Heat treatment of tungsten and its alloy
Tungsten and its alloy have high hardness and high work hardening speed, so heat treatment must be carried out. For example, for 0.3x200mm thin tungsten plate, the stress shall be eliminated immediately after warm processing, otherwise it may crack by itself, and the cracking is more serious if the processing amount between two annealing is too large (such as 80-90%). In addition, annealing can reduce or eliminate the inhomogeneity of microstructure. The properties of tungsten alloy with phase transformation can also be improved by heat treatment.
Stress relief annealing, recrystallization annealing and thermomechanical treatment are mainly used for tungsten and its alloys.
1. Stress relief annealing
Stress relief annealing is often used in the production of tungsten and its alloys in order to eliminate the internal stress caused by cold working, reduce hardness and improve plasticity. The stress relief annealing temperature is generally about 20 ℃ below the initial recrystallization temperature.
The hardness of tungsten at room temperature is high, and the hardness decreases rapidly with the increase of temperature. In order to process at a higher temperature without recrystallization, the method of stress relief annealing can be used to reduce the internal energy of metal, increase the recrystallization temperature, and then increase the pressure processing temperature.
2. Recrystallization annealing
Recrystallization annealing is often used to reduce or eliminate the inhomogeneity of the structure of tungsten and its alloy in the breaking stage. For example, the recrystallization annealing at 1750 ° C and 60 minutes is carried out for the Φ 9.4mm wal5 swaging bar. This treatment is not applicable to the near finished product and the finished product annealing, otherwise the plastic brittle transition temperature will be increased.
3. Thermomechanical treatment
The solubility of W-Hf-C and W-Re-Hf-C increases with the increase of temperature. Therefore, the carbide can be dissolved in the early processing by means of thermomechanical treatment, and then the final processing can be completed at a practical low temperature, so as to induce any dissolved carbide to precipitate into fine particles as much as possible, and introduce fine sub crystal structure as much as possible through cold processing. Annealing or aging at intermediate temperature should be avoided in thermomechanical treatment, because it will increase HFC particles and reduce its high temperature strength.
4. Surface pollution during heat treatment
Tungsten and its alloys are easy to be polluted by oxygen, nitrogen, carbon and so on at high temperature, so they must be heat treated in protective atmosphere or vacuum, and hydrogen protection is generally used. If pollution has formed, it must be removed.
