What galling is, and how to stop stainless threads seizing
Galling is a cold-welding failure in which two threads seize and fuse solid part way through tightening, wrecking the fastener and the nut together so that neither can be undone. It is not corrosion and it is not ordinary overtightening. It happens most to stainless steel, which surprises people who bought stainless expecting the easy life.
The pattern is always the same. The nut runs down normally, then gets progressively harder, then stops. More torque snaps the fastener or strips the thread. Backing off does not release it. This page covers the mechanism, the site conditions that cause it, and the handful of measures that reliably prevent it.
- Galling is adhesive wear: clean, oxide-free metal surfaces cold weld under pressure.
- Austenitic stainless (A2 and A4) is the worst offender, with a very thin oxide film and a habit of work hardening.
- Speed and friction heat are the biggest triggers, so impact drivers on stainless nuts cause much of it.
- Anti-seize compound, a hardness difference between nut and bolt, and slow tightening prevent nearly all of it.
- Once a fastener has galled it normally has to be cut off.
What galling actually is
Surfaces that look smooth are rough at a microscopic level, and under load two threads touch only at their high points. As they slide against each other those high points scrape through the thin oxide film and expose clean, bare metal. Bare metal pressed against bare metal and moving will cold weld. The weld then tears, drags more material with it, and the process runs away within a turn or two.
What the fitter feels is a torque reading climbing steeply and unrepeatably, well before the fastener is anywhere near its correct preload. Because the seizure is mechanical rather than chemical, it happens just as readily to brand new fasteners in a clean workshop as to weathered ones on site.
Why stainless does it and zinc-plated steel mostly does not
Several properties of austenitic stainless combine against it:
- A very thin passive film. The chromium oxide layer that protects stainless is a fraction of the thickness of the oxide on aluminium or titanium, so it is easily rubbed through.
- Work hardening. Austenitic grades harden where they are deformed, which raises contact pressure exactly where the rubbing is worst.
- Poor thermal conductivity. Friction heat does not spread away, it concentrates in the thread flank.
- Matched hardness. A stainless nut and a stainless bolt of the same grade are equally hard, so neither surface acts as the sacrificial one.
Zinc-plated carbon steel rarely galls because the plating itself behaves as a soft sacrificial layer between the threads. Strip the coating and carbon steel can gall too. Aluminium and titanium threads are also prone, for much the same reasons as stainless.
What triggers it on site
- Speed. Running a stainless nut down with an impact driver or a cordless gun generates heat far faster than it can dissipate. This is the single most common cause.
- Grit and swarf. Dirt in the thread concentrates pressure and breaks the oxide film early.
- Damaged or burred threads. One dented lead thread starts the tear that takes the rest with it.
- Long engagement. Studding, long-reach nuts and deep tapped holes give far more sliding distance before the joint is tight.
- Larger diameters. M12 and above are noticeably more prone than small sizes.
- Hot conditions. Anything that adds heat before tightening starts reduces the margin.
How to prevent it
- Use an anti-seize compound. Nickel, molybdenum disulphide, graphite or PTFE based, applied to the male thread. This is the single most effective measure. Check compatibility first, since copper-based compounds are usually avoided where copper contamination or later chloride exposure matters.
- Break the hardness match. A nut of a different grade or hardness gives one surface that resists tearing. Guidance is usually a difference of around 50 Brinell points between the two parts.
- Slow down. Run down by hand or on a low-speed driver, then tighten with a spanner or torque wrench. No impact tools on stainless nuts.
- Keep threads clean and undamaged. Blow out tapped holes, chase damaged threads, deburr cut ones.
- Prefer rolled threads, which have a smoother, work-hardened surface and gall less readily than cut threads.
- Use the lubricated torque figure. A lubricated thread reaches a much higher preload for the same applied torque, so a dry torque value applied to a greased fastener will overload it.
What to do when one has already galled
Once the threads have fused there is usually no recovery. Do not apply more torque and do not work the nut back and forth, which simply welds more surface together. Cut the nut off with a nut splitter or an abrasive disc, scrap both parts, and check the tapped hole and the clamped component for damage before refitting.
One caution when applying the hardness-mismatch rule in a corrosive environment: dropping to an A2 nut on an A4 bolt does break the match, and it also creates a corrosion weak point in the joint. Where chlorides are present, use a proprietary anti-galling nut in the correct grade, a coated nut, or a different grade combination that keeps both halves fit for the environment. Grade selection is covered in our fastener selection guide.
Common questions
Does anti-seize change the tightening torque?
Yes, significantly. Most of the torque applied to a fastener is spent overcoming friction, so lubricating the thread means far more of it turns into clamp load. Applying a dry torque figure to a lubricated stainless fastener can overload or snap it. Use the lubricated torque value from the manufacturer or the specification, and be consistent about whether every fastener in the joint is lubricated.
Is A4 less likely to gall than A2?
No. Both are austenitic stainless steels with the same thin passive film and the same work-hardening behaviour, so both gall readily. Paying more for A4 buys chloride resistance, not easier assembly. If anything, A4 is used in the larger, more heavily loaded marine and coastal fixings where galling is most likely, so it needs anti-seize as a matter of routine.
Can a galled bolt be freed?
Rarely. Once the surfaces have cold welded, the metal has genuinely joined and heat or penetrating oil will not release it in the way they release a corroded joint. Working the nut backwards and forwards makes it worse. The practical answer is to split or cut the nut off, discard both parts, and inspect the threads of anything that stays in place.
Does galling only affect stainless?
No. Aluminium and titanium threads gall for similar reasons, and plain carbon steel will gall if the plating is missing or worn through. Stainless dominates the reports because it is often assembled without lubricant on the assumption that it does not need protection, and because zinc plating on ordinary steel happens to act as a lubricant between the threads.
Related
Back to the Fixings, fasteners & threads glossary · All glossary terms
Need this checked against a real specification?
Definitions only get you so far. Send us the actual requirement - a PPE schedule, a fixings list, a PQQ question or a marking you cannot make sense of - and the TES team will come back with what matches it. Trade pricing is on the B2B portal.