Issue 37

D. Krastev et alii, Frattura ed Integrità Strutturale, 37 (2016) 280-286; DOI: 10.3221/IGF-ESIS.37.37 283 layer characteristics, its homogenous structure and thickness, are of dependence on the electric current parameters and on the duration of treatment. By lower voltage the obtained recast white layer is inhomogeneous and local deposited on the metal surface. The treatment time increasing in this case shows an insignificant influence on process. In Fig. 3 is shown an optical micrograph of surface microstructure after electrical discharge treatment in electrolyte at 150 V for 3 minutes. The thickness of obtained layer is under 0.01 mm. Figure 3: Optical micrograph of modified HS 6-5-2 steel surface at 150 V for 3 min, 800x. The investigations show that it is possible to obtain a white layer on the high-speed tool steel surface at voltages of about 100 V, Fig. 4, but the electrical discharges energy is insufficient for dissolving of carbides and the hardness of the modified surface can not receive the expected high level. Figure 4: SEM micrograph of modified HS 6-5-2 steel surface at 100 V, 2 min. The voltages above 200 V give a very high intensity of sparking on the metal surface with energy enough for melting of the metal and dissolving of the carbides and it is possible to obtain compact recast layer with homogeneous structure. By the high speed quenching from liquid state the solubility of the alloying elements remains very high in a supersaturated solid solution and after nonequilibrium phase transformations metastable structures with high hardness and wear resistance are formed. In Fig. 5 an optical micrograph of the recast layers obtained after 3 minutes treatment at 200 V is shown. The thickness of the obtained white layers is about 0.05 - 0.06 mm. The modified surface has a very high corrosion resistance and the microhardness of the white layer in these cases reaches more than HV 1500. The XRD investigation of the modified by electrical discharge treatment in electrolyte high-speed steel surface shows a significant difference between the recast layer and bulk material (Fig. 6 and Fig. 7). The modified surface has typical diffraction patterns for nanocrystalline structures. The XRD analysis also proves the mass transfer of boron from the electrolyte and its diffusion in the surface layer. In the modified steel surface along with carbides typically for the high- speed steel structure Me 2 B is also presented.