Evaluation of structure and properties of boride coating with combined method of boron-copper plating of 45 steel

Abstract

The complex diffusion saturation of the surface of steels with boron and metal by the solid-phase method can improve the functional properties of parts and tools. Thus, the combined process of simultaneous saturation with boron and copper (boron-copper plating) with the use of an exothermic reduction reaction of their oxides in the process of thermal chemical-treatment (TCT) provides a greater layer thickness compared to pure boron plating and makes it possible to increase the plasticity of the coating. The aim of the work was to evaluate the effect of the time of saturation with boron and copper on the structure and mechanical properties, including the fragility (plasticity) of the diffusion layer during the processing of 1045 steel. The boron-copper plating process was carried out at 1123 K for 3, 4 and 5 hours. As a result, diffusion layers with a thickness of 60, 160, and 180 µm were obtained with an uneven distribution of microhardness along the length of the coating. The microhardness of the surface layer is no more than 15 GPa. The maximum value of microhardness reaching HV 20 GPa after 4 and 5 hours of treatment is determined at a distance of up to 15...20 µm from the treated surface of the samples and with a length of up to 20...25 µm. Then the microhardness decreases monotonically and at the end of the layer has the values 17...18 GPa. The diffusion coefficient for the combined method of boron-copper plating was (6.8...7.1)•10–13 m2 /s, the optimal time for TCT was set at 4—5 hours. The results of a study to determine the fragility of diffusion layers showed that the plasticity of layers based on boron and copper is up to 2 times higher compared to pure boriding. Elemental analysis showed a high con centration of boron (22 % wt.) and copper (more than 2 % wt.) in the surface layer of the diffusion coating, which can confirm the formation of an iron-copper alloy on the surface of the treated sample and indicate the presence of a high concentration of higher boride on the surface. Then, a decrease in the boron concentration from the surface to the interface is observed, which indicates the formation of a lower boride. An increased concentration of carbon under the boron-copper layer is determined due to the displacement of carbon from the surface deep into the base, enhanced by the copper-containing component.