parameters of the electric current, type and composition of the electrolyte, movement of the workpiece and electrolyte. The high-alloy steel gives an opportunity for higher effectiveness of treatment on structure and properties of modified surfaces after the nonequilibrium phase transformations from liquid state. Under the high temperature of discharge column, the surface layer can dissolve activated elements formed in it from the electrolyte and receives structure with different characteristics and properties. For the experiments between the electrodes is applied direct current with voltage from 80 to 240 V. The time of treatment is from 1 to 10 minutes. The investigations show that the optimal time is 3 minutes. Obtained layers have been investigated by light microscopy, SEM, XRD and Hanneman microhardness testing. R ESULTS AND DISCUSSION he electric discharges generate an enormous amount of heat, causing local melting on the workpiece surface and thereupon it is rapidly quenched from the liquid state by the electrolyte. This recast area has a specific structure, which can be composed of several microscopic metallurgical layers, depending of machining conditions. The high rate of the recasting process gives opportunities for formation of metastable phases and considerable decreasing of grain size. The electrolyte type is of great importance for the chemical composition, microstructure and properties of the recast layer. The melted and resolidified layer during this process is also referred as the “white zone”, since generally no etching takes place in these areas at the metallographic preparation. The structure of recast layer can be seen after SEM investigation on Fig. 2. Figure 2: SEM micrograph of recast layer on HS 6-5-2 steel after electrical discharge treatment in electrolyte At short times of treatment it is not observed diffusion of elements from the electrolyte in the modified surface, but it is available diffusion process inside the workpiece between the white layer and the matrix – Tab. 1. The strong carbide- formed elements such as Mo, W, and V diffuse from the white layer to the matrix and Cr, Co in the opposite side. Chemical element Matrix of workpiece White layer Si <0.01 <0.01 Mo 5.58 4.87 V 2.30 1.63 Cr 4.25 4.52 Co <0.01 0.19 Ni <0.01 <0.01 W 8.34 5.75 Table 1: EDS analysis of modified workpiece from HS 6-5-2 steel The specific properties of recast white layer in a case of tool steels are the remarkable high hardness, strength and corrosion resistance related to the nonequilibrium phase transformations in the high alloyed metallic system. The white T