Strengthening the Inner Surfaces of Long Holes in VT22 Titanium by Glow Discharge Nitriding

In terms of its structure, the VT22 titanium alloy belongs to the class of two-phase high-strength (α + β) alloys and is widely used in aircraft manufacturing due to its high specific strength and corrosion resistance. It is used to manufacture fuselage components, wings, landing gear, control systems, aircraft power components, as well as weapon components for the Armed Forces of Ukraine, etc. However, the reliability of these products often depends not only on their strength but also on their tribological properties. Many studies have noted that among the numerous surface hardening methods that improve wearand fretting resistance of titanium alloys, gas nitriding is effective and economically viable; it is technologically simple, ensures stable physicochemical surface characteristics, and allows for the treatment of complex-shaped parts, including those with holes. To intensify the surface hardening process of titanium and its alloys, the method of anhydrous nitriding in a glow discharge (BATR) is promising. Waterless nitriding in a glow discharge (BATR) was performed on the UATR-1 unit, which corresponds to a diode-type model operating on direct current and is additionally equipped with heating elements located in the gas discharge chamber, which made it possible to freely-adjust the power parameters – voltage U and current density ϳ (the ratio of current to the total area of the anode and cathode).This work examines the influence of the technological characteristics of anhydrous nitriding in a glow discharge (temperature, composition of the saturating medium, pressure of the saturating mixture in the gas discharge chamber, and hardening time) on the physicochemical properties of the nitrided layers of the VT22 titanium alloy. The samples for the research were manufactured in the form of bushings, based on the requirements for geometric similarity to the customer’s products (ZSU). Dependencies were identified between the physicochemical properties of the inner surfaces of the sleeves (surface microhardness, its distribution along the depth of the hardened layer, and the axial force of sleeve mandrel) and the technological parameters of the nitriding process.As a result of the studies, it was established that the optimal composition of the saturating mixture is 75 % N2 (nitrogen) + 25 % Ar (argon), and the optimal temperature for the VT22 alloy is 700-750 °C, which allows for a balance between diffusion depth, hardness, and the preservation of bulk mechanical properties. To ensure optimal formation of the nitride phase on the surface and nitrogen penetration into the depth, it is necessary to create an average pressure (60-70 Pa) in the chamber to achieve a balance between the formation of the hardened layer and the depth of nitriding.