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Short answer: Galling (cold welding) happens when the protective oxide film on austenitic stainless threads, such as 304 or 316 (A2 or A4), is wiped away under load. The exposed metal surfaces then adhere and tear. To prevent it, use a suitable thread lubricant or anti-seize, pair dissimilar materials or hardnesses, consider coated or plated nuts or inserts, keep threads clean and undamaged, and tighten at a slow, steady speed. In cleanrooms or vacuum, choose a lubricant or coating that the environment allows.
Why stainless galls
- Austenitic stainless steels are ductile and work-harden. Their corrosion resistance comes from a thin chromium oxide film.
- Under thread contact pressure and sliding, that film breaks. The fresh metal surfaces bond locally, then tear, and the torn material raises friction further. The result can be a nut that seizes partway on and a bolt that snaps when you try to remove it.
- Fast tightening (power tools), fine pitch, damaged or dirty threads, and same-material pairs all make it worse.
Prevention steps
1. Lubricate the threads with a product suited to stainless. Common choices are anti-seize compounds (for example nickel or molybdenum-disulfide based) or dry-film coatings. Check what the application allows:
- Vacuum and UHP or cleanroom: low-outgassing, approved lubricants only, or none, using plated or coated hardware instead.
- Food or pharma: food-grade products.
- Oxygen service: oxygen-compatible products only.
2. Use dissimilar mating materials where corrosion allows. For example, use a different stainless grade or a hardened nut. Different hardness reduces adhesion.
3. Use plated or coated parts. Silver-plated nuts are common on stainless gas fittings for exactly this reason. Other options are nitrided, PTFE-coated or proprietary anti-galling coatings.
4. Consider coarse threads. They are generally more tolerant than fine threads, and a looser thread class gives more clearance.
5. Control tightening. Tighten slowly and steadily. Avoid impact drivers on stainless where possible. Don't back off and re-run repeatedly.
6. Keep threads clean. Remove chips, and protect threads during handling and cleaning.
7. Adjust the torque spec for the lubricant. Lubrication changes the friction coefficient a lot, and with it the preload you get at a given torque. Use torque values for the lubricated condition, from the fastener or lubricant supplier, or establish them by test.
Worked example
A frame for a cleanroom tool uses M8 A4 (316) screws into A4 nuts. The assemblers report that some nuts seize during assembly with cordless drivers.
Investigation:
- The fasteners are clean, dry and the same material, and they are run in fast with a driver. All the galling conditions are present.
Changes made:
- Switch the assembly method to a hand wrench or a low-speed driver for final tightening.
- Specify anti-galling coated nuts instead of lubricant, because the cleanroom spec restricts loose lubricants near the process area.
- Recalculate the assembly torque for the coated condition from the supplier's data and update the drawing note.
- Add a drawing note: "Do not reuse galled fasteners. Inspect threads before reassembly."
The seizures stop. The torque change matters: the original dry-thread torque, applied to a lower-friction coated nut, could have over-stressed the screws.
Common mistakes
- Using dry-condition torque with lubricated threads. This risks over-tensioning and thread stripping.
- Applying anti-seize in UHP, vacuum or oxygen areas without checking compatibility.
- Reusing galled fasteners. The damaged threads gall again.
- Fine pitch on stainless without a functional reason.
- Assuming "stainless" means "won't seize." Austenitic grades are among the most prone.
- Ignoring tapped holes. A stainless screw galling in a stainless tapped hole is worse than a nut, because the part itself is damaged. Consider thread inserts.
Design checklist
- State the fastener material and property class (for example ISO 3506 A4-70) on the drawing or BOM.
- State the thread lubricant, coating or "assemble dry" explicitly, not "as required."
- Give the tightening torque for that exact condition.
- Consider thread inserts for stainless or aluminum tapped holes that will be serviced repeatedly.
- Add a note banning impact tools on stainless fasteners where that is your practice.
- For cleanroom, vacuum or UHP areas, reference the site's approved materials list for lubricants and coatings.
- Keep spare fasteners in the service kit, because galled ones must be discarded.
Signs galling is starting
Watch for these during assembly: torque rising sharply before the nut is seated, a grinding or squeaking feel, or the nut stopping partway along the thread. Stop at once. Forcing it on usually ends with a seized joint or a snapped bolt. Back it off slowly if it will move, discard both parts, and look for the cause (dirty threads, missing lubricant or tightening too fast) before you try again.
FAQ
Is 316 less prone to galling than 304?
Both austenitic grades gall readily. Changing grade between these two is not a reliable fix by itself. Use lubrication, coating, or different hardness or material.
Do thread inserts help?
Yes. Coated or dissimilar-material inserts in tapped stainless or aluminum holes reduce galling and protect the parent part.
Can I free a galled nut?
Sometimes, by working it slowly with penetrating lubricant. Often the fastener has to be cut off. Replace both parts.
What about titanium?
Titanium also galls readily and needs similar precautions.