Molybdenum crucible usually needs to be annealed, which is a key step in the manufacturing process, aiming to eliminate processing stress, optimize organizational properties and increase service life. The following is an explanation of the necessity, process points and impact analysis of annealing:
- Necessity of annealing
Relieve processing stress
Molybdenum crucibles will generate internal stress during the forming process (such as powder metallurgy, forging, and welding). Annealing can release the stress and prevent cracking due to stress concentration during subsequent use.
Optimizing organizational performance
Annealing can adjust the grain structure of molybdenum, make it uniform, improve high-temperature strength and creep resistance, and adapt to high-temperature environments such as crystal growth.
Improve machining accuracy
After stress relief, the dimensional stability of the crucible is enhanced, which can reduce deformation in subsequent machining (such as turning and grinding).
- Annealing process points
Temperature control
Recommended temperature: 1200-1500℃ (the specific temperature needs to be adjusted according to the thickness and material of the crucible).
Heating rate: ≤100℃/h, to avoid cracking caused by thermal shock.
Holding time: 1-4 hours (proportional to the crucible size).
Cooling method
Furnace cooling: cooling rate ≤50℃/h to prevent abnormal grain growth.
Protective atmosphere: filled with argon or vacuum environment throughout the process to avoid oxidation.
Equipment selection
It is recommended to use a vacuum heat treatment furnace or an argon protection furnace to ensure the annealing quality.
- Effect of annealing on the performance of molybdenum crucible
| Performance index | Before annealing | After annealing | Improvement effect |
|–||–|-|
| Hardness | HV220 -260 | HV180 -220 | Reduce by 15%-25% |
| Tensile strength | 400- 500MPa | 350- 450MPa | Reduced by 5%-10% |
| Elongation | 5%-8% | 10%-15% | Increase 50%-100% |
| High temperature strength | Easy to deform after long-term use | Enhanced creep resistance | Improved stability |
- Precautions for annealing process
Avoid overburning
When the temperature exceeds 1600°C, the molybdenum grains will grow rapidly, resulting in increased brittleness, and the temperature needs to be strictly controlled.
Prevent oxidation
A protective atmosphere is required throughout the annealing process, otherwise molybdenum oxide (MoO ₃) will be generated on the molybdenum surface, affecting the purity.
Dimension monitoring
After annealing, the dimensional changes of the crucible need to be checked to ensure that the design tolerances are met (such as diameter deviation ≤ ± 0.1mm).
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