Abstract
The available literature describing the effects of high heating and cooling rates on the properties of gray cast iron with electroplates seems to be rather limited and ambiguous. Therefore, the problem of identifying structural regularitites and studying the composition factors and their effect on the properties of the material under examination is of high relevance in both academic and practical terms.
This research focused on composite chromium-based electrochemical coatings and the interaction area between a coating and a substrate (i.e. the surface modified layer). Composite coatings were applied in a self-alloying sulphate electrolyte (CrO3 – 250 g/l, SrSO4 – 6 g/l) at 60°C and the current density of 50 A/dm2 to ensure the formation of a hard wear-resistant chromium deposit. Coatings with thicknesses of 15 to 130 µm were produced, determined by the electrolysis time (60 to 180 min).
Gray cast iron of grade SCh-20 and a double-layer roll foundry iron with a working layer made of LPKhd-70 were used as substrates for plasma electrolytic diffusion coatings. The casting was plated with chromium at 60°C and the current density of 50 A/dm2 to obtain 100 and 300 µm thick coatings. After that the chromium deposit was plasma-treated under the following process parameters: the current density of 7 to 8 MA/m2, the flow rate of plasma-forming argon of 5 to 6 l/min, the specific power of the plasma torch of 240 to 250 MW/m2, the diameter of the nozzle of 8 mm, the linear speed of the plasma torch of 70 m/s. The hardened layers were formed with no forced cooling applied but merely due to heat dissipation in the metal. The above process parameters were designed to cause the surface layer of the casting to melt.
The physical and mechanical properties of chromium coatings can be enhanced through the application of heat treatment (HT). Various HT techniques are applicable at high heating and cooling rates, with plasma treatment being one of them. HT helps improve the adhesion strength of coatings and achieve an excellent wear resistance of the surface modified layer due to hard finely dispersed chromium carbides.
Keywords
Diffusion coatings, wear resistance, plasma treatment, double-layer rolls, solid solution.
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