Titanium is a highly active metal that can interact with almost all elements. At high temperatures, it can also react with gas compounds CO, CO2, water vapor, NH4 and many volatile organic compounds. During the heating process, the reaction between metallic elements and the titanium surface results in surface contamination and changes in chemical composition. Some gas elements will not only form compounds on the titanium surface, but also enter the metal lattice to form interstitial solid solutions. Under an industrial atmospheric pressure, the oxygen absorption and nitrogen absorption curves of pure titanium will change with various atmospheric environments.
Titanium and its alloys react with oxygen when heated in air or an oxygen-containing atmosphere. When heated below 428°C, a protective oxide film is formed. As the temperature increases, the thickness of the oxide film increases. When the temperature rises above 538°C, the oxide film begins to lose its protective effect. Oxygen diffuses through the film into the interior of the metal, forming obvious outgassing. layer. If it rises above 815°C, a layer of loose oxide scale will form on the surface of the titanium alloy.
The effect of hydrogen and titanium alloy is related to the heating temperature and time. When the temperature is lower than 427°C, if there is an oxide film on the surface of the titanium alloy, it can prevent the inhalation of hydrogen. When the temperature is higher than 427°C, hydrogen begins to penetrate the oxide layer and enter the interior of the alloy structure. The extent of the impact of hydrogen inhalation on the properties of titanium alloys is also directly related to the structural state of the alloy. Since the solubility of hydrogen atoms in the β phase is much greater than that in the α phase, the amount and shape of the β phase of the alloy determines hydrogen contamination. one of the main factors.
In addition, oil stains and stains on the workpiece are causes of carbonization. Sweat droplets can also easily cause the adhesion of chloride during heating, thus causing hot salt stress corrosion in subsequent use. The increase in the content of interstitial elements not only directly affects the mechanical properties of titanium and titanium alloys, but also affects the a+β/β phase transformation point and some phase transformation processes of titanium alloys. Therefore, preventing contamination during the heating process is a very important issue for titanium and titanium alloys.
For β-type titanium alloys with thin wall thickness, high surface brightness requirements and strong susceptibility to hydrogen embrittlement, vacuum forming is the most ideal. Vacuum forming does not necessarily require expensive vacuum heating equipment.
Therefore, in order to reduce various influences in the atmospheric environment, vacuum quenching furnaces and vacuum annealing furnaces are generally used for heating. The inert gas in the vacuum furnace can protect titanium and titanium alloy materials from contamination during the heating process.
