Machined parts for vacuum and cryogenic systems are made from low-outgassing metals, cleaned of all machining residue and dimensioned for the temperature they run at. In ultra-high vacuum, 10⁻⁹ Pa up to below 10⁻⁶ Pa under ISO 3529-1:2019, gas released from surfaces limits the pressure; between 293 K and 4 K, 304 stainless steel shrinks by 0.300 % and 6061-T6 aluminium by 0.415 %.
What pressure ranges does vacuum technology define?
ISO 3529-1:2019 sets the ranges below. Compared with the 1981 edition, it lowered the limit of high vacuum by a factor of 10 to 10⁻⁶ Pa and defined extreme-high vacuum for the first time. The range decides how much the material, the seal and the surface preparation matter: they hardly matter in rough vacuum and decide everything in UHV. Typical users are vacuum and semiconductor equipment builders and research groups with cryostats and beamlines.
Table 1. Vacuum ranges in ISO 3529-1:2019 and what each needs
| Pressure | Range | Hardware and preparation |
|---|---|---|
| 100 Pa to atmospheric | Low (rough) | Simple materials, positive displacement pumps |
| 0.1 Pa up to below 100 Pa | Medium (fine) | Simple materials, positive displacement pumps |
| 10⁻⁶ Pa up to below 0.1 Pa | High | Stainless steel, elastomer seals acceptable, no or only a short, mild bake-out |
| 10⁻⁹ Pa up to below 10⁻⁶ Pa | Ultra-high (UHV) | Metal seals, bake-out at 150 °C or more, low-carbon stainless steel, special surface preparation and cleaning |
| below 10⁻⁹ Pa | Extreme-high | Vacuum-fired low-carbon steel, extensive surface preparation and cleaning, getter pumping |
Source: ISO 3529-1:2019 as tabulated and described in Jousten, IMEKO TC-16, 2023, Table 2 and section II.
Why does outgassing decide the material and the process?
In a leak-free system the pressure is set by the gas released from the walls divided by the effective pumping speed. A machined metal surface carries an outer layer of oil, grease and dirt from machining and handling, an adsorbed layer of water vapour and hydrocarbons, an oxide of 1 to 10 nm, and below it a damaged skin of 10 to 100 µm in which gas is trapped (Chiggiato, Fig. 1). After cleaning, water vapour dominates. For unbaked stainless steel, copper, aluminium and titanium the water outgassing rate follows about 3×10⁻⁹ / t mbar·l/(s·cm²), with t in hours. Hydrogen dissolved in the bulk takes over once the water is gone.
Table 2. Specific outgassing rates measured at room temperature, mbar·l/(s·cm²)
| Case | Gas | Pumping time | Rate |
|---|---|---|---|
| Neoprene | H₂O | 10 h | ≈10⁻⁵ |
| Unbaked austenitic stainless steel | H₂O | 10 h | 3×10⁻¹⁰ |
| Unbaked austenitic stainless steel | H₂O | 100 h | 3×10⁻¹¹ |
| Austenitic stainless steel, baked 150 °C, 24 h | H₂ | not applicable | 3×10⁻¹² |
| Aluminium alloys, baked 100 to 200 °C, about 20 h | H₂ | not applicable | ≈10⁻¹³ |
| OFS copper, baked 200 °C, 24 h | H₂ | not applicable | 3×10⁻¹⁴ |
Source: Chiggiato, CERN Accelerator School 2017, Table 3.
Chiggiato's example: a 1 cm² piece of neoprene in a 1 m² stainless steel vessel, 0.01 % of the area, raises the pressure after 10 h by one order of magnitude. Bake-out is effective for metals when it lasts at least 12 h above 120 °C. Standard electropolishing does not change water outgassing significantly, while dedicated electropolishing treatments reduce it by at least a factor of 10.
Material choice follows. Cd, Zn, Mg, Li, Ba, Pb and Ca have vapour pressures incompatible with high vacuum, so alloys containing them, such as zinc-bearing brass and leaded grades, are used only after detailed investigation. Taborelli lists stainless steel, OFE copper (UNS C10100), OFS copper (C10700) and aluminium of the 6000 and 5000 series as the usual vacuum chamber substrates; Jousten names low-carbon stainless steel for UHV. Grade data for the steels is in stainless steel for machined parts.
How clean must a machined vacuum part be?
Taborelli names the residues machining leaves behind: lubricants, fingerprints, marker traces and chips. They cause strong hydrocarbon outgassing, so every part for UHV (typically below 10⁻⁵ Pa) goes through precision cleaning, in most cases a water-based detergent bath or an organic solvent, and afterwards is handled, packaged and stored with care. A finish is the last treatment in the sequence; no handling step follows it.
The machining process has to make that cleaning possible. Excess cutting fluid is removed early, before precision cleaning, and solid lubricants (graphite, MoS₂, BN) are avoided because their dust pollutes the cleaning bath. Grinding, brushing and blasting are restricted, and where unavoidable the abrasive must not embed in the surface.
For organic materials such as elastomers, adhesives and lubricants, the screening test is ASTM E595. NASA's database applies limits of 1.0 % total mass loss (TML) and 0.10 % collected volatile condensable material (CVCM) after 24 h at 125 °C, with the collector plate at 25 °C. CERN uses the test only to exclude materials, never to accept them: a null CVCM does not prove that condensable contamination is negligible in UHV.
Cryogenic vessels in oxygen or other oxidising service add ISO 23208:2017 (EN ISO 23208:2019, amended 2020). It sets minimum surface and particle cleanliness for surfaces that touch the cryogenic fluid, to reduce malfunction and ignition risk.
What changes in cryogenic service?
Two things: the metal must stay tough, and its dimensions change. Face-centred cubic metals, among them aluminium alloys and austenitic stainless steel, stay ductile at low temperature. Body-centred cubic ferritic and carbon steels show a ductile-brittle transition (Duthil). Fracture toughness stays high for stainless steels 316 and 310 and drops for nickel steels and Ti-6Al-4V (Duthil, Fig. 18).
The dimensional change is large against drawing tolerances. NIST's fit equations give the contraction relative to the length at 293 K. Below 23 K (304) and 18 K (6061-T6) the fits are constant, so the 4 K values are 0.300 % for 304 stainless steel and 0.415 % for 6061-T6 aluminium. NIST states that the 304 fit deviates from its data by 5 %. In Duthil's plot most of the contraction has occurred by 50 to 77 K, and OFHC copper lies close to stainless steel.
Take an aluminium housing with a 200 mm stainless steel insert. At 4 K the aluminium bore shrinks by about 0.83 mm and the insert by about 0.60 mm, so the bore closes on the insert by about 0.23 mm. A general tolerance of ISO 2768-m allows ±0.5 mm in that size range (ISO 2768 tables), so a fit that must work cold needs an individual tolerance calculated from the contraction, not a general one. Cooling also helps vacuum: at 125 K the saturated vapour pressure of water is 10⁻¹¹ mbar, so cold surfaces hold water, though venting to air undoes the benefit.
What do vacuum and cryogenic requirements change in machining?
Table 3. Requirement, reason and effect on the machining process
| Requirement | Reason | Effect on the process |
|---|---|---|
| Low-outgassing metal | Cd, Zn, Mg, Li, Ba, Pb and Ca have vapour pressures incompatible with high vacuum | Stock limited to low-carbon stainless steel, OFE copper, 5000 and 6000 aluminium |
| Residue-free machining | Oil, grease and dirt from machining form the outer contamination layer | Minimal cutting fluid, removed early; no graphite, MoS₂ or BN; no stickers or marker on functional faces |
| Cleanable geometry | Traps and pockets cannot be rinsed | Few small blind tapped holes; no non-penetrated welds or voids |
| No embedded abrasive | Inclusions and porosity retain contamination | Grinding, brushing and blasting restricted |
| Burr-free edges | Overhanging burrs on turned stainless flanges are a suspected cause of poor coating adhesion | Deburr along the tool path |
| Dimensions at operating temperature | 0.300 % (304) and 0.415 % (6061-T6) contraction from 293 K to 4 K | Individual tolerances on cold-critical dimensions |
| Standard flange faces | Parts from different suppliers must mate | ISO 1609:2020, ISO 2861:2020 for clamped couplings, ISO 3669:2026 for knife-edge flanges |
| Leak-tight joints | A leak path must be found before assembly | Tracer-gas test to ISO 20485:2017 with the acceptance rate on the drawing |
Source: Chiggiato 2017; Taborelli 2024; Duthil; NIST; ISO 1609, 2861, 3669, 20485.
A blind tapped hole also traps gas that escapes slowly and imitates a leak. Where one cannot be avoided, agree a vent path with the designer.
What belongs on the drawing and in the request for quotation?
A supplier can only meet what the drawing states. Put these on it:
- The vacuum range, bake-out temperature and duration, and the lowest operating temperature.
- The material designation, product form and heat treatment.
- The cleaning method, the cleanliness requirement and how it is verified, plus packaging and storage.
- The dimensions that must hold at operating temperature, with that temperature.
- The flange standard with its edition.
- The leak test method, tracer gas and acceptance rate.
The technical drawing checklist covers the other title block entries. Our network machines by milling and turning, works to ISO 2768-m as standard and tighter where the drawing specifies it. Cleaning, packaging and leak-test requirements are confirmed per drawing before quotation.
Frequently asked questions
- What is ultra-high vacuum?
- ISO 3529-1:2019 defines ultra-high vacuum as the range from 10⁻⁹ Pa up to below 10⁻⁶ Pa. Below 10⁻⁹ Pa the standard speaks of extreme-high vacuum, a range ISO defined for the first time in 2019.
- Which metals are used for UHV parts?
- Low-carbon stainless steel, oxygen-free copper (UNS C10100 and C10700 in accelerator practice) and aluminium alloys of the 5000 and 6000 series. Alloys containing volatile elements such as zinc, cadmium or lead are used only after detailed investigation.
- How much does a part shrink when cooled to 4 K?
- Measured against its length at 293 K, 304 stainless steel contracts by 0.300 % and 6061-T6 aluminium by 0.415 % at 4 K (NIST fit equations). A 200 mm length shrinks by 0.60 mm in stainless steel and 0.83 mm in aluminium.
- What do TML and CVCM limits of 1.0 % and 0.10 % mean?
- They are the NASA screening limits for total mass loss and collected volatile condensable material in ASTM E595, measured after 24 hours at 125 °C in vacuum. CERN uses the test only to exclude organic materials, never to accept them for UHV.
- Is ISO 3669 still the 2020 edition?
- No. ISO 3669:2020 was withdrawn on 24 June 2026 and replaced by ISO 3669:2026, Dimensions of knife-edge flanges. Drawings should cite the edition they were made to.
Sources
- Jousten, ISO standards for vacuum metrology, IMEKO TC-16 conference 2023 (ISO 3529-1:2019 ranges, Table 2)
- BS ISO 3529-1:2019, Vacuum technology, Vocabulary, Part 1 (BSI, status current, supersedes ISO 3529-1:1981)
- Chiggiato, Outgassing properties of vacuum materials for particle accelerators, CERN Accelerator School 2017
- Taborelli, Surface Finishing and Coatings for Accelerator Vacuum Applications, CERN Accelerator School 2024
- Duthil, Material Properties at Low Temperature, CERN Accelerator School
- NIST Cryogenic Materials Data: 304 Stainless Steel, linear expansion fit
- NIST Cryogenic Materials Data: 6061-T6 Aluminum, linear expansion fit
- NASA Goddard, Outgassing Database user guide (ASTM E595 screening limits)
- EVS-EN ISO 23208:2019, Cryogenic vessels, Cleanliness for cryogenic service (ISO 23208:2017)
- ISO 1609:2020, Dimensions of non-knife edge flanges (EVS)
- ISO 3669:2026, Dimensions of knife-edge flanges, valid from 2026-06-24 (EVS)
- ISO 2861:2020, Dimensions of clamped-type quick-release couplings (EVS)
- EVS-EN ISO 20485:2018, Leak testing, Tracer gas method (ISO 20485:2017)
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