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Surface hardening treatment on titanium alloy (1)

May 29, 2018

The surface hardening of titanium is to improve abrasion resistance, and eliminate the risk of co-adhesion between components when abrasion exists. When hardness is improved, corrosion resistance and fatigue strength are also might be improved. Here enhancement of surface hardness is firstly discussed and then the technique itself and its influence on enhancement of surface hardness are discussed. Now a kind of oxidation-based double treatment is found, which could produce optimal hardness on Ti-6Al-4V. This kind of boost / diffusion oxidation (BDO) treatment could produce a thick subsurface to enhance wear-resistant thermal oxidation (TO) layer on the thin surface. This technique is similar to vacuum carburization treatment on steel and it is developed by the company. The required element, which is oxygen here, is formed into a highly concentrated surface layer with great potential energy, and then is diffused to the substrate in vacuum. The treatment following such requirements will exactly produce C-shaped hardened layer, which could not be produced through common diffusion treatment. Thermal oxidation treatment forms a thin layer of rutile (relative to octahedrite) titanium dioxide (TiO2) and a narrow area of oxygen diffusion. The surface layer has good abrasion resistance, but it needs support of thicker BDO lower lining to bear greater stress load.
At high temperature, titanium will react with oxygen and nitrogen to lead to hardening. With nitrogen treatment at high temperature (800-900℃), the surface’s Vickers hardness will be more than 700; by adding appropriate nitrogen or oxygen into argon through overlay welding, its surface hardness could be improved by 2 to 3 times; by producing a layer of approximately 5-micrometer-thick titanium nitride through ion electroplating, the surface’s Vickers hardness could be up to 16000-20000; chromium plating and so on. This kind of treatment should be controlled well in the stove with pressure protective atmosphere. It could conveniently change the gas composition when treatment is completed, to produce even and pore-free rutile layer. Its result is similar to TO treatment. In this way, it completes double treatment with one step and does not need three steps required by BDO/TO treatment, thus significantly saving energy. This technique only uses inert gases – argon and oxygen, so it is environment-friendly, has no toxic gases, and won’t cause global warming effect. Although this technique is good, the cost for vacuum treatment is expensive, and two-step oxidation/diffusion treatment has obvious control problem. Even if diffusion time in the vacuum is fixed, subtle changes of oxide content which forms in the first step could also make significant difference in final distribution of hardness. This technique completely depends on experienced control. On the other hand, during the process of gas diffusion, zirconium oxide electrode could be used to directly control the oxygen potential, thus relatively easily forming required stable shape of distribution curve. Therefore, an alternative method is found. It is more similar to gas carburization treatment. The process of treatment is controlled by forming the added element – oxygen – into controlled potential energy in the form of gas.

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