Impact of microarc carburizing and boriding of steel on diffusion layer structure

Abstract

Intensification of carburizing and boriding of steel parts is achieved by microarc surface alloying. For carburizing, the parts are immersed in coal powder followed by electric current passing. For boriding, a coating with diffusant is used. Acceleration of diffusion is achieved by action of microarc discharges on the steel surface. The aim of this work was to study the effect of diffusion parameters on thickness, structure, and phase composition of coatings. The samples were made of 20 steel. Surface current density was 0.45 – 0.53 A/cm2. Duration of the process was 2 – 8 min. At the beginning of heating, temperature of the samples increases, and then stabilizes at 930 – 1250 °C due to cessation of micro-formation during combustion of coal particles. After carburizing, a eutectoid mixture is formed on the surface, then, the zone with ferrite-perlite structure is located, which transfers into the original structure. The maximum layer thickness (60 – 390 microns) is reached after 6 – 7 min of heating and then does not increase due to a decrease in the carbon potential during combustion of coal particles. Similar relationship is obtained when boriding: the maximum layer thickness (60 – 340 microns) is reached after 6 – 7 min and then does not increase due to depletion of diffusant source in the coating. At current density of 0.45 A/cm2, the layer consists of a base (a dispersed ferrite-carbide mixture) containing fine inclusions of iron borides and boron carbide. At current densities of 0.49 and 0.53 A/cm2, the layer has heterogeneous structure, with areas of high-hard boride eutectic located at the base. At high current density, diffusion of carbon and boron along the grain boundaries forms Fe – C – B triple eutectic. At lower current density, surface temperature is lower than eutectic formation temperature, so heterogeneous coating structure is not formed. The work results make it possible to choose modes of microarc heating to obtain the required parameters of diffusion layer.