FORMATION OF CARBIDE TYPE COATING IN THE PROCESS OF MICROARC STEEL VANADATION

The high-energy impact on the material surface is used for the intensifi cation of surface diff usion saturation of steel. The article describes the process of microarc thermochemical treatment in which a steel product was immersed in a container fi lled with coal powder, and was heated by passing electric current. Microdischarges are formed in a powder environment. They are concentrated around the product and create a region of gas discharge. This leads to surface heating of the product and the surrounding powder environment. The pyrolysis of coal in a container formed a carbon-containing environment, which enables to carry out carburizing of steel. In addition, there is a possi bility of formation of carbide type surface due to the simultaneous diff usion of carbon and alloying element. Its source is a conductive coating containing the powder of diff usant. The authors have investigated the possibility of forming the coatings of carbide type by diff usion saturation of low carbon steel with vanadium. The cylindrical samples of steel 20 with a diameter of 12 mm and length 35 mm were used. The powder of low carbon ferrovanadium was used as the source of diff usant. The current density on the surface of the samples was 0.53 A/cm² , the temperature was increased from ambient to 1250 °С. To study the structure and phase composition of the diff usion layer optical microscopy, scanning electron microscope with energy dispersive microanalysis system, X-ray phase analysis, atomic force microscopy, microhardness analysis were used. After treatment for 3 min it was detected the formation of the diff usion layer with thickness of 170 – 180 μm. The basis of the diff usion layer is a solid solution of vanadium in α-iron with a vanadium concentration of 3 – 4 wt. % and the ratio of the number of atoms in the unit cell is 9:1, corresponding to α-Fe9 V with microhardness of 8.0 – 9.0 GPa. The layer base comprises the multiple nanoscale carbide inclusions, as well as carbides of the VC_0.863 type with the size up to 10 microns with vanadium concentration about 64 wt. % and microhardness of 21.65 – 25.75 GPa, undergoing the atomic ordering with the formation of the cubic V₈ C₇ superstructure.

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