Heat treatment in air oxidises the surface you have just spent time electropolishing. Doing the same operation under vacuum keeps the surface bright and leaves nothing to clean off afterwards, which is why it is the standard route for vacuum components.
Three different jobs people call vacuum heat treatment
The first is conventional metallurgical heat treatment — annealing, stress relief, hardening — performed under vacuum to avoid oxidation and decarburisation. The second is vacuum brazing, where the furnace is the joining tool. The third is bake-out: heating an assembled chamber under vacuum to 150–400 °C to drive off adsorbed water vapour and hydrogen before it is sealed. The temperature and the atmosphere requirements are different in each case and the cycles should not be confused.
Bake-out: the step that makes UHV possible
After electropolishing, the dominant residual gas in a chamber is water. Water desorbs slowly at room temperature but rapidly above 150 °C, and hydrogen starts to leave the steel above roughly 250 °C. A controlled bake at 250–400 °C with continuous pumping removes most of it. A chamber that is only leak-tight but never baked will sit at high vacuum indefinitely.
What we run
Annealing and stress relief for stainless fabrications, vacuum brazing cycles, and chamber bake-out for UHV service. Thermocouples are placed on the workpiece rather than only in the furnace, so the recorded profile reflects the part and not the chamber atmosphere.
Effect on the rest of the process chain
Bake-out goes after electropolishing and before final sealing, because heating a polished surface does not harm the finish but heating a contaminated one bakes the contamination in. Stress relief, where required, goes before final machining so the part does not move after the last cut.
What we control in-house
- 304 / 316L stainless steel and 6061 / 5083 aluminium fabrication
- Clean-environment TIG welding with full penetration and no micro-porosity
- Helium mass-spectrometer leak testing to 1 × 10⁻⁹ mbar·l/s
- In-house electropolishing and passivation — not subcontracted
- Chamber bake-out to 150–400 °C for UHV outgassing control
- Precision-machined ISO-KF, ISO-K and CF (ConFlat) flanges
- Zeiss CMM verification to ±0.02 mm across large work envelopes
- 12 m gantry CNC machining — 12 m × 4 m × 2 m envelope
- SEMI-referenced documentation and inspection protocols
Frequently asked questions
What temperature should a UHV chamber be baked at?
Typically 250 °C for chambers with elastomer seals and 350–400 °C for all-metal-sealed chambers. The limit is set by the seals and by the braze or weld filler, not by the steel.
How long does a bake-out take?
The soak is usually 12–24 hours at temperature, then a controlled cool-down under vacuum. Rushing the cool-down re-adsorbs water and undoes much of the benefit.
Does vacuum heat treatment change the surface finish?
Under vacuum the surface stays bright because there is no oxide to form. It will not restore a polish that was not there to begin with, but it will not degrade an electropolished surface either.
Can you heat treat aluminium under vacuum?
Aluminium is usually heat treated in air or in an inert atmosphere rather than vacuum, because the oxide that forms is protective rather than harmful for most applications and vacuum adds cost without benefit.
Is bake-out needed if the chamber already passes helium leak testing?
Yes. Leak tightness and outgassing are independent. A chamber can be perfectly leak-tight and still fail to reach UHV because water desorbs from its internal surfaces.
Related capabilities
Need this made to spec?
Send the drawing or the specification and we will come back with a DDP price, lead time and inspection plan.