Influence of the proportion of additives of the emitter of electrons and anions in the technological filling on the acceleration of the processes of thermal diffusion chromium plating

The results of experiments on thermal diffusion chromium plating of structural steel samples with the use of an internal emission field created in technological backfill during heating are presented. The possibility of controlling the efficiency of the thermionic field by changing the proportion of the additiveemitter in the technological filling is shown. So, to create a field, instead of inert aluminum oxide, a mixture of the following composition was introduced into the separating part of the base technological backfill: powders of scheelite (CaWO4), serpentine (Mg3Si2O7), MgO, dicalcium silicate (γ-2 CaO•SiO2). Variable compositions were formed on the basis of the base mixture with the addition of ferrotungsten powder in one case, and ferrotungsten powder and an additional portion of scheelite in the second case. The choice of these materials was based on the results of preliminary experiments on the measurement of thermionic current in metal and oxide components. Saturation of samples of steel 35Kh2N3 with chromium was carried out at a temperature of 1000 °C for 24 hours. The chemical composition of the diffusion layer on metal samples was controlled using a JEOL JSM-6460 LV universal scanning (scanning) electron microscope. The microhardness of the coatings was measured using an FM-800 microhardness tester. The results obtained show that an increase in the proportion of tungsten-containing additive-emitter (ferrotungsten and scheelite CaWO4) in the technological backfill provides an increase in the chromium diffusion rate due to the intensification of the process of transporting atomic chromium to the saturated surface, providing an increase in the chromium diffusion depth by 1.3 times compared to filling the base composition. The use of strong carbide-forming elements as emitter additives causes a change in the phase composition of chromium coatings and an increase in the proportion of CrN nitride phases and tungsten-based carbide phases in them. The hardness of the chromium-plated surface in the technological backfill with a low content of emitter additive (10 wt. %) is 744 HV, with an increase in the proportion of the additive to 18 and 30 wt. %, the hardness increases to 808 HV, which is associated with an increase in the concentration of chromium on the surface.