What Is Thread Galling?

Thread galling is a form of cold welding that occurs when mating bolt and nut threads seize together under pressure and friction during tightening. The contacting thread surfaces generate enough heat to fuse locally, tearing material from one surface and transferring it to the other. Once galling begins, further rotation worsens the damage, often resulting in a permanently locked or broken fastener.

Why Galling Occurs

Galling is primarily a concern with austenitic stainless steels (304, 316), nickel alloys (Inconel, Monel), aluminum, and titanium fasteners. These materials form thin, soft oxide layers that break down easily under sliding contact, exposing bare metal surfaces that bond under pressure.

Contributing Factor	Mechanism	Risk Level
Material type	Soft oxide layers on SS, Ni alloys	High for austenitic SS, Ni alloys
Surface finish	Rough threads increase friction	Higher with coarse machining
Tightening speed	Fast rotation generates more heat	Risk increases with speed
Lubrication	Absence of anti-seize compound	Severe without lubrication
Thread fit	Tight tolerances increase contact	Higher with Class 3 fit
Contamination	Dirt or debris in threads	Moderate

Materials Most Susceptible to Galling

Material	Galling Resistance	Notes
Carbon steel (A193 B7)	Good	Hard oxide layer resists galling
Stainless steel 304 (A193 B8)	Poor	Most common galling problem in piping
Stainless steel 316 (A193 B8M)	Poor	Same susceptibility as 304
Monel (A193 B865)	Poor	Nickel-copper alloy, prone to seizing
Inconel (A193 B446)	Poor	Nickel-chromium alloy, high risk
Alloy steel B16	Good	Cr-Mo-V, similar to B7

Prevention Methods

The most effective strategies to prevent thread galling on stud bolts and nuts:

1. Apply anti-seize compound: Molybdenum disulfide (moly) or nickel-based anti-seize on all stainless steel and nickel alloy threads. This is the single most effective prevention measure.
2. Slow the tightening speed: Reduce RPM on impact wrenches or use hand-torquing for stainless steel bolts.
3. Use different hardness levels: Specify nuts 1-2 HRC points harder or softer than the stud to minimize adhesion (e.g., strain-hardened Class 2 nuts with Class 1 studs).
4. Specify coatings: Zinc plating, PTFE coating, or silver plating on threads reduce metal-to-metal contact.
5. Clean threads: Remove all dirt, rust, and debris before assembly.
6. Avoid reusing galled fasteners: Any fastener showing galling damage must be replaced.

Common Mistake

Applying excessive torque to overcome initial galling resistance will catastrophically worsen the seizure. If resistance is felt during tightening, stop immediately, disassemble, inspect for damage, and apply anti-seize before retrying.

Anti-Seize Compound Selection

Compound Type	Base	Temperature Range	Best For
Molybdenum disulfide	MoS2	Up to 450 degC	General SS bolting
Nickel-based	Nickel + graphite	Up to 1,315 degC	High-temp, Ni alloys
Copper-based	Copper flake	Up to 980 degC	Stainless steel, brass
PTFE-based	Teflon	Up to 260 degC	Low-temp, food grade

Pro Tip

When using anti-seize on bolt torque applications, reduce the target torque value by 15-25% to compensate for the lower friction coefficient. Consult the torque charts for adjusted values.

Read the full guide to stud bolts for flanges