Influence of Tool Steel Chemical Composition on Structure and Phase Composition of a Diffusion Layer after Microarc Boriding
Yu. M. Dombrovskii, M. S. Stepanov
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The effect of alloy tool steel chemical composition on structure and phase composition of a diffusion layer after microarc boriding is studied. Specimens of die steels 5KhNM and Kh12F1, and high-speed steel R6M5K5 are studied. Current density at a specimen surface is varied from 0.45 to 0.53 A/cm2. Diffusion impregnation duration is from 2 to 8 minutes. During steel 5KhNM microarc boriding diffusion layer microstructure is governed by surface current density. With current density of 0.53 A/cm2 there is formation of an inhomogeneous diffusion layer structure that consists of a base in the form of a fine ferrite-carbide mixture with microhardness of 7.0–8.0 GPa, within which there are carbide and boride phase inclusions and areas of carboboride eutectic with microhardness of 12.0–12.5 GPa. Eutectic content is reduced compared with boriding steel not containing carbide-forming elements within the composition This is explained by the effect of strong carbide-forming elements within the steel composition on displacement of the formation of carboboride eutectic process into a higher temperature region. For the same reason, microarc boriding of Kh12F1 and R6M5K5 steels with all values of surface current density does not lead to formation of a carboboride eutectic due to a higher content of strong carbideforming elements within the composition. The greatest boride layer thickness is observed for maximum values of surface current density and process duration.