In many mechanical systems, engineers face the challenge of metal parts seizing or locking together during service. This sudden failure is not caused by fracture or corrosion but by a specific adhesive wear phenomenon known as galling. Galling has been reported in industries from aerospace fasteners to oilfield bearings, leading to expensive downtime and component replacement.
Understanding the definition of galling, its mechanism, the materials most susceptible to it, and practical prevention strategies is essential for anyone involved in design, machining, or maintenance of metal assemblies.
What is Metal Galling?
Metal galling is a form of adhesive wear in which two metallic surfaces slide against each other under pressure, causing local cold welding, material transfer, and eventual surface damage.
In practical terms, galling means surfaces that should slide or tighten smoothly suddenly begin to seize. Unlike normal wear, which is gradual and predictable, galling often develops abruptly. Once surfaces begin to seize, they may lock up entirely, a condition engineers refer to as “galled metal.”
Microscopic contact points (asperities) between metals weld together, and as sliding continues, fragments tear off, forming raised lumps and scratches. Over time, the surface becomes rough and damaged, and in severe cases, the parts cannot move or be disassembled without destruction.
Mechanism of Galling Formation
The galling process is more than simple friction damage. It is a chain of tribological events, influenced by pressure, temperature, material chemistry, and surface finish.
Asperity Contact
No surface is perfectly smooth. Under magnification, even polished metals reveal microscopic peaks and valleys. When two surfaces are pressed together, these asperities bear most of the load.
Oxide Film Breakdown
Many alloys, such as stainless steel and aluminum, naturally form protective oxide layers. Sliding under high load disrupts these thin films, exposing fresh, reactive metal beneath.
Local Adhesion and Cold Welding
Once oxide protection is lost, clean metallic surfaces come into direct contact. Under pressure and frictional heat, atoms bond across the interface, creating localized “cold welds.”
Material Transfer
As sliding continues, the welded junctions tear apart. Material fragments are pulled from one surface and smeared onto the other. These transferred lumps increase surface roughness, which accelerates further adhesion.
Growth of Damage and Seizure
With repeated cycles, raised areas grow larger and more numerous. Engineers often observe components that run smoothly at first, then suddenly seize after a short operating period once adhesion passes a threshold.
Materials Susceptible to Galling
Stainless Steel Galling
Austenitic stainless steels (304, 316) are highly ductile and corrosion resistant, but they are also the most common victims of stainless steel galling. Their toughness, combined with strong affinity for oxygen, makes them prone to adhesion once the passive oxide film breaks down. Fasteners made of stainless steel are especially vulnerable.
Aluminum Galling
Aluminum and its alloys are relatively soft, with strong chemical reactivity. During sliding, the oxide film ruptures and aluminum quickly adheres to counter surfaces. Aluminum galling is a frequent issue in forming tools and sliding components.
Titanium and Nickel Alloys
Titanium alloys, valued for high strength-to-weight ratio, also suffer adhesive wear due to their reactivity. Nickel-based alloys, though corrosion resistant, can exhibit galling under pressure and sliding motion.
Resistant Materials
Hardened tool steels, nitrided components, and carburized surfaces resist galling because of their higher hardness and chemically stable surfaces.
Applications Where Galling Occurs
Galling is not a laboratory curiosity; it is a field problem seen across industries. Some of the most common applications include:
Thread Galling
Stainless steel bolts and nuts often seize during tightening. Once threads are galled, disassembly is almost impossible without cutting. This problem appears frequently in aerospace, marine, and chemical industries, where stainless fasteners are common for their corrosion resistance.
Bearings
In plain bearings, galling occurs when lubrication breaks down. Shafts and bearing surfaces weld locally, leaving deep scores. This reduces load capacity and accelerates heat generation, often leading to seizure. Maintenance records from compressors and turbines show galling as a leading cause of unplanned shutdowns.
Shafts and Bushings
Rotating shafts that run directly in bushings may gall under heavy load. Once galling starts, shafts lock and machines stop abruptly, causing long downtime in production environments.
Molds and Dies
Forming operations with stainless steel or aluminum often leave deposits on die surfaces. These deposits are fragments transferred during galling. Each subsequent part formed carries visible scratches or surface defects, which reduces yield.
Guides and Sliding Surfaces
Machine tool ways and linear slides can gall if lubrication is inadequate. In such cases, engineers report not only machine damage but also loss of precision in parts being produced.
Preventing and Reducing Metal Galling
Because galling stems from adhesion and material transfer, preventive measures must focus on how the surfaces interact—both chemically and mechanically. The first step is often material selection. When two similar stainless steel components are paired, galling risk is high. Engineers typically avoid using identical alloys in fasteners and instead combine stainless bolts with bronze or coated nuts. Some alloy systems are also designed with elements such as nitrogen or sulfur to reduce adhesive tendencies.
Another effective strategy is surface modification. Heat treatments like nitriding, carburizing, or case hardening produce a hardened surface layer, making adhesive welding less likely. Coatings such as titanium nitride (TiN), chromium nitride (CrN), DLC films, or even simple hard chrome plating can act as a physical barrier between metals. These treatments are especially useful in tooling and aerospace components, where downtime caused by galling would be extremely costly.
Lubrication remains one of the most practical and widely used defenses. Stainless fasteners, for example, almost always benefit from anti-seize compounds. In aerospace and heavy machinery, greases containing molybdenum disulfide (MoS₂), graphite, or PTFE are standard practice. Proper lubrication ensures a thin film separates the surfaces, preventing direct metallic bonding.
The role of surface finish is often underestimated. Galling does not always decrease with smoother surfaces. If surfaces are too rough, asperities lock mechanically; if they are too smooth, the actual contact area becomes large enough to promote adhesion. For sliding parts, an Ra between 0.2 and 0.8 µm is generally recommended. Optimizing surface finish, therefore, is as important as choosing the right alloy or lubricant.
Finally, engineers also rely on design practices to minimize galling. Increasing contact area lowers unit pressure, while using coarse-pitch or rolled threads helps prevent seizing in bolts. Correct torque application is equally important; over-tightening stainless fasteners is one of the quickest ways to trigger thread galling. Field tests at JeekRapid once showed that unlubricated M12 stainless bolts began to seize at about 75% of design torque, while the same bolts treated with anti-seize reached full torque smoothly. This simple example illustrates how prevention is often a combination of material choice, surface treatment, lubrication, and good design discipline.
Engineering Example
In assembly trials at JeekRapid, stainless steel M12 fasteners without lubrication began to seize at around 75% of the design torque. When the same bolts were coated with anti-seize compound, they reached full torque smoothly with no signs of galling. This illustrates how relatively simple preventive measures dramatically change performance in real applications.
How to Repair Worn Metal Surfaces?
Before attempting any repair, engineers must first evaluate the extent of wear or galling. Light scoring is often limited to the surface, but in severe cases, material transfer or cracks may extend deeper. Good practice is to measure depth and distribution of the damage before deciding whether the part should be repaired or scrapped.
If the component is still structurally sound, several CNC-based repair methods can be applied:
CNC Re-machining
Damaged material on shafts, bushings, or bearing seats can be removed with controlled turning or milling operations. By machining away the galled layer, the surface can be restored to a uniform finish. Engineers typically specify a target surface roughness between Ra 0.2–0.8 µm for sliding components. This ensures the repaired part regains both dimensional accuracy and resistance to further adhesive wear.
Precision Grinding and Polishing
For minor wear, fine grinding or polishing is sufficient. CNC surface grinders and lathes equipped with fine abrasives can eliminate shallow scratches and restore the required surface finish. This is often applied to sealing surfaces, valve seats, and precision fits where smoothness is critical.
Thread Re-machining (Re-tapping or Re-threading)
When galling damages internal or external threads, CNC lathes or tapping operations can re-cut the profile. This method works for light to moderate thread damage. To avoid repeat galling, engineers often recommend using lubricants or selecting dissimilar nut-and-bolt material combinations after repair.
In practice, CNC repair machining is most effective when the base material is valuable alloy steel or stainless steel. Replacing such parts may be expensive and time-consuming, whereas re-machining extends their service life while preserving tight tolerances. For example, a stainless steel bearing housing with galled surfaces can often be restored within ±0.01 mm accuracy using CNC boring and finishing, allowing it to return to service without the cost of full replacement.
Conclusion
Metal galling is a serious adhesive wear process that causes sudden failure in fasteners, bearings, shafts, and forming tools. It is most problematic in stainless steel, aluminum, and titanium assemblies. By optimizing surface finish, applying lubricants, selecting the right material combinations, and adopting surface treatments, engineers can greatly reduce the risk of galling.
At JeekRapid, prevention of galling is part of everyday practice. With ISO 9001 and ISO 13485 certifications, more than 20 years of machining experience, and advanced CNC facilities, JeekRapid manufactures stainless steel, aluminum, and alloy steel parts with optimized surface quality. Customers benefit from both precision machining and expertise in surface engineering.
Upload your CAD files to request a free quotation from JeekRapid. Our team reviews materials, tolerances, and surface finish requirements before production.
References (text citation only): Please consult for more information.
ASM Handbook, Vol. 18: Friction, Lubrication, and Wear Technology
FAQ
1. What does galling mean in metals?
It refers to adhesive wear where sliding surfaces cold weld and transfer material, leading to scoring and seizure.
2. What is galled metal?
“Galled metal” describes a surface that has suffered from galling, showing lumps, scratches, and transferred material.
3. Why is stainless steel galling common?
Because stainless steels are ductile and their protective oxide film, once ruptured, allows strong adhesion.
4. How to prevent thread galling?
Use anti-seize lubricants, avoid over-tightening, and pair stainless bolts with dissimilar nuts.
5. Is aluminum galling a concern?
Yes. Aluminum is soft and reactive, prone to adhesion in forming and sliding contact.
6. Galling vs normal wear – what is the difference?
Normal wear is gradual abrasion; galling is sudden adhesion and material transfer that can lock parts together.
7. Can bearings gall?
Plain bearings without lubrication are highly susceptible. Adhesive transfer quickly leads to seizure.
