I. Reaction with Gases
1. Oxygen
- Titanium is highly reactive with oxygen in high-temperature environments. When the temperature rises close to the melting point of titanium (about 1,668 °C), titanium reacts rapidly with oxygen in the surrounding environment. This reaction forms an oxide film on the surface of titanium, and the oxide film continues to thicken as the temperature rises and the time increases. This not only consumes the titanium material itself, but also changes the chemical composition of titanium, affecting its purity and performance. For example, in some open high-temperature furnaces, if effective protection measures are not taken, the oxidation of the titanium surface will lead to a decline in the quality of the final product and fail to meet high-end applications (such as the requirements for titanium alloy parts in the aerospace field).
2. Nitrogen
- Titanium is also prone to react with nitrogen at high temperatures. Nitrogen molecules decompose and combine with titanium atoms at high temperatures to form titanium nitrides. The formation of these nitrides changes the crystal structure of titanium, making it more brittle. This embrittlement phenomenon is very unfavorable during high-temperature processing, such as casting or forging titanium products. It may cause titanium products to crack or even break during subsequent processing or use, reducing the reliability and service life of titanium materials.
3. Hydrogen
- Hydrogen in high temperature environments will also affect titanium. Hydrogen is easily absorbed by titanium and forms hydrides inside titanium. The presence of hydrides can cause hydrogen embrittlement of titanium, significantly reducing the toughness and ductility of titanium. In some high-temperature and high-pressure chemical reaction vessels, if titanium materials are used, the infiltration of hydrogen may cause serious safety hazards, because the titanium material after hydrogen embrittlement may not be able to withstand the pressure inside the container and break.
2. The influence of impurities
1. Impurities in raw materials
- The melting of titanium usually starts with the extraction of titanium-containing ores (such as ilmenite or rutile). These ores often contain other impurity elements, such as iron, manganese, silicon, etc. During the high-temperature melting process, these impurity elements may form low-melting point eutectics with titanium. The presence of these eutectics will change the melting characteristics of titanium, reduce the actual melting temperature range of titanium, and may cause local overheating or overcooling during the melting process, affecting the uniformity of the solidification structure of titanium, and thus affecting the mechanical properties of titanium products.
2. Introduction of impurities during processing
- During the melting process of titanium, impurities may be introduced by the furnace materials and tools used. For example, if the refractory material of the furnace is of poor quality, some components may dissolve into the titanium liquid at high temperatures. These foreign impurities will interfere with the normal melting and solidification process of titanium, and may cause inclusions in titanium, reducing the quality of titanium. In some applications that require extremely high purity of titanium materials, such as medical implants, the presence of these impurities is not allowed because they may trigger an immune response or other adverse health effects in the human body.
III. Difficulty of temperature control
1. Local overheating
- When melting titanium in a high temperature environment, local overheating is prone to occur due to the relatively poor thermal conductivity of titanium. For example, in the process of melting titanium using an electron beam, the energy of the electron beam is highly concentrated. If the scanning path or power is not properly controlled, the temperature of the local area of titanium will be too high. Local overheating will cause the microstructure of titanium to be uneven and produce coarse grains, thereby reducing the strength and toughness of titanium. In industries such as aerospace that have strict requirements on the performance of titanium materials, this microstructural inhomogeneity may cause early failure of parts during use.
2. Temperature gradient
- Maintaining a suitable temperature gradient during the melting process of titanium is also a challenge. If the temperature gradient is too large, it will cause greater thermal stress during the solidification process of titanium. This thermal stress may cause defects such as deformation and cracks in titanium products. When manufacturing large titanium structural parts, such as titanium alloy aircraft engine casings, due to the complex structure and large size, it is very difficult to control the temperature gradient throughout the melting and solidification process, requiring precise heating and cooling systems and advanced process control technology.
