My Stainless Bolt Seized Halfway In — Galling

A stainless bolt that stops halfway and will neither tighten nor back out. It is a common stainless fastening failure on site, known in English as galling.

Galling is adhesive wear: under high contact pressure and relative motion, the metal surfaces stick together and tear away. The metal does not melt, which is why it is also called cold welding, but the frictional heat generated during tightening can accelerate it.

Why stainless galls so readily

When friction wipes away the thin passive film that protects stainless, the exposed bare metal surfaces readily stick to each other. Galling also occurs in titanium and aluminium, but it is a particular problem with stainless fasteners, which are usually used without plating.

1. Threads rub under high contact pressure as the bolt is tightened
2. The friction wipes off the thin passive film
3. Bare metal now contacts bare metal directly
4. Stainless is ductile, so the contacting surfaces adhere
5. Continued turning tears the adhered points and strips the thread

Most carbon steel bolts carry a plated finish, and the plating keeps bare metal from meeting bare metal; heat-treated high-strength grades also tend to gall comparatively less. Stainless bolts are often used unplated and are highly ductile, which works against them.

Conditions that invite galling

Practical ways to reduce galling

1. Control driving speed. On site this is often the easiest step that makes a difference. High-speed power driving can raise frictional heat and galling risk, so manage rotation speed and torque, and finish by hand if needed. Speed alone does not always solve it, so manage the items below as well.
2. Lubricate. Molybdenum disulphide (MoS₂), nickel-based anti-seize pastes and similar. But lubrication lowers friction, so the same tightening torque can produce a higher clamp load. Do not reuse the dry-condition torque; confirm a tightening torque suited to the target preload and the lubricated condition (see tightening torque and friction). Choose the lubricant for service temperature, corrosive environment, cleanliness and the customer's material and contamination requirements; anti-seize containing metal particles may be restricted where metallic contamination is controlled (batteries, for example).
3. Make the bolt and nut different. Different materials, surface hardness or finishes (such as a lubricant coating) can reduce the risk. An A2 (304) bolt with an A4 (316) nut alone does not guarantee prevention: both are austenitic and the hardness difference may be small, so check the actual hardness difference and the lubrication. When combining different steel grades, also check the corrosion resistance of the joint as a whole and the service environment, not just galling resistance. ISO 3506-1 (6.4) recommends mating bolts and nuts of the same grade for corrosion resistance, and where other combinations are used, the component with the lowest corrosion resistance governs.
4. Consider coarse threads first. They often resist galling better than fine threads.
5. Keep threads clean and do not assemble contaminated parts.
6. Check the fit. Confirm the pitch and dimensions of the bolt and mating thread and check for thread damage, so that the parts are never forced together.

Why rolled threads help

Rolled threads are formed by plastic deformation rather than cutting. Compared with cut threads they generally have smoother flanks and a grain flow that follows the thread form, which can help reduce galling. Our screw threads are rolled. Rolled threads can still gall, however, so lubrication and tightening conditions still need to be managed.