Issue 37

D. Krastev et alii, Frattura ed Integrità Strutturale, 37 (2016) 280-286; DOI: 10.3221/IGF-ESIS.37.37 285 Homogeneous structure of the recast layer can be observed by a higher magnification on SEM micrograph in Fig. 8. The modified surface has a very high corrosion resistance and can not be etched. The microhardness of the white layer in these cases reaches HV 1500–1600 compared to HV 780–820 of the core. Figure 8: SEM micrograph of modified punch surface. Combination of high voltages with increasing duration of the electrical discharge treatment causes a significant heating of the workpiece, melting the surface and penetrating of the diffusion process in depth to the grain boundaries (Fig. 9). The surface recast layer in this case has ledeburitic structure with higher concentration of carbon and boron. Figure 9: Optical micrograph of the recast layer on HS 6-5-2 steel after electrical discharge treatment in electrolyte at 230 V for 10 min, 800x C ONCLUSIONS he high energy process of electrical discharge treatment in electrolyte and the nonequilibrium phase transformations in the high alloyed metallic system of HS 6-5-2 steel causes considerable modifications of the metallic surface and obtaining of recast layers with finecrystalline and nanocrystalline structure. The modified surfaces have high hardness, strength, wear- and corrosion resistance related to the supersaturated solid solution of alloying elements in the obtained recast white layer and the nonequilibrium phase transformations by the high quenching speed from liquid state. Investigations show that obtained recast layers reach a thickness about 0.05 mm. The hardness of the modified layers can vary considerably and depend of the treatment conditions, electrolyte composition and microstructure, but in principle it is higher then the hardness of the typical microstructure of this steel. The microhardness of the recast layers measured by Hanneman test shows values after the different treatments up to 1600 HV which are very higher than the microhardness of HS 6-5-2 steel microstructures after the typical heat treatment. The experiments show that tools with such surface hardness have higher wear resistance and working capacity. T