The structure, phase composition, and residual stresses of diffusion boride layers formed by thermal-chemical treatment on the die steel surface

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 2 2021 Introduction The quality of the surface layer of machine parts and tools is determined by many parameters (roughness, waviness, hardness, residual stresses, etc.). It is ensured by a sequence of mechanical and heat treatment op- erations. Thermal-chemical treatment (TCT) of metals is used to harden the surface layers of forming tools, such as dies for stamping processes and injection molding [1-4]. In this case, the technological equipment durability and the quality of the products made with its help depend on the quality of the obtained diffu- sion layers. That determines the need for an integrated approach to assess the properties of diffusion layers, including its’ physical and mechanical properties, structural, phase, and stress state. It is evident that after TCT there is a difference in the specific volumes of the diffusion layer and the base steel, under the influence of which residual stresses (RS) arise, which have a significant effect on the per - formance of the coating and the product as a whole. It is known that compressive RS are more preferable in terms of fatigue crack retardation, which is also true for TCT processes [5, 6]. Thus, the authors [6, 7] found out that the processes of carburizing, nitriding, boriding, and borosiliconizing positively affect the distribu- tion of RS on alloy structural steel (0.25 % C, 0.8 %Mn, 0.03 % Ti), and VKS-5, EP718, VNS-17 alloys. RS measurement by Davydenkov’s method for combined processes, including carburizing and sub- sequent grinding of 17CrNi6-6 alloy steel (EN 10084-2008 standard), is considered in [8]. The authors discovered that common and low-temperature carburizing leads to a favorable distribution of RS in the diffusion layer. Subsequent fine grinding with a CBN grinding wheel made it possible to maintain the RS distribution. A positive effect of this tool on the roughness of die steel (0.3 % C, 2 % Cr, 8 % W, 0.2 % V) after boroaluminizing is well known [9]. Thus, the initial roughness after TCT was reduced from 7.7 μm to 0.43 μm in terms of the R a parameter. In the research [10, 11], the results of a favorable distribution of RS in the diffusion layer after combined treatment, including boriding followed by ultrasonic treatment of carbon steel, are also presented. Thus, when developing methods for diffusion treatment on the surface of steel products it is necessary to focus on such RS distributions that ensure the desired performance properties of the machine units. A literature review of recent publications on the RS revealed a lack of publications on the RS determination after die steel (0.3 % C, 2 % Cr, 8 % W, 0.2 % V) boriding. This work aims to determine the diffusion boriding temperature, which contributes to a favorable RS distribution in the surface layer of die steel (0.3 % C, 2 % Cr, 8 % W, 0.2 % V). Methodology Samples of die steel (0.3 % C, 2 % Cr, 8 % W, 0.2 % V) (Table 1) were subjected to TCT in powder mixtures using furnace heating. The powder method was carried out in containers with a fusible seal (Fig. 1) [12]. The parts were packed in a container with a fusible glass sealed lid. The container was then loaded into a preheated furnace. After the exposure, the container was cooled outside the furnace in still air. Boriding was carried out in a powder mixture of boron carbide and sodium fluoride as an activator (96 % B 4 C + 4 % NaF) at 950 °C and 1050 °C for 2 hours. The microstructure was investigated using optical and scanning electron microscopy (SEM). The microhardness was determined using a PMT-3M microhardness tester at a load of 100 gf. EDS analysis was carried out on the JSM-6510LV JEOL SEM with an INCA Energy 350 microanalysis system at an Ta b l e 1 Chemical composition of 3Kh2V8F/3Х2В8Ф steel, wt. % C Si Mn P S Cr Ni Cu W V 0.3–0.4 0.15–0.4 0.15–0.4 > 0.03 > 0.03 2.2–2.7 > 0.35 > 0.03 8.5–10.0 0.3–0.6