THE CHROMIUM AND COBALT DISTRIBUTION IN THE DIFFUSION LAYER OF STEEL X35CrNi2-3 DURING THERMОDIFFUSION SATURATION

Abstract

For parts of machines and equipment operating under the influence of aggressive media, abrasive, high temperatures, the creation of a hard, wear-resistant surface is an urgent task. Cobalt is used as a binder to create hard alloy materials that work under these conditions. It was of interest to evaluate the possibility of using cobalt as an additive in complex thermal diffusion chromium plating of structural steel. The article describes the features of thermal diffusion saturation of steel X35CrNi2-3 with chromium and cobalt at a temperature of 1000°C. A technique for analyzing the obtained coating is presented, based on the possibilities of X-ray spectral microanalysis of the diffusion layer on transverse microsections of the obtained samples. The elemental composition of the diffusion layer was monitored using the JEOL JSM-6460 LV universal scanning (scanning) electron microscope. The microstructure was studied using the optical metallographic microscope Axio Observer D1.m. The X-ray phase analysis was carried out with the Rigaku Ultima IV diffractometer. The hardness measurement was carried out with the FM-800 microhardness tester at a load of 50 g. Data were obtained on the distribution of chromium and cobalt in the surface layer of steel. It is shown that the resulting thermal diffusion coating consists mainly of a substitutional solid solution based on Cr‒Fe‒Co with bcc lattice and chromium carbides. The average diffusion coefficients of chromium in the α-phase of chromium were determined to be DCr = 3,22·10‒15 м2/с; in the γ-phase of iron DCr = 9,41·10‒15 м2/с. The diffusion coefficient of cobalt in the γ-phase of iron DCo = 7,89·10‒15 м2/с. The depth of the outer coating on the sample was about 15–20 µm, the depth of the diffusion layer of chromium in the base metal was about 70 µm, and that of cobalt was about 40 µm. The maximum hardness of the outer surface layer was 1740 HV, the diffusion layer in the base metal was 900–560 HV, and the base metal was about 510 HV.