Results of steel microarc diffusion impregnation with boron combined with carbide-forming elements

Microarc surface alloying of steel is performed in carbon powder under electric current flow conditions, which leads to accelerated surface material impregnation with carbon. For surface impregnation with other alloying elements, a coating containing a diffusion agent is used, which is previously applied to a strengthened component. The purpose of this work is a study the effect of microarc diffusion impregnation temperature of steel with boron combined with carbide-forming elements on coating structure, phase composition and microhardness. Cylindrical steel 20 specimens are subjected to multicomponent impregnation with B + Cr, B + Mo, B + V, B + W at 950, 1100 and 1250 °C. Duration of the impregnation process is varied from 2 to 8 min. A coating containing boric acid H₃BO₃ powder and ferroalloy powders of carbide-forming elements in a ratio of 1:1 by volume based upon an electrically conductive binder is used for alloying. It is found that after microarc multicomponent impregnation at 950 and 1100 °C a diffusion layer consists of a base in the form of a fine ferrite-carbide mixture with microhardness up to 9.4 GPa, within which there are high hardness finely dispersed inclusions of iron and carbide-forming element carbides and borides having a higher microhardness (up to 16.5 GPa) compared with single-component boriding. Then there is a carburized layer with a eutectoid structure, passing into the original ferrite-pearlite structure of steel 20. After multicomponent impregnation at 1250 °C a diffusion layer additionally contains sections of carbide forming eutectic existing within the Fe–C–B system, with an increased content of chromium and boron as a result of predominant diffusion along grain boundaries. Therefore, microarc surface impregnation of steel with boron combined with a carbide-forming elements increases the temperature of boride eutectic formation, which may lead to increased layer brittleness compared with single-component boriding. The greatest diffusion layer thickness is obtained after 6 min of the microarc multicomponent impregnation process and then it hardly increases due to carbon particle combustion and depletion of the diffusion agent source within a coating.