Structure, phase composition, mechanical properties and wear resistance of steel after microarc boriding and vanadation

Abstract

Boriding is a common method of thermo-chemical treatment of steel products. It increases their hardness and wear resistance, but also increases the surface fragility, has a long duration and high labor intensity. The combined coating with boron and vanadium is used to improve the properties, and it is possible to apply microarc surface alloying to reduce the duration of the diffusion saturation process. This makes it possible to intensify the diffusion of alloying elements by forming a gas discharge zone at the surface of the steel product. The aim of this work was to study the structure, phase composition, mechanical properties and wear resistance of steel after boriding and vanadation. During the experiments, a lubricant containing boron carbide powders B4C and ferrovanadium FeV80 were used, which was applied to the surface of the steel sample. During boriding and vanadation of steel a surface layer with a thickness of 150 – 190 μm is formed. It has a base with microhardness of 7.8 – 8.3 GPa and light grey granular inclusions and eutectic areas with microhardness of 13.5 – 14.0 GPa. Further there is a carbonized layer of eutectoid concentration, passing into the original ferrite-perlite structure. The content of alloying elements in the characteristic points of the surface layer was determined, which confirmed the increased content of carbon, vanadium and boron in the base layer, areas of eutectic and carbide phase. X-ray phase analysis revealed the presence of iron borides FeB and Fe2B, vanadium borides VB2 and V2B3 and vanadium carbide VC0.88 in the surface layer. Mechanical properties of coatings were studied by microindentation of its cross-section with registration and analysis of deformation diagram under loading and subsequent unloading of the indenter. Hardness at indentation in the base layer increased to 7.95 GPa, in dispersed inclusions – to 13.90 GPa. The modulus of elasticity for indentation in the base and inclusions is 238 MPa and 340 MPa, respectively. Creep and proportion of the plastic component in microindentation is naturally reduced with increase in hardness. Fine inclusions of iron borides, vanadium borides and carbides significantly increase the steel wear resistance. It has increased in 4 times during friction against the fixed abrasive particles in comparison with the initial state.