Thread galling is a form of cold welding that happens when metal threads seize together during tightening, fusing a bolt and nut into a single stuck mass. It occurs most often with stainless steel, aluminum, and titanium fasteners, and once it happens, the fastener is usually destroyed. Unlike normal wear that develops over time, galling can strike on the very first use of a brand-new bolt.
How Thread Galling Happens
Metals like stainless steel naturally form a thin oxide layer on their surface. On stainless steel, this chromium oxide coating is only a few angstroms thick (thousands of times thinner than a human hair). That layer acts as a barrier between mating surfaces.
When you tighten a bolt, pressure builds between the contact points of the male and female threads. That pressure shears away the oxide coating. With the protective layer gone, bare metal high points on one thread are now pressing directly against bare metal on the other. Friction spikes. The combination of exposed metal and increasing friction generates enough heat to literally fuse the two surfaces together. The nut and bolt become one piece. At that point, you can’t loosen or tighten the fastener without shearing it apart.
This can happen in a fraction of a second. You’ll often feel it as a sudden increase in resistance partway through tightening, followed by the fastener locking completely. If you try to force it further, the bolt can snap.
Which Metals Are Most Vulnerable
Materials that form natural protective oxides are, paradoxically, the most prone to galling. The same thin oxide layer that protects these metals from corrosion is also easy to strip away under thread pressure. Stainless steel, aluminum, and titanium all fall into this category.
Stainless steel is the worst offender by far. Decades of industry experience show that the vast majority of thread galling cases involve both components made from 300 series stainless steel (the most common type, used in everything from food equipment to marine hardware). When two pieces of the same soft, adhesive alloy are threaded together, the conditions for cold welding are ideal.
Carbon steel and hardened alloy steels gall far less frequently. Their surfaces are harder and less prone to the adhesive bonding that drives the process.
Why It Matters Beyond a Stuck Bolt
A galled fastener isn’t just an inconvenience. If the threads seize before you’ve applied enough torque, the joint is underclamped, meaning the connection is weaker than designed. In structural, marine, oil and gas, aerospace, or chemical processing applications, an underclamped joint can lead to catastrophic failure.
Galled fasteners also create expensive downtime. Industrial machines are designed to run thousands of hours with minimal interruption. When a galled bolt forces a production stoppage, the cost of lost output often dwarfs the cost of the fastener itself. And because a galled fastener usually requires destructive removal (cutting, drilling, or splitting), repairs take longer and risk damaging surrounding components.
How to Prevent It
Most galling is preventable with a few straightforward practices.
Use Lubrication or Anti-Seize Compound
Applying anti-seize compound to the threads before assembly is the single most effective prevention method. These products contain solid lubricant particles that maintain a barrier between metal surfaces even under high pressure. Common formulations use molybdenum disulfide, nickel, or copper-based compounds. A thin, even coat on the bolt threads is enough.
Slow Down Installation
High-speed installation generates heat, which accelerates galling. For stainless steel and titanium fasteners, avoid power tools. Use controlled hand tools or low-RPM torque drivers instead. The slower you go, the less frictional heat builds between threads.
Use Different Metals for the Bolt and Nut
Two identical stainless steel alloys threaded together are a recipe for galling. Using a different alloy or hardness for the nut versus the bolt reduces the tendency for the surfaces to bond. For example, pairing a 316 stainless bolt with a 304 stainless nut, or using a bolt with a hardened surface treatment, creates enough material difference to resist cold welding.
Don’t Use Bolts to Pull Joints Together
A bolt’s job is to clamp, not to draw misaligned parts into position. When you use a bolt to force components together, you’re applying side loads to the threads that dramatically increase friction and galling risk. Align the joint first, then insert and tighten the fastener.
Check Thread Quality
Rough, damaged, or dirty threads have more high points where metal-to-metal contact concentrates. Inspect threads before assembly and discard fasteners with visible nicks or burrs. Even debris from a previous installation can trigger galling.
Removing a Galled Fastener
If a fastener has already galled, your options depend on how severe the seizure is.
For mild cases where the bolt is stuck but not fully fused, apply penetrating oil and let it soak for at least 10 to 15 minutes. You can also carefully apply heat with a heat gun to expand the metal and break the bond. Alternate between heating and applying penetrating oil for the best chance of freeing it.
For fully seized fasteners, destructive removal is usually the only option: cutting, drilling, or splitting the bolt or nut. Once you’ve removed it, assess the internal threads. Minor damage can be cleaned up with a tap and die set to restore the thread profile. If the threads are stripped or severely deformed, threaded inserts can create a fresh, strong thread inside the damaged hole. In the worst cases, the entire component may need replacement.
Whatever the outcome, the removed fastener should never be reused. Replace it with a new one and apply anti-seize compound before reassembly.
How Galling Resistance Is Measured
Engineers quantify a material’s resistance to galling using a standardized test called ASTM G98. A button-shaped specimen is pressed against a flat block of the test material under a set load, then rotated one full revolution. Both surfaces are inspected for galling. If no galling occurred, a fresh pair of specimens is tested at a higher load. This continues until galling appears.
The result is a number called the threshold galling stress: the average of the highest load that didn’t cause galling and the lowest load that did. It’s treated as a binary outcome. There are no degrees of galling in this test; it either happened or it didn’t. This threshold helps engineers select material pairings and surface treatments for applications where galling risk is a design concern.