The body-centered cubic crystal structure of tungsten gives it a high Tungsten Ductile Brittle Transition Temperature (DBTT), the temperature at which the material fracture changes. When the temperature of the tungsten material rises to 200°C and above, the material fracture type transforms from brittle to ductile fracture. In contrast, when the temperature drops to 500°C, a ductile fracture gradually transforms into a brittle fracture. DBTT is a crucial factor for analyzing material stress changes in machining operations. The tungsten’s forming temperature range is its DBTT up to its recrystallization temperature. Most tungsten formings, including bending, punching, stamping, spinning, and cutting, must operate below its recrystallization and above the DBTT temperature.
The five major factors determining tungsten DBTT fluctuation are fabricating method, impurities content, tungsten grain size, surface condition, and material deformation rate.
1. Effect of tungsten fabrication on DBTT
Powder metallurgically sintered tungsten exhibits the highest DBTT. Tungsten material produced by chemical vapor deposition has the lowest DBTT. In addition, tungsten’s DBTT goes up when it is under a high annealing temperature, long annealing duration, or high cold work degree.
2. Impurities impact on tungsten DBTT
Carbon and oxygen can significantly increase the tungsten’s DBTT because they are interstitial impurities in tungsten. Furthermore, carbon can embrittle intergranular structures so to increase dislocations of different elements in tungsten. In addition, carbide particles can generate intercrystalline counter force to each other. Compared with carbon, oxygen can improve the DBTT of single-crystal and polycrystalline tungsten when it reaches the same molar concentration as carbon. Oxygen segregation existing among grain boundaries reduces the tungsten’s surficial energy and breaks its intergranular structure. Oxygen has a smaller effect on the tungsten yield stress in single-crystal or polycrystalline tungsten. The increase in the carbon content significantly improves the yield stress of tungsten.
3. Grain size impact on tungsten DBTT
Tungsten with medium-sized grains has the highest DBTT.
4. Surface condition impact on tungsten DBTT
Electropolishing and oxidation of tungsten surfaces improve the ductility of tungsten materials. Surface scratches and cracks caused by machining are removed by electropolishing the surface of tungsten workpieces. Tungsten workpieces are oxidized in air to remove carbonaceous layers or surface scratches. Moreover, jet machining, grinding, chemical etching cleaning, and other treatments of tungsten materials improve the DBTT of tungsten.
5. Deformation rate impact on tungsten DBTT
Increasing tungsten’s deformation rate can reduce tungsten DBTT. Take tungsten sheet material as an example. The DBTT of a thin tungsten sheet is lower than a thicker sheet with the same length and width as the thin sheet. In addition, because the deformation rate applied for bending is higher than the rate for punching, DBTT increased by bending is much more than punching. Therefore, tungsten is too brittle to be deformed at a temperature lower than DBTT.