When a stainless steel bolt suddenly "seizes" during tightening and further force either breaks it or strips the threads—this is galling (also called cold welding). It is particularly 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 passive 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, and wear debris does not easily detach but instead embeds between threads;
- Very thin passive film: only a few nanometers, 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 may 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 welding between identical metals;
- Alternatively, use copper alloy or coated nuts (but be mindful of galvanic corrosion and strength).
③ Thread Forming Method:
- Thread rolling (flat-die rolling) is superior to machining: rolling induces work hardening and compressive stress in the surface layer, resulting in a smoother surface and significantly 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/power tools to tighten at high speed all the way in one pass—high-speed frictional heat is a catalyst for galling;
- Tighten by hand first, then use a torque wrench;
- Reapply lubricant each time for repeatedly disassembled joints;
- 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—breaking a bolt is ten times more troublesome than removing a nut;
- Try slight back-and-forth reversal (tighten a little first, then loosen), which can sometimes break the weld points;
- Local heating (mind heat effects and seals), penetrating oil soaking;
- Last resort: cut out/drill out, which is the most costly.
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, resulting in actual preload exceeding design value and overloading the bolt;
- Purchasing only specifies "304 bolt" without specifying thread forming method or whether anti-galling treatment is required.
Tell us the specifications, material, operating environment (high temperature/food contact), and assembly method, and we will advise whether anti-seize compound is needed, material pairing recommendations, and the choice between thread rolling and machining—a single galled bolt often costs an entire flange.
