The effect of borocoppering duration on the composition, microstructure and microhardness of the surface of carbon and alloy steels

Abstract

Introduction. Borocoppering is one of the methods of thermochemical treatment (TCT) aimed at forming diffusion layers with high physical and mechanical properties on the surface of carbon and alloy steels. The thickness of the diffusion layer is the most important characteristic of the TCT, which determines the depth of hardening. Consequently, the intensity and main characteristics of the TCT (layer thickness, alloying element concentration profile) depend on the process conditions (temperature, duration, and amount of alloying element). The purpose of this work is to determine the temperature-time parameters of diffusion borocoppering, which contribute to the formation of diffusion layers with a maximum thickness. The paper considers the results of surface hardening of carbon and alloy steels (for example, Steel 45 (0.45% C), Steel U10 (1.0% C), and 0.5C-Cr-Ni-Mn steel) by high-temperature soaking in powder mixtures containing boron and copper. Borocoppering was carried out in sealed containers with the powder mixture consisting of boron carbide, copper oxide, and sodium fluoride as an activator at a temperature of 950 °C for 3–5 h. The resulting specimens with a diffusion layer were examined using an optical microscope and a scanning electron microscope (SEM); the microhardness, elemental and phase composition of the layers were also determined, as well as the roughness of the obtained surfaces. Results and discussions. The microstructure of the obtained diffusion layers is studied; diagrams of the changes in the layers’ thickness and the microhardness distribution over the layers’ thickness are shown. It is established that with an increase in the soaking time from 3 to 5 h, the thickness of the diffusion layer increases from 120 to 170 μm on Steel 45 (0.45% C); from 110 to 155 µm on Steel U10 (1.0% C) and from 130 to 230 µm on 0.5C-Cr-Ni-Mn steel. A gradual decrease in the concentration of boron and copper along the layer thickness from 15–16% and 2–3% on the surface, respectively, to zero values at the boundary with the base metal is revealed. It is established that borocoppering to the formation of more thick boride layers on the surface of carbon and alloy steels compared to pure boriding. Moreover, an increase in the duration of soaking during the process contributes to the greatest increase in the thickness of the layer on 0.5C-Cr-Ni-Mn steel. A study of microgeometry is carried out, microtopographies and profilograms of specimens’ surfaces are shown before and after borocoppering. It is established that the roughness after borocoppering increases by 2-3 times compared to the initial one, and an increase in the duration of the process does not have a significant effect on the roughness.