When a stainless steel bolt suddenly "seizes" during tightening and further force either snaps it or strips the thread — that is galling (also called cold welding). It is especially common on stainless steel fasteners and has nothing to do with strength grade or thread accuracy.
One-line diagnosis
Galling is friction surfaces "welding together" under high pressure: the passive film is worn through, bare metal contacts bare metal, and local welding occurs under pressure and frictional heat. There are four countermeasures: lubrication, material pairing, thread-forming method and assembly habits.
Why stainless steel galls so easily
- High plasticity, rapid work hardening: contact points harden quickly under load, wear debris does not readily fall away but instead embeds between the threads;
- The passive film is very thin: a few nanometres, worn through at the slightest abrasion, after which metal contacts metal directly;
- Poor thermal conductivity: frictional heat concentrates at the contact points, and the temperature rise promotes welding;
- Same-material pairing: when both bolt and nut are 304/316, cold welding between identical metals is most likely.
Four countermeasures (ordered by ease of implementation)
① Lubrication (most direct):
- Use an anti-seize compound specifically for stainless steel (containing molybdenum disulfide, graphite or ceramic particles), applied to the threads and bearing surface;
- Note: ordinary grease is unsuitable in high-temperature or vacuum environments; food/pharmaceutical applications require food-grade or dry-film lubrication;
- Lubrication changes the torque-preload relationship, so assembly torque must be recalibrated, otherwise overload will occur.
② Material pairing:
- Use different grades or different hardnesses for bolt and nut (e.g. 304 bolt + 316 nut, or an A2 + A4 combination) to reduce welding between identical metals;
- Alternatively use a copper-alloy or coated nut (but watch for galvanic corrosion and strength).
③ Thread-forming method:
- Rolled (thread-rolled) threads are better than machined: rolling work-hardens the surface layer and induces compressive stress, giving a smoother surface and markedly better galling resistance;
- Machined threads have tool marks on the surface — the "starting line" for galling.
④ Assembly habits:
- Do not drive it home in one high-speed pass with pneumatic/electric tools — high-speed frictional heat is the catalyst for galling;
- Tighten by hand to position first, then use a torque wrench;
- Reapply lubricant each time at locations that are repeatedly dismantled and reassembled;
- Avoid burrs, iron swarf and grit on the threads (dirt is an accomplice to galling).
What to do if it has already galled
- Do not force it — a snapped bolt is ten times more troublesome than a removed nut;
- Try slight back-and-forth reversal (tighten a little first, then loosen); this sometimes breaks the weld points;
- Local heating (mind the heat effect and seals) and penetration oil soaking;
- Last resort: cut out/drill out, the highest-cost option.
Common pitfalls
- Dry tightening (no lubrication) of large-size stainless steel bolts — galling is almost inevitable;
- Applying carbon steel bolt torque values to stainless steel bolts (different friction coefficients);
- Adding lubrication without adjusting torque, so actual preload exceeds the design value and the bolt is overloaded;
- Purchase specifications that only say "304 bolt" without stating the thread-forming method or whether anti-seize treatment is required.
Tell us the size, material, service environment (high temperature/food contact or not) and assembly method, and we will advise whether an anti-seize compound is needed, recommend material pairing, and help you choose rolled or machined threads — a single galled bolt often costs an entire flange.
