FEATURES OF FORMATION OF THE STRUCTURE AND PROPERTIES OF STEEL 09Cr3NIMo3VNBr DURING THERMOCHEMICAL TREATMENT

Abstract

Purpose. The article is aimed to investigate the structural state formation of the new 09Cr3NiMo3VNbr steel after cementation and nitration with subsequent heat treatment. To determine the rational mode of thermochemical treatment, which will ensure the creation of a uniform diffusion layer and high mechanical and operational characteristics. Methodology. Thermochemical treatment was carried out on primary and secondary hardening in four versions using the SIEMENS and ACEA furnaces. Nitration was carried out in two stages. To identify the microstructure of pilot production steel the nitric acid etching in a 4% solution of nitrous acid was carried out, the metallographic studies were performed on the cross section of the samples on Axio Observed.D1m microscope. To determine the effective depth of the diffusion layers after cementation and nitration, microhardness was measured on LECO LM248AT instrument, and hot hardness measurements were performed on Wilson Rockwell Hardness Tester instrument. Findings. To obtain a homogeneous structural state of steel 09Cr3NiMo3VNbr and maximize the results of mechanical and performance characteristics, thermochemical treatment was performed according to the following mode: cementation at 930°C with a carbon potential of 0.22, 13 hours, annealing 650°C, 5-7 hours, hardening from 1 030°С with cooling in oil, three-time tempering 530°С, cold treatment – 70°С with a holding time of 1 h. 30 min. and subsequent tempering at 510°C, 1 hour. After cementation, hardening was carried out in the temperature range of 1000–1030°C to prevent oversaturation of the working diffusion layer and formation of a carbide mesh. Subsequent three-time tempering at 530°С and cold treatment of – 70°С for 1 h. 30 min. contribute to the decomposition of residual austenite and the formation of fine-grained tempering martensite. Originality. We developed the optimal mode of thermochemical treatment, which will ensure maximum mechanical and operational properties of steel 09Cr3NiMo3VNbr due to a predetermined ratio of the structural components of the cemented layer. Practical value. The tested mode of thermochemical treatment of multicomponent alloyed 09Cr3NiMo3VNbr steel, produced by Motor Sich JSC and Ivchenko–Progress, which was developed for the gears of aviation gearboxes of gas turbine engines of the new generation, significantly increases heat and power load.