Metallurgy of tungsten and its processing technology
Tungsten is a metallic element with chemical symbol W, atomic number 74, melting point 3410℃, boiling point 5927℃, density 19.35g/cm, belonging to the VIB group of the sixth cycle (second longest cycle) in the periodic table of elements. After smelting, tungsten has a silver-white luster and is characterized by good high-temperature strength, high elastic modulus, small expansion coefficient, low vapor pressure, excellent electrical conductivity, extremely high melting point, high hardness, and chemical stability, and is not subject to air erosion at room temperature, and is mainly used for manufacturing filaments and high-speed cutting alloy steel, super-hard molds, and also for optical and chemical instruments.
China is the world's largest tungsten storage country. Since the reform and opening up, China has made great progress in the development and utilization of tungsten with high speed, but compared with developed countries, there is still a considerable gap in the processing level and production equipment of tungsten in China.
At present, tungsten has made great progress in the following four applications. First, tungsten is an important alloy element in high-speed steel, hot work steel, cold work steel and impact-resistant steel, and plays a major role in heat-resistant steel (super alloy) and Stellites with good surface wear resistance; second, tungsten-based alloys, tungsten in the electronics industry (such as emitters, contacts, certain heating elements), tungsten-based high-density alloys in counterweight and radiation resistance Secondly, among the tungsten-based alloys, tungsten has an irreplaceable role in the electronics industry (e.g., emitters, contacts, certain heating elements) and tungsten-based high-density alloys in weight distribution and radiation resistance; again, tungsten carbide used in cemented carbide has shown very excellent wear and heat resistance in various tools and dies; finally, in the chemical industry, certain compounds of tungsten can be used as catalysts, lubricants and added to coatings.
The main methods of preparing dense tungsten are powder metallurgy methods and smelting methods. Because of its high melting point, early methods of preparing tungsten were limited to powder metallurgy, and Coolidge laid the foundation for modern powder metallurgy of tungsten in 1909 by reducing tungsten acid with hydrogen at 800-900°C, followed by pressing, pre-sintering at 1000°C, and draping the tungsten powder. Although various improvements have been made to the powder metallurgy method over the century, this method has remained the main method for preparing dense tungsten metal to date. With the development of melting technology, high quality tungsten blanks of relatively high purity can be prepared using electron beam melting technology, especially electron beam local melting technology has opened up a new world for the preparation of single crystals of tungsten. However, the preparation of tungsten by melting method has some disadvantages that cannot be overcome by itself (e.g., the grain size is relatively large), which brings great difficulties to the further plastic processing of tungsten.
1.1 Powder Metallurgy
The advantages of the powder metallurgy method are: uniform organization of the billet, small grain size, which is conducive to further processing; small metal loss, high yield; small investment, fast results, production has great flexibility, can directly produce finished products and meet certain size requirements of the billet.
1.2 Melting method
With the development of electron beam melting technology and regional purification technology, the original tungsten, which could only be produced by powder metallurgy methods, has become possible using smelting technology. However, since the melt alloy has a relatively low yield and the coarse grain size of the billet makes further processing more difficult, the powder metallurgy method is still the most dominant method for producing tungsten and its alloys.
On the other hand, electron beam area melting can purify tungsten metal, making its impurity content much lower, and also providing the possibility of single crystal preparation. As early as 1970, the improvement of the equipment has enabled the production of tungsten single products of Φ10 mm × 100 mm, meeting the needs of the development of the aerospace industry.
Powder metallurgical tungsten usually has fine grains, and its opening billets are generally selected by high-temperature forging and rolling methods, with temperatures generally controlled between 1500 and 1600°C. After billet opening, the tungsten can be further rolled, forged or spin-forged. The pressure processing is usually performed below the recrystallization temperature because the grain boundaries of the recrystallized tungsten material are brittle, which limits the processing performance, so the deformation temperature decreases accordingly as the total amount of tungsten processed increases.
2.1 Tungsten rods and wires
After opening the billet tungsten rod according to its deformation, the temperature of further forging is generally chosen in the range of 1100~1400℃. Forging hammers are generally selected from air hammers, rotary forging machines or other forms of forging accordingly, and tungsten rods with diameters of 3~60mm can be obtained by forging. Recrystallization is generally not allowed in forging, and more than 80% of the finished bars need to be processed by forging to ensure uniformity of bar deformation and stress relief treatment at 1150°C.
Tungsten wires with diameters less than 3mm need to be deformed by single or multiple passes of stretching on a drawing die, and the pass-through heating and stress relief method is generally chosen, with graphite milk as the lubricant.
2.2 Tungsten plate and strip
Besides tungsten wire, tungsten plates and strips are the most commonly used tungsten materials. Because the deformation resistance of tungsten is very large, the deformation temperature is generally in the range of 1000~1400℃, so the rolling mill generally requires a large enough rolling force, and the rolls not only require high temperature resistance, but also must meet a certain high temperature strength. Therefore, it is very difficult to roll a relatively wide tungsten plate.
After the opening of the tungsten plate processing again, with the increase in processing volume, the need to continuously reduce the processing temperature, when the total amount of processing is greater than 90% later, you can choose 200 ~ 600 ℃ for processing, processing of the channel deformation of about 10%. The total deformation is about 80% need to be stress relieved annealing treatment.
There are two production methods for tungsten strip: when the width of tungsten strip is wide, the method of rolling foil and then slitting is usually used; when the width of tungsten strip is small, slitting becomes difficult. Rolling the strip directly from the tungsten wire becomes a viable method. Although the tungsten wire has undergone a large deformation before rolling, it is still a brittle material and cannot be rolled at room temperature, so the strip is still rolled from the wire by heating. After reaching a certain amount of deformation, the tungsten strip needs suitable stress relief and electrolytic polishing.
2.3 Cutting, stamping, welding and machining of tungsten
After pressure processing, the rods, wires, strips and sheets need further processing to become qualified parts for use. These further processing methods include: cutting, stamping, bending, welding, polishing and some mechanical processing. Considering the high plastic-brittle transition temperature and the high hardness of tungsten materials, cutting, bending and stamping of tungsten materials require heating, and the corresponding processing temperature can be reduced as the thickness of tungsten plates decreases.
Tungsten is commonly riveted, bolted and welded. Welding is usually done in a vacuum or protective atmosphere chamber, and the welded part usually has coarse grains and is therefore more brittle than the base metal. Riveted and bolted joints are usually suitable for joining different metals, but once used at high temperatures, tungsten exhibits brittle recrystallization. In addition, tungsten is also suitable for certain brazing methods.
Machining of tungsten materials usually includes turning, milling, grinding and drilling. Because of the brittleness of tungsten at room temperature and its high hardness, carbide tools are generally used for machining with coolant, and it is worth mentioning that the machining performance of high-density tungsten alloys and tungsten alloys doped with rare earths is greatly improved.
3、Surface treatment of tungsten
In most occasions, the surface of tungsten needs to be treated, these treatments include both mechanical such as sandblasting, chemical treatments such as acid washing, alkaline washing and electrolytic polishing, and also include electroplating, vapor deposition and other more complex metallurgical processes.
Conventional sandblasting, pickling, alkaline washing and electrolytic polishing treatments can significantly improve the surface finish of tungsten, and are also essential pre-processes for deposition or plating of tungsten surface. Depending on the use requirements, sandblasting can be done either dry or in water; pickling generally uses aqueous solutions of nitric acid and hydrofluoric acid; alkaline washing is generally done in molten sodium hydroxide solution.
In recent years, the emergence and wide application of new tungsten composites, the gradual promotion of new forming and sintering technologies, and the continuous exploration of new processing technologies have provided new horizons for the development of tungsten materials. We believe that through the efforts of our tungsten counterparts, a better development situation will emerge in the tungsten industry.
