A stainless bolt that spins freely by hand can suddenly lock solid halfway down the thread and shear off — without ever reaching its torque target. This is galling, and it is one of the most frustrating failures on site because the bolt was never overloaded. It is a surface-interaction problem, not a strength problem.
Galling is cold welding of stainless steel threads under torque, causing seizure without overload — the bolt appears to tighten normally then suddenly locks and the thread is destroyed on removal. It occurs because stainless steel has low surface hardness and high friction coefficient, especially at higher property classes. Prevention: use anti-seize compound (copper-based or Molykote) on all stainless-to-stainless thread engagement; reduce tightening speed; use a calibrated torque wrench not an impact gun. Once galled, the fastener must be cut off and the mating thread repaired.
- Best for
- Engineers and technicians installing stainless steel fasteners (A2/A4 bolts in SS clamps, SS eye bolts in SS brackets) in offshore wind nacelles or J-tubes where galling risk is highest
- Not suitable for
- Using anti-seize compound on carbon steel fasteners with HDG coating — it is not necessary and can reduce friction below the assumed value, causing over-preload at the specified torque
- Prevention steps
- 1 — Apply copper anti-seize to all SS-to-SS threads → 2 — Use calibrated torque wrench at slow speed → 3 — Stop if resistance increases suddenly → 4 — Never use impact gun on stainless structural fasteners → 5 — Allow cooling between attempts on seized bolts
- RFQ information
- Fastener grade and material, thread diameter and pitch, anti-seize compound required, torque target with friction factor adjustment
§ 01 What galling actually is
Galling (also called cold welding or seizing) happens when two sliding metal surfaces under pressure adhere to each other at their high spots, tear, and transfer material. In a threaded joint, the bolt and nut threads micro-weld together as they turn, the welds tear and build up, friction climbs steeply, and the assembly locks — often so hard that the bolt twists off before it can be backed out.
§ 02 Why stainless is especially prone
Stainless steel galls far more readily than carbon or coated steel for two linked reasons:
- The passive film is thin and self-repairing — the same property that gives corrosion resistance means that when sliding pressure rubs it off, fresh reactive metal is exposed that readily cold-welds to the mating surface.
- Austenitic stainless work-hardens and has poor inherent lubricity — A2 (304) and A4 (316) are tough and "sticky" in sliding contact, with no protective oxide or coating to keep the surfaces apart.
This is unique to the bare metal-on-metal contact: a zinc-flake coated carbon-steel bolt has a built-in low-friction layer, whereas a bare A4 bolt and A4 nut have nothing between them.
§ 03 Conditions that trigger it
| Trigger | Why it galls |
|---|---|
| No lubricant | Bare A4-on-A4 threads cold-weld immediately |
| High tightening speed | Frictional heat builds faster than it dissipates |
| Same grade nut & bolt | Matched hardness welds more readily |
| Dirt / grit in threads | Debris scores the surface and starts adhesion |
| Thread damage | Burrs concentrate pressure and tearing |
§ 04 How to prevent it
- Use an anti-seize lubricant on the threads (nickel- or PTFE-based) — the single most effective measure. Note the lubricant changes the friction factor, so the torque target must be adjusted accordingly (see preload & torque).
- Tighten slowly — keep speed low to limit frictional heat; avoid high-speed impact drivers on bare stainless.
- Mix hardness — pair the nut and bolt from different grades/hardness (e.g. a harder nut) so the surfaces do not weld as easily.
- Keep threads clean and undamaged; chase out grit before assembly.
- Consider grade — some duplex grades and specially treated stainless resist galling better than plain A4; see duplex / super duplex.
§ 05 If a bolt seizes mid-installation
Once galling starts, forcing it usually makes it worse — the welds tear further and the bolt shears. Stop turning, let it cool, and back it out slowly with steady pressure if it will move. A seized fastener that will not release generally has to be cut out; do not reuse it. Then fix the root cause (lubricant, speed, grade pairing) before installing the replacement.
Galling is purely a surface/installation issue and should not be confused with corrosion staining, which has different causes — see rust on stainless fasteners.
Evidence and decision boundary
- Direct evidence
- ASTM G98 ranks material couples under a defined laboratory galling test and explicitly warns against using the result as a final quantitative design value. ISO 3506-1 defines stainless-fastener properties; ISO 16047 defines torque/clamp-force test conditions; ISO 4042 addresses coating systems and lubrication context.
- Engineering inference
- For a clamp assembly, control grade pairing, thread finish, lubricant, tightening speed and installation procedure together. A material grade or passivation statement alone does not establish a safe torque or galling-free assembly.
- Typical or indicative value
- Lubricant factors and torque reductions are product- and joint-specific. Confirm them by supplier data or representative torque/clamp-force testing; do not apply a universal percentage.
Primary sources checked
- ASTM G98-23 - Galling resistance of materials
- ISO 3506-1:2020 - Stainless-steel fastener properties
- ISO 16047:2005 - Torque/clamp-force testing
- ISO 4042:2022 - Fastener coating systems and lubrication
Related commercial route: Compare wind-turbine clamp systems and RFQ inputs.
Related evidence: 304 vs 316 stainless fasteners and rust on stainless fasteners.