Issue 37

D. Krastev et alii, Frattura ed Integrità Strutturale, 37 (2016) 280-286; DOI: 10.3221/IGF-ESIS.37.37 280 Focused on Fracture Mechanics in Central and East Europe Recast layers on high speed steel surface after electrical discharge treatment in electrolyte D. Krastev, V. Paunov, B. Yordanov University of Chemical Technology and Metallurgy, Sofia, Bulgaria A BSTRACT . In this work are discussed some experimental data about the obtaining of recast layers on the surface of high speed tool steel after electrical discharge treatment in electrolyte. The electrical discharge treatment of steel surface in electrolyte produces a recast layer with specific combination of structure characteristics in result of nonequilibrium phase transformations. The modification goes by a high energy thermal process in electrical discharges on a very small area on the metallic surface involving melting, alloying and high speed cooling in the electrolyte. Obtained recast layers have a different structure in comparison with the metal matrix and are with higher hardness, wear- and corrosion resistance . K EYWORDS . Recast Layers; High Speed tool steel; Electrical discharge treatment in electrolyte. I NTRODUCTION he recast layers on metals and alloys are mainly created by treating the surface with high energy stream such as laser, ion beam or electrical discharge for a very short time and pulse characteristics. The high energy attack on the surface involves local melting and in many cases vaporizing of metal microvolumes. After the cooling, on the treated metal surface a recast layer with different structure and properties from the substrate is formed. This recast layer can be with the same chemical composition as the substrate or with different one if in the thermal process suitable conditions for surface alloying are created. When the recast process is not controlled there are on the surface microcracks and pores which have negative influence on the surface properties and the recast layer must be removed. In the controlled recast processes it is possible to produce surface layer with determinate chemical composition, thickness, structural characteristics and properties, which are unique for the material with the very high hardness, corrosion- and wear resistance. The basic techniques that give opportunities in this direction are laser surface treatment, electrical discharge machining and plasma electrolysis. Laser surface treatment is widely used to recast and modify localized areas of metallic components. The heat generated by the adsorption of the laser light provides a local melting and after controlled cooling is obtained a recast layer on the metal surface with high hardness, wear resistance and corrosion resistance. The laser surface melting is based on rapid scanning of the surface with a beam focused to a power density scale of 10 4 W/cm 2 to 10 7 W/cm 2 . Quench rates up to 10 8 - 10 10 K/sec provide the formation of fine structures, the homogenization of microstructures, the extension of solid solubility limits, formation of nonequilibrium phases and amorphous phases or metallic glasses, with corrosion resistance 10–100 time higher compared to crystalline [1]. Laser surface melting is a simple technique as no additional materials are introduced, and it is especially effective for processing ferrous alloys with grain refinement and increase of the alloying T