My Stainless Bolt Seized Halfway In — Galling
The most common failure in stainless fastening. Causes and practical prevention.
A stainless bolt that stops halfway and will neither tighten nor back out. It is the most frequent stainless-specific failure on site, known in English as galling. Despite the association with heat, it is not thermal: it is cold welding, where mating surfaces fuse under pressure and friction at ambient temperature.
Why it is so severe in stainless
Paradoxically, because of the passive film that makes stainless stainless.
- Threads rub under high contact pressure as the bolt is tightened
- The friction wipes off the thin passive film
- Bare metal now contacts bare metal directly
- Stainless is ductile, so the contacting asperities weld on the spot
- Continued turning tears the welded points and strips the thread
Most carbon steel bolts carry a plated finish, and the plating layer keeps bare metal from meeting bare metal; heat-treated high-strength grades are too hard for ductile welding to take hold easily. A stainless bolt is the opposite on both counts — unplated bare metal, and high ductility.
Conditions that invite galling
| Condition | Why |
|---|---|
| Fast power-tool driving | No time for frictional heat to dissipate, accelerating welding |
| Like against like (stainless bolt + stainless nut) | Equal hardness, so both surfaces tear |
| Prevailing-torque nuts (nylon insert type) | Thread friction is high by design, raising the risk |
| Contaminated threads (debris, chips) | Local pressure concentration |
| Fine pitch threads | Greater contact area over a longer rubbing path |
| Repeated reuse | Cumulative damage to the passive film |
Prevention, in order of effectiveness
- Slow down. The most effective measure and it costs nothing. Avoid high-speed power driving and finish by hand.
- Lubricate. Molybdenum disulphide (MoS₂), nickel-based anti-seize pastes and similar. But lubrication raises clamp load sharply at the same torque, so the tightening torque must be reduced. Always use the torque value for the lubricated condition.
- Use dissimilar combinations. Different steel groups for bolt and nut (for example an A2 bolt with an A4 nut), or a plated part on one side.
- Prefer coarse threads. They resist galling better than fine threads.
- Keep threads clean and do not assemble contaminated parts.
Turning further destroys the thread completely and makes removal harder. Stop immediately. Let the joint cool, apply a penetrating lubricant, allow time for it to work, then try backing out slowly. If that fails, cutting or drilling the bolt out costs less than persisting.
- Pickling — removes debris and scale from the threads, controlling surface contamination (one of the galling triggers) at the process stage.
- Specification review — where galling recurs, we will look at adding a finish such as a lubricant coating, or changing the bolt/nut material combination (for example an A2 bolt with an A4 nut).