HOW TO PREVENT
GALLING.

What galling actually is

Galling is a form of adhesive wear. When two metal surfaces slide against each other under load, microscopic high points on each surface make contact, weld together under pressure, and then tear apart as sliding continues. Each cycle transfers material from one surface to the other, roughening both. On a threaded fastener the process is self-accelerating: the rougher the threads get, the higher the contact pressure, the more welding occurs.

In its terminal form the fastener seizes completely. The nut will neither advance nor back off, and the joint is scrap. On larger assemblies that often means drilling out the fastener and repairing the tapped hole — a repair that costs many times the price of the fastener.

Why stainless steel is uniquely vulnerable

Stainless is not soft, so its galling tendency surprises people. Four properties combine against it:

• The passive oxide layer is thin and self-healing. Chromium oxide is what makes stainless corrosion resistant, but it is only nanometres thick. Sliding contact scrapes it off, exposing clean, chemically active metal that bonds readily to the mating surface.
• Austenitic grades work-harden aggressively. 304 and 316 harden rapidly at the point of deformation, concentrating stress instead of distributing it.
• Mating parts are usually the same alloy. Identical metals have identical lattice structures and similar hardness, which is the ideal condition for cold welding.
• Stainless has poor inherent lubricity. Unlike leaded steels or cast iron, there is no free graphite or soft phase to act as a natural release agent.

Austenitic grades — the 300 series — are the worst offenders. Duplex and precipitation-hardening grades gall less readily but are not immune. Titanium and aluminium share the same failure mode for the same reasons.

Where it shows up

Galling concentrates wherever there is sliding contact under load with limited lubrication. In practice that means threaded fasteners above all — particularly during power installation, where speed generates heat and heat accelerates welding. It also appears on valve stems and seats, sliding guides and ways, bushings and bearing journals, and any press-fit or slip-fit interface assembled dry.

The five methods that work

1. Use dissimilar materials or hardnesses

The simplest fix is to break the material symmetry. Pairing a 316 bolt with a nut of a different grade, or hardening one member well above the other, prevents the matched-lattice cold welding that drives galling. A hardness difference of roughly 50 HB is a common rule of thumb. The limitation is obvious: your design may not permit a material change, and mixing alloys can introduce galvanic corrosion concerns in wet service.

2. Control installation speed and torque

Galling is heat-driven, and impact drivers generate a great deal of heat very quickly. Installing stainless fasteners by hand or at low RPM gives the interface time to shed heat. Correct torque matters just as much — over-torquing raises contact pressure past the point where any surface treatment can help. This costs nothing to implement but depends entirely on operator discipline, which makes it unreliable at production volume.

3. Improve thread form and surface finish

Rolled threads gall less than cut threads. Rolling produces a smoother, work-hardened surface with a continuous grain flow, while cutting leaves torn micro-peaks that are exactly the asperities galling feeds on. Specifying rolled threads and a finer surface finish reduces the number of contact points available to weld. This helps, but on its own it delays galling rather than preventing it.

4. Anti-seize compounds

Nickel, copper, and moly-based pastes put a physical barrier between the surfaces and work well on first assembly. Their weaknesses are practical rather than technical: they are messy, they attract abrasive dust and swarf, they migrate away from where they were applied, they wash off in service, and coverage depends entirely on the person applying them. In food, medical, optical, and cleanroom environments they are frequently prohibited outright. They also change the torque-tension relationship, so torque specs derived for dry fasteners no longer apply.

5. Thermally cured dry film lubricant

A dry film lubricant is a solid lubricant — typically PTFE or molybdenum disulphide — suspended in a resin binder, sprayed onto the part and then cured with heat so the binder cross-links and bonds to the substrate. The result is a thin, dry, non-migrating layer with a low coefficient of friction that stays exactly where it was applied.

For production fastener work this is usually the right answer. It cannot be applied incorrectly on the line because it is applied before the parts ever reach assembly. It does not attract contamination. It survives handling, storage, and multiple assembly cycles. Film thickness is controlled and repeatable, so the torque-tension relationship stays consistent from part to part — which for a torque-critical joint is often the deciding factor.

Coatings in this class are commonly qualified to MIL-PRF-46010, the specification covering heat-cured solid film lubricants, along with related standards such as MIL-L-46147 and AS5272.

Choosing between them

For a one-off repair, anti-seize is fine. For a prototype, a material or hardness change may be the fastest route. For anything that runs in volume, has a torque specification, must be disassembled and reassembled, or operates where contamination matters, a cured dry film lubricant is the method that holds up.

The failure mode to watch for is combining approaches badly — applying anti-seize on top of a dry film coating, for example, which adds contamination without adding protection and destroys the torque consistency the coating was specified to provide.

Testing it on your own parts

Galling behaviour depends on your specific alloy pairing, thread form, torque spec, and installation method, and no article can predict it exactly. The reliable approach is to coat a small batch and run your own assembly test. We will coat a sample set at no charge so you can measure the result on your actual hardware before committing to anything.

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