Complex chromium alitization of carbon steels in chlorine environment

Abstract

The authors investigated the influence of the nature of the chlorine-containing activator on the structure, chemical composition, and some properties of diffusion layers formed during complex chromium-alloying of carbon steels in a chlorine environment. On the basis of thermodynamic calculations, the quantitative ratio of saturating elements (chromium and aluminum) and rational coating temperatures were determined. It was determined that for the complex chromoalitization of steels, an increased amount of chromium in the composition of the saturating mixture is necessary. Based on the data obtained, the temperature intervals for complex chromoalitization of carbon steels in a chlorine environment were recommended to be 800-1100°C. Coatings were applied at a temperature of 950-1050°C for 2 hours in a saturating mixture of the following composition: 45 % wt. chromium, 5 % wt. aluminum, 47 % wt. Al2O3 and 3.0 % by weight of the activator. The chlorine-containing compounds NH4Cl and NiCl2 were used as activators. The structure, chemical composition, and distribution of microhardness along the cross-section of the obtained chrome-aluminum coatings were investigated. It was found that the distribution of structural components in both types of coatings obtained at a temperature of 950 °C is almost identical. At the same time, the structure of the diffusion zone obtained by using NiCl2 as an activator shows the presence of a two-phase layer of dark inclusions, the main component of which is small columnar crystals. The thickness of the obtained coatings is 23-32 microns. An increase in the saturation temperature to 1050°C during complex chromoalitization using NH4Cl as an activator leads to an increase in the thickness of the resulting coatings to 200-220 microns, with the amount of aluminum on the surface being 16.59% by weight and chromium 4.36% by weight. Keywords: chromium, aluminum, carbon steel, activator, diffusion coatings.