When a stainless steel bolt suddenly "seizes" during tightening and further force either snaps it or strips the threads — this is galling (also called cold welding). It is especially common on stainless steel fasteners and has nothing to do with strength grade or thread precision.
One-sentence diagnosis
Galling is friction surfaces "welding together" under high pressure: the passivation film is worn through, bare metal contacts directly, 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 is especially prone to galling
- High plasticity, rapid work hardening: contact points harden quickly under load, wear debris does not easily detach and instead embeds between threads;
- Very thin passivation film: a few nanometres, worn through easily, after which metal contacts directly;
- Poor thermal conductivity: frictional heat concentrates at contact points, and rising temperature promotes welding;
- Same-material pairing: when both bolt and nut are 304/316, cold welding is most likely between identical metals.
Four countermeasures (ordered by ease of implementation)
① Lubrication (most direct):
- Use anti-seize compounds specifically for stainless steel (containing molybdenum disulfide, graphite, or ceramic particles), applied to threads and bearing surfaces;
- Note: ordinary grease is unsuitable for 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 hardness for bolt and nut (e.g. 304 bolt + 316 nut, or A2 + A4 combination) to reduce same-metal welding;
- Or use copper alloy/coated nuts (but watch for galvanic corrosion and strength).
③ Thread forming method:
- Rolled threads (thread rolling) are better than machined threads: rolling produces work hardening and compressive stress in the surface layer, giving a smoother surface and markedly better galling resistance;
- Machined threads have tool marks on the surface, which are the "starting line" for galling.
④ Assembly habits:
- Do not use pneumatic/electric tools to drive all the way at high speed in one go — high-speed frictional heat is a catalyst for galling;
- Tighten by hand to position first, then use a torque wrench;
- Reapply lubricant each time at locations that are repeatedly disassembled;
- Avoid burrs, iron chips, and sand particles on threads (dirt is an accomplice to galling).
What to do if galling has already occurred
- Do not force it — snapping a bolt is ten times more troublesome than removing a nut;
- Try slight back-and-forth in the reverse direction (tighten a little first, then loosen), which can sometimes break the weld points;
- Local heating (watch for heat effects and seals), penetrating oil soaking;
- Last resort: cut out/drill out, the highest cost.
Common pitfalls
- Dry tightening (no lubrication) of large-size stainless steel bolts — almost certain to gall;
- Applying torque values for carbon steel bolts to stainless steel bolts (different friction coefficients);
- Adding lubrication without adjusting torque, so actual preload exceeds design value and the bolt is overloaded;
- Purchasing only specifies "304 bolt" without specifying thread forming method or whether anti-seize treatment is required.
Tell us the specifications, material, operating environment (high temperature/food contact or not), and assembly method, and we will advise on whether anti-seize compound is needed, material pairing recommendations, and the choice of rolled vs machined threads — a single galled bolt often costs an entire flange.
