Investigation of the quality of sulfoalitizing coatings on steel surfaces obtained by electrospark alloying method

Abstract

The method sulfoalitizing of work surfaces by an environmentally friendly method of electrospark alloyingis proposed. The method is comprised of applying sulphur ointment to the surface and subsequent ESA by an aluminium electrode. Steels 20 and 40 were used as the substrate materials. After processing, the roughness of the surface layer was determined. Analysis of the surface profiles of the samples after sulfoalitizing by EIL and the roughness parameters of the studied surfaces is showed that parameters Ra, Rz, Rmax are increased with increasing discharge energy, as well as the carbon content in the steel. Microstructural, durometric and local energy dispersive analyses are carried out. Metallographic and durometric analyses of the obtained coatings are showed zones on microstructures: a near-surface, not continuous loose layer with thickness of 20-40 µm, a hardened layer (20-80 µm), diffusion zone and a base metal with a ferrite-pearlite structure. Replacing the substrate material from steel 20 to steel 40 the hardness of the upper layer (1670 and 2240 MPa at discharge energies of 0.13 and 3.4 J, respectively) and of the reinforced layer (5147 and 10380 MPa at discharge energies of 0.13 and 3.4 J respectively) are increased. The parameters of the coating thickness, microhardness, continuity are increased with an increase discharge energy. Local energy dispersion analysis is showed that the sulfur is concentrated in the surface layer, which characterized the layer of reduced microhardness and is distributed at a depth of up to 15 μm. The diffusion zone of aluminum is 30-80 μm, depending on the energy parameters of the EIL process. The aluminum is concentrated on coating areas located at a distance of 7-15 μm from the surface. The near-surface layer is enriched with sulfur, the hardened layer is aluminium.