Technological basis for the process of application of diffusion coatings in liquid metal melts with use of electric furnaces with air atmosphere

Abstract

Diffusion saturation of the components surfaces in the melts of low-melting metals is the most efficient among known technologies for application of coatings from a liquid phase. However, as soon as this technology is realized at present time with use of special units with protective media, it restricts principally its wide practice. This work presents the results of examination of application processes for diffusion coatings in the melts of low-melting metals using electric furnaces with air atmosphere. Use of this process in shaft or muffle furnaces, which are widely applied at the modern machine-building production facilities, means introduction of the important innovations in the existing technology due to the effect of electric furnace oxidizing medium on quality of obtained coatings. Thereby two following aims of the conducted investigations were formulated: comprehensive study of coatings forming regularities with use of liquid metal melts in the conditions of oxidizing effect in electric furnaces with air atmosphere, as well as development of practical recommendations for elimination of harmful influence of high-temperature oxidation of melt and processing product on quality of coatings. Testing of suggested technological routes for forming of diffusion metallic coatings on steel products in low-melting metal melts using electric furnaces with air atmosphere is an actual problem. Protection of final melt and product surface from oxidation using different fluxes is proposed. Use of the mix of NaCl-Na 2 CO 3 salts didn’t provide protection of liquid metal bath and steel product surface from high-temperature oxidation. Use of Na 2 B 4 O 7 as a protective flux also didn’t provide efficient protection from high-temperature oxidation, because porous inhomoge-neously distributed diffusion nickel coating with thickness 9-10 μm was obtained. The best result was achieved using CaO-Li 2 CO 3 -B 2 O 3 flux. As a result, single-layered, uniform, porous-free, homogeneously distributed diffusion nickel coating with thickness 20-22 μm was obtained.