What is Galling?
Galling is a form of cold welding where stainless steel threads fuse together during tightening. The friction generates heat, the protective oxide layer breaks down, and bare metal welds to bare metal. Once galled, the fastener is destroyed—you can't remove it without cutting.
Why Stainless is Prone to Galling
- Soft oxide layer: Stainless's chromium oxide protection is thin and easily damaged under pressure
- Ductile material: Stainless deforms easily, creating more surface contact
- Heat buildup: Poor heat conductivity traps friction heat in threads
- Similar materials: Stainless-on-stainless galls more than dissimilar metals
Prevention Strategies
1. Use Anti-Seize Lubricant (Most Important)
Apply anti-seize compound to threads before assembly. This is the single most effective prevention method.
- Nickel-based anti-seize: Best for stainless, rated to 2600°F
- Copper-based anti-seize: Good general purpose, avoid on some alloys
- Molybdenum disulfide (moly): Excellent for high loads
Note: Anti-seize changes torque values. Reduce torque by 20-25% when using lubricant.
2. Slow Down
High-speed assembly (power tools) generates more heat. Use hand tools or slow power tool settings for stainless.
3. Use Different Alloys
Using dissimilar stainless grades reduces galling. For example:
- 304 bolt with 316 nut
- 18-8 bolt with 316 nut
The slight hardness difference reduces friction welding.
4. Choose Gall-Resistant Materials
- Nitronic 60: Stainless alloy designed to resist galling
- Silicon bronze: Naturally gall-resistant
- Brass nuts with stainless bolts: Good combination
5. Proper Torque
Over-torquing dramatically increases galling risk. Use a calibrated torque wrench and follow specifications.
6. Clean, Undamaged Threads
- Inspect threads before assembly
- Don't force cross-threaded fasteners
- Remove debris and burrs
Signs of Galling Starting
- Increasing resistance while tightening
- Squeaking or grinding sound
- Fastener suddenly locks up
- Heat buildup in fastener
If you feel these signs, STOP immediately. Back off and restart with lubricant. Forcing it will weld the threads.
If Already Galled
Unfortunately, severely galled fasteners cannot be saved. Options:
- Apply penetrating oil and try to back off slowly
- Apply heat (may damage surrounding material)
- Cut off the bolt head and extract
- Drill out and retap (last resort)
Quick Reference
- Always: Use anti-seize on stainless threads
- Never: Use power impact tools at full speed
- Best practice: Hand-start all stainless fasteners
- Buy extra: Keep spare stainless fasteners for galling casualties