Saturation of titanium alloys from graphite in a nitrogen-oxygen-containing medium
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SATURATION OF TITANIUM ALLOYS FROM GRAPHITE IN A NITROGEN-OXYGEN-CONTAINING MEDIUM I. M. Pohrelyuk, O. I. Yas’kiv, V. M. Fedirko, and I. V. Dyuh
UDC 669.295:621.785
We study the kinetics of carbonitriding of VT1-0, OT4, and VT14 titanium alloys in the course of their saturation from graphite in nitrogen. We propose to use nitrogen with an admixture of oxygen, transporting gaseous carbon to the surface of the metal, as a gaseous medium. This intensifies phase formation and enables one to obtain the carbonitride phases at low saturation temperatures (below 850°C), which increases substantially the microhardness and decreases the roughness of the surface. The additional diffusion of oxygen favors an increase in the depth of the gas-saturated layer, and that of carbon leads to its hardening.
The optimization of the conditions of formation of carbonitride compounds is a necessary component of the technology of thermodiffusion saturation, intended for enhancing the functional properties of the surface of titanium alloys. High temperatures (1100°C and higher), at which the carbonitride phase is formed in the course of saturation from graphite in the atmosphere of deoxygenated molecular nitrogen [1], are unacceptable since they worsen catastrophically the mechanical properties of the titanium matrix (its ultimate strength is reduced by a factor of 3 to 5, and elongation by a factor of 7 to 8). However, at lower temperatures, the carbonitride phases are not formed on the surface of titanium alloys [1]. Hence, in order to decrease the temperature of carbonitride formation, it is necessary to use different approaches or technological methods. In what follows, we propose saturation from graphite in a nitrogen-oxygen-containing medium, where oxygen transports gaseous carbon to the surface of the metal since, according to thermodynamic calculations, the Gibbs thermodynamic potential of reaction for the formation of TiCx Ny Oz titanium carbonitrooxide is lower than that of TiCx Ny carbonitride. In addition, with decrease in the saturation temperature, the probability of formation of TiCx Ny Oz grows (Fig. 1). The aim of the present work is to study the kinetics of carbonitriding of titanium alloys by their saturation from graphite in a nitrogen-oxygen-containing medium and to investigate some characteristics of the formed carbonitride layers. Materials and Experimental Procedure We tested specimens made of commercially pure VT1-0 titanium, OT4 pseudo-α-titanium alloy of the Ti – Al – Mn system, and VT14 ( α + β ) -alloy of the Ti – Al – Mo – V system, 10 × 15 × 1 mm in size. The surface of the specimens was polished, washed in alcohol, and dried. The specimens were saturated from graphite in a flow of commercially pure molecular nitrogen (not more than 0.4 vol. % of oxygen and 70 mg / m3 of water vapor), rarefied to 1 Pa. The saturation temperature was 850°C, and the time of isothermal holding was 5, 10, or 20 h. For comparison, we also used deoxygenated molecular nitrogen: prior to feeding into the reaction space of the furna
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