What is Thread Galling?
What is Thread Galling?
Thread galling (also called "cold welding") is a form of severe adhesive wear that occurs when fastener threads seize together during tightening. The friction between mating threads generates enough heat to cause material transfer — essentially welding the bolt and nut together. Once galled, fasteners are typically destroyed during removal.
Other Names: Cold welding, thread seizure, pick-up, adhesive wear
Why Galling Happens
Galling occurs when:
1. Friction generates heat — Metal-to-metal contact creates localized heating
2. Protective oxide layer breaks — Exposing bare metal
3. Material transfer occurs — Metal welds to mating surface
4. Damage propagates — Each thread damages the next
5. Complete seizure — Fastener locks up mid-tightening
Materials Most Prone to Galling
| Material | Galling Risk | Why |
|---|---|---|
| Stainless Steel | Very High | Soft oxide layer, work hardens |
| Aluminum | High | Soft, transfers easily |
| Titanium | Very High | Similar to stainless |
| Nickel Alloys | High | Soft, adhesive |
| Brass | Moderate | Softer, some natural lubricity |
| Carbon Steel | Low | Hard oxide layer |
| Alloy Steel | Very Low | Hard, stable surface |
Why Stainless Steel is the Worst
Stainless steel is notorious for galling because:
- Thin oxide layer — Chrome oxide is thin (vs thick rust on carbon steel)
- Work hardening — SS hardens as you work it, increasing friction
- Soft base metal — Austenitic SS (304, 316) is relatively soft
- Self-mating — SS bolt + SS nut = same material = maximum adhesion
Warning Signs of Galling
During installation:
- Increasing resistance despite correct torque
- "Sticky" feeling when threading
- Visible metal shavings or flakes
- Threads feel rough or damaged
- Fastener suddenly locks up
Prevention Strategies
1. Lubrication (Most Effective)
Apply anti-seize or thread lubricant before assembly:
| Lubricant Type | Best For | Temp Range |
|---|---|---|
| Copper anti-seize | High temp, general | -30°C to 980°C |
| Nickel anti-seize | Stainless, high temp | -30°C to 1315°C |
| Moly (MoS2) | General, high pressure | -45°C to 450°C |
| Graphite | Moderate temps | -200°C to 650°C |
| Zinc anti-seize | Aluminum, galvanic protection | -45°C to 760°C |
| Thread lubricant oil | Light duty, easy removal | -20°C to 150°C |
Application: Thin coat on male threads only. Avoid excess.
2. Slow Installation Speed
| Method | Galling Risk | Why |
|---|---|---|
| Power tool, high speed | High | Generates heat quickly |
| Power tool, low speed | Medium | Less heat buildup |
| Hand tools | Low | Minimal heat generation |
Recommendation: For stainless, start by hand, finish with low-speed power tool or hand wrench.
3. Different Materials
Using different alloys reduces galling risk:
| Bolt | Nut | Galling Risk |
|---|---|---|
| 304 SS | 304 SS | Very High |
| 304 SS | 316 SS | High (but better) |
| 304 SS | Bronze nut | Low |
| 316 SS | Monel nut | Low |
| Titanium | Titanium | Very High |
| Titanium | Stainless | Moderate |
4. Surface Treatments
| Treatment | How It Works | Effectiveness |
|---|---|---|
| Silver plating | Lubricious layer | Excellent |
| PTFE coating | Dry film lubricant | Good |
| Phosphate coating | Holds lubricant | Good |
| Nitriding | Hard surface layer | Good |
| Waxed/pre-lubed | Built-in lubrication | Good |
5. Proper Thread Fit
| Thread Class | Fit | Galling Risk |
|---|---|---|
| 4H/4g (loose) | Clearance | Lower |
| 6H/6g (standard) | Normal | Normal |
| 5H/4g (transition) | Snug | Higher |
Tighter fits = more friction = higher galling risk. Specify standard fit unless precision requires tighter.
6. Coarse vs Fine Threads
| Thread Type | Galling Risk | Reason |
|---|---|---|
| Coarse pitch | Lower | Fewer threads, more clearance |
| Fine pitch | Higher | More threads, more friction |
7. Installation Technique
Do:
- Start threads by hand
- Verify alignment before power tools
- Back off and restart if resistance increases
- Use slow, steady tightening
- Apply lubricant to clean, dry threads
Don't:
- Cross-thread (misalignment)
- Run down at high speed
- Force past resistance
- Reuse galled fasteners
- Over-torque
Removing Galled Fasteners
If galling has already occurred:
Step 1: Stop Immediately
Don't keep forcing — you'll make it worse.
Step 2: Apply Penetrating Oil
Soak the joint. Products like:
- Kroil
- PB Blaster
- Liquid Wrench
- WD-40 Specialist Penetrant
Wait 15-30 minutes (longer is better).
Step 3: Apply Heat (Carefully)
Thermal expansion can break the weld:
- Heat the nut (expands it away from bolt)
- Don't overheat stainless (affects corrosion resistance)
- Propane torch or heat gun
Step 4: Impact or Vibration
- Impact wrench (short bursts)
- Tap with hammer while turning
- Ultrasonic vibration (industrial)
Step 5: Cutting/Drilling (Last Resort)
If all else fails:
- Split nut with nut splitter
- Cut bolt with saw
- Drill out bolt
Reality: Severely galled fasteners usually need to be destroyed for removal.
Material Selection for Galling Resistance
| Application | Best Bolt | Best Nut |
|---|---|---|
| Marine (corrosion + galling) | 316 SS, lubricated | Silicon bronze or lubricated 316 |
| High temp | Inconel | Monel or different Inconel alloy |
| Cryogenic | 304/316 SS lubricated | Same, with silver plating |
| General SS assemblies | A2/A4 bolts | Different alloy nut or lubricated |
Galling-Resistant Fastener Options
Pre-Lubricated Fasteners
Some manufacturers offer factory-applied dry lubricant:
- PTFE-coated threads
- Wax-coated fasteners
- Molybdenum disulfide coating
Special Alloys
- Nitronic 60 — Galling-resistant stainless steel
- Nitronic 50 — High-strength, galling-resistant
- Waukesha 88 — Bronze alloy for stainless bolts
- Silicon bronze — Natural galling resistance
Quick Reference: Galling Prevention Checklist
☐ Using stainless steel? Apply anti-seize
☐ Threads clean and undamaged?
☐ Alignment verified before power tool?
☐ Starting by hand?
☐ Slow installation speed?
☐ Not reusing previously galled fasteners?
☐ Correct torque (not over-tightening)?
☐ Thread fit appropriate (not too tight)?
FAQ
Q: I'm using stainless bolts and nuts — what should I do?
A: Apply copper or nickel anti-seize before assembly. This is the single most effective prevention.
Q: Does thread locker prevent galling?
A: Not really. Thread lockers are designed to prevent loosening, not galling. Some have lubricating properties, but dedicated anti-seize is better.
Q: Can galled threads be repaired?
A: Minor galling can sometimes be cleaned with a thread chaser. Severe galling = replace the fastener.
Q: Why doesn't carbon steel gall like stainless?
A: Carbon steel has a thick, hard oxide layer (rust) that acts as a barrier. Stainless has a thin chrome oxide layer that breaks easily under friction.
Q: Does plating prevent galling?
A: Zinc plating provides minimal help. Silver plating is excellent. The goal is to create a barrier between mating surfaces.
Q: At what torque does galling typically start?
A: There's no specific torque — it depends on speed, lubrication, material, and fit. Galling can occur at surprisingly low torques with stainless.
Prevent costly galling failures by stocking anti-seize compounds and galling-resistant fastener materials. Browse our stainless steel fasteners and anti-seize products.