Influence of the Initial Structure on Nitriding of VT22 Titanium Alloy
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INFLUENCE OF THE INITIAL STRUCTURE ON NITRIDING OF VT22 TITANIUM ALLOY О. H. Luk’yanenko,1, 2 I. М. Pohrelyuk,1 T. М. Кravchyshyn,1 and V. S. Trush1
UDC 669.295:621.785.532
We study the process of nitriding of VT22 titanium alloy with different initial structural state (coarseand fine-grained structures) at temperatures of 750, 800, and 850°С, for holding times of up to 8 h, and under a nitrogen pressure of 10 5 Pa. It is shown that the initial structural state of the alloy affects the kinetics of interaction with nitrogen, the process of growth of the hardened subsurface zone, and the formation and properties of surface nitride films. Keywords: titanium alloy, initial structural state, nitriding, diffusion layer, hardness, surface roughness.
Introduction At present, titanium alloys are widely used both in the critical components of aircraft and space engineering and in the domestic equipment. The worldwide titanium industry proposes a great variety of titanium alloys. Thus, high-strength two-phase titanium alloys of different systems (namely, Ti–6Al–2Sn–2Zr–2Mo–2Cr; Ti–6Al–5Mo–5V–1.5Cu–1Fe; Ti–3Al–8V–6Cr–4Zr–4Mo, et al.) are regarded as promising for applications in the aircraft building [1]. The VT22 deformable (α+β)-titanium alloy based on the Ti–А1–Mo–V system with additions of Fe and Сr is one of these alloys. It is used in the production of various parts of the aircrafts [2]. Titanium alloys produced in different countries have similar compositions and properties [2] but may differ by their initial structural states. However, the choice of alloys with different initial structural states is complicated by the absence of comparative information about their properties obtained under identical conditions of chemical-and-thermal treatment (CTT). Despite definite advantages of titanium alloys as compared with the other structural materials, they have a very important disadvantage, which complicates their active implementation in the every-day practice of machine building, namely, they are characterized by low wear resistance and susceptibility to “seizure” caused by the cold microwelding with conjugated material in the course of friction [3]. Due to the growth of production and extension of the fields of application of titanium alloys, it becomes necessary to develop new methods and technological processes that would enable us to enhance the wear resistance of these alloys [3–6]. Among these methods, we can especially mention the process of nitriding because, in this case, there is no adhesion in the course of friction of nitrided titanium surfaces [7]. Gas-thermal nitriding is a traditional procedure of СTT of metal products and has the following advantages: it can be combined in a single technological cycle with hardening thermal treatment of the products 1 2
Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine. Corresponding author; e-mail: [email protected].
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 55, No. 5, pp. 115–121, September–October, 2019. Or
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