Vacuum Furnaces and Heat Treatment
Heating metal in air oxidises and decarburises its surface. A vacuum furnace removes the air first, so parts come out bright and clean, and processes like vacuum brazing and sintering become possible without fluxes or protective atmospheres.
Processes
Typical vacuum heat treatment processes include:
- Hardening and tempering of tool steels, followed by high-pressure gas quenching with nitrogen or helium.
- Vacuum brazing, for example of aluminium heat exchangers or stainless steel assemblies, without flux.
- Sintering of metal powders and metal injection moulded parts.
- Annealing, stress relieving and degassing of titanium, superalloys and medical implants.
Partial pressure operation
Some alloying elements, such as chromium or manganese, would evaporate in high vacuum at high temperature. Furnaces therefore often run at a controlled partial pressure of argon or nitrogen, typically somewhere between about 0.1 and a few mbar, instead of the lowest possible pressure.
Leak-up rate
Furnace tightness is checked with a leak-up (rate-of-rise) test: the hot or cold chamber is isolated and the pressure rise per hour is recorded. The rate of rise, multiplied by the chamber volume, gives the leak rate, which our leak rate converter calculates directly.
Typical pressures
| Process stage | Pressure |
|---|---|
| Hardening, annealing (typical) | ≈ 10⁻² – 10⁻³ mbar |
| Brazing, titanium, high-purity work | ≈ 10⁻⁴ – 10⁻⁶ mbar |
| Partial pressure (Ar, N₂) | ≈ 0.1 – a few mbar |
| High-pressure gas quenching | Up to about 20 bar overpressure |
Suitable vacuum pumps
References & further reading
- K. Jousten (ed.). Handbook of Vacuum Technology, 2nd ed.. Wiley-VCH (2016). Standard reference on gas flow, conductance, pumps and gauges.
- J. F. O’Hanlon & T. A. Gessert. A User’s Guide to Vacuum Technology, 4th ed.. Wiley (2023). Practical design of vacuum systems, pump-down and outgassing.
- ISO/TC 135. ISO 20484:2017 Non-destructive testing — Leak testing — Vocabulary. International Organization for Standardization (2017). Standard terms and units for leak rates.
Frequently asked questions
Why heat treat in a vacuum furnace?
Without oxygen the surface does not oxidise or decarburise, so parts come out bright and need no cleaning. Vacuum also allows flux-free brazing and removes dissolved gases from the metal.
What pumps does a vacuum furnace use?
A roughing pump (rotary vane or dry screw), usually a Roots booster for speed in the medium vacuum range, and a diffusion pump for high-vacuum furnaces. Turbomolecular pumps are used on smaller or cleaner systems.
What is the leak-up rate of a vacuum furnace?
The pressure rise per hour after isolating the pumps, usually quoted in µm/h (microns per hour). Multiplied by the chamber volume it gives the total gas load from leaks and outgassing.
Other applications
HVAC Evacuation: Pulling a Deep Vacuum
How to evacuate an HVAC/R system: removing air and moisture, why 500 microns, decay tests, hose and pump tips, and the physics of boiling water out.
Vacuum Distillation
Vacuum distillation lowers boiling points to protect sensitive products: choosing the pressure, the delta-20 rule for rotavaps, industrial columns and pumps.
Freeze Drying (Lyophilization) Under Vacuum
How freeze drying works: sublimation below the triple point, chamber and condenser pressures, ice vapour pressure, drying phases and the right vacuum pumps.
Vacuum Pressure Impregnation (VPI)
Vacuum pressure impregnation for windings, porous castings and wood: evacuation, resin flooding, pressurisation, curing, typical pressures and pumps.
Vacuum Sewer Systems
How vacuum sewers move wastewater with air pressure instead of gravity: interface valves, the vacuum station, typical pressures, pumps and where they fit.