Why Galling Occurs on Threaded Fasteners
Thread galling is a frustrating problem for engineers working with bolted joints. If you’ve ever had a nut suddenly seize on a bolt during tightening, you’ve witnessed galling. In severe cases, the nut and bolt are effectively “cold welded” together – removal becomes impossible without cutting the fastener off. Such seized fasteners can lead to costly downtime, as equipment must be disassembled with torches or splitters, causing schedule delays and lost production. This article provides an engineering overview of why galling occurs on threaded fasteners, how the galling process works, and – most importantly – how to prevent galling (with the Velocity Washer emerging as the number one solution).
What is Thread Galling?
Thread galling is a form of adhesive wear that occurs when two metal surfaces in sliding contact effectively stick together due to friction and material transfer. In the context of fasteners, galling refers to the seizing or locking of mating threads (bolt and nut) during installation or removal. It is often compared to a cold-welding process: as the bolt and nut are tightened under high pressure, portions of the metal surfaces shear off and fuse together, bonding the threads. Once a fastener has seized from galling, it can no longer turn and usually cannot be removed without destroying the bolt or nut (e.g. by cutting the bolt or splitting the nut).
In less severe cases of galling, the threads may only be mildly damaged (light scoring) and the assembler might notice increased resistance – if tightening is stopped early, the fastener can sometimes be backed off. However, if galling continues, it will strip or weld the threads completely. The end result is often a ruined bolt and nut that must be scrapped - but that is the least of the concerns. It leads to standby time, safety issues, and lost production as well. Galling is commonly observed in stainless steel fasteners (it’s sometimes even called “stainless steel seizing”), but it can affect other materials like steel, titanium and aluminum alloys as well.
Why Does Galling Occur on Threaded Fasteners?
Galling doesn’t happen under normal low-stress contact – it requires a specific combination of conditions that frequently occur in bolted joints. The process is driven by adhesion and friction between the mating threads under high load. Here’s a closer look at why threaded fasteners are prone to galling:
High Contact Pressure and Friction: When a nut is tightened on a bolt, the flanks of the male and female threads are pressed together with tremendous force (especially as the bolt reaches high preload). The surface contact occurs primarily at microscopic high points (asperities) on the metal threads. As the nut turns, these asperities slide against each other under pressure, producing friction. If friction is high, significant heat can be generated in the thread interface. The combination of high pressure and heat causes local adhesion – the asperities can begin to shear and lock together, rather than sliding smoothly. In essence, small bits of one thread weld onto the other. This creates even more friction and a cascading effect: the longer you continue turning under these conditions, the more material transfers and binds, until the threads seize completely.
Removal of Protective Oxide Films: Many metals used in fasteners (notably stainless steels, as well as aluminum and titanium) are protected by a thin oxide layer on their surface. This oxide film normally prevents direct metal-to-metal contact and also reduces friction in the early stages of tightening. However, under high pressure and sliding, the oxide coating can be scraped off at the thread peaks. Once bare, reactive metal is exposed on both the bolt and nut thread surfaces. Exposed fresh metal has a strong tendency to adhere or “grab” the mating surface. In the case of stainless steel, which is a relatively soft and ductile metal, this adhesion quickly leads to galling once the oxide barrier is gone. The metal atoms from one side actually jump to the other side (material transfer), especially as frictional heating softens the material, leading to that galling “lock-up” phenomenon.
Material Properties (Soft, Ductile Metals): The materials of the fastener play a big role in galling propensity. Austenitic stainless steels (like 304 or 316 stainless bolts and nuts) are notorious for galling because they are ductile and work-hardened surfaces can readily bond. They also self-generate oxide layers (as mentioned above) which, once damaged, expose bare metal that likes to stick. Titanium and aluminum fasteners show similar behavior for the same reasons. However, galling can occur on any fastener, including standard low steels, after they have been exposed to things like cyclic heat, which can bake off the flow lubricants and prime them for galling during disassembly.
Thread Design and Condition: The geometry and condition of the threads influence galling. Fine threads (UNF) have more threads per inch and shallower thread depth than coarse threads (UNC). This means more surface contact area and tighter clearances, which can increase friction and make fine-threaded fasteners more susceptible to galling. Additionally, any thread damage or roughness will act like sandpaper and raise the friction between threads. Burrs, nicks, or dirt on thread surfaces create high friction spots that can initiate galling. For this reason, standard practice is to ensure threads are clean and undamaged before tightening. A smoothly rolled thread (as opposed to a cut thread with rough edges) will have fewer asperities to cause trouble. Proper thread fit is also important: threads that are too tight (class 3 fit or poorly toleranced) can have excessive friction. In fact, overtightening a fastener beyond its yield can gall the threads because the threads deform and dig into each other.
Lack of Lubrication: “Dry” assembly of bolts and nuts (with no lubrication) significantly increases the chances of galling. Without lubrication, the coefficient of friction between metal threads is high, leading to more heat and adhesion. Threads that are well-lubricated (with anti-seize compounds, for example) have a much lower tendency to gall because the lubricant separates the surfaces and dissipates heat. Industry standards such as ASME PCC-1 (Guidelines for Pressure Boundary Bolted Flange Joint Assembly) strongly emphasize proper thread lubrication during assembly to control friction and achieve correct preload. We’ll discuss lubrication more in the prevention section, but it’s worth noting here: a lack of lubricant is a major contributing factor to galling. However, the reality is you can do everything “perfectly” during assembly, including lubricants, but after being in service for months (or years) the threaded fastener is already primed for galling, as the flow lubricants may no longer be present.
Fastener Usage Factors: Certain assembly practices can inadvertently promote galling. One common mistake is using the bolt to “pull” misaligned joints together. If two flange plates aren’t snug and you force them closed by tightening the bolts, you put huge side loads and extra friction on the threads, almost guaranteeing galling. High-speed tightening is another culprit – using an impact gun or spinning a nut down at high RPM generates rapid frictional heating. The faster the installation, the less time there is for heat to dissipate, so temperatures at the thread interface spike, making galling more likely. This is why many manufacturers recommend against using power tools on stainless steel fasteners. Additionally, be cautious with prevailing-torque lock nuts (like nylon insert locknuts or all-metal locknuts). These nuts intentionally add friction to prevent loosening, but that added friction means higher risk of galling during installation. In fact, galling problems are most frequently observed with stainless steel lock nuts. We’ve read a piece from Fastenal’s engineering team that notes nylon insert in stainless locknuts creates so much friction that it significantly increases galling incidence, so they often wax-coat these nuts to reduce friction.
How to Prevent Galling on Threaded Fasteners
Galling may be common – but it’s not inevitable. By understanding the causes, engineers can take preventative measures at both the design stage and during installation to avoid galling. In general, anything that reduces friction, heat, or metal-to-metal adhesion in the joint will help. Below are several proven strategies for mitigating thread galling (with the Velocity Washer as the top recommendation):
Use an Anti-Galling Washer (Velocity Washer): The most effective way to prevent galling is to eliminate the conditions that cause it in the first place – namely, the combination of high pressure and friction while turning the nut. The Velocity Washer is a patented solution designed to do exactly this. It’s a special washer that goes under the nut and temporarily relieves the compressive load on the threads when you start to loosen the fastener. In practice, the Velocity Washer “pops” to break the static friction, allowing the nut to spin off freely without galling. By removing thread pressure during disassembly, it prevents the adhesive wear mechanism from ever getting started. This technology has been deployed on critical bolted flanges and has achieved a 100% success rate in preventing galling across all global installations. In other words, no more seized nuts and no more torch cutting during disassembly. For engineers dealing with chronic galling problems, simply switching to Velocity Washers on the studs can be a game-changer in ensuring bolts come apart smoothly.
Apply Proper Lubrication: Lubricate, lubricate, lubricate – this cannot be overstated as a galling prevention measure. A high-quality thread lubricant or anti-seize compound is one of the simplest and most effective ways to reduce galling risk. Lubricants (such as molybdenum disulfide grease, copper nickel anti-seize, PTFE-based paste, etc.) create a film between the male and female threads that reduces friction by lowering the coefficient of friction and preventing direct metal contact. This in turn keeps the heat down and stops micro-welding. ASME PCC-1 bolting guidelines specifically recommend applying lubricant to both the threads and the nut bearing surface for any critical bolted joint – doing so not only ensures you achieve the correct preload at the specified torque, but also greatly lowers the chance of galling in the process. (Be aware that using lubrication will change the torque-tension relationship: a lubricated fastener will require less torque to achieve the same tension. Adjust your tightening specifications or use the manufacturer’s K-factor recommendations when lubrication is applied. Note: your k-factors will stay the same if you choose Velocity Washer technology, as it is designed to replicate the same friction as a standard hardened washer.)
Slow Down the Installation Speed: Avoid high-speed tightening, especially with galling-prone materials. Frictional heat is a major contributor to galling, so by slowing down the turning of the nut, you allow heat to dissipate and prevent localized hot spots. In practice, this means do not use impact drivers or air wrenches on stainless steel or titanium fasteners if you can help it. Instead, tighten by hand or with a controlled-speed tool, particularly for the final snugging. If you must use power tools to run the nut down, finish the last turns slowly and watch for any binding. This is especially important when using lock nuts or thread locking inserts, which inherently create more friction as they engage. By tightening at a deliberate pace (and even pausing if you feel resistance), you’ll greatly reduce the chance of galling due to heat buildup.
Ensure Clean, Damage-Free Threads: A surprisingly common cause of galling is simply dirty or damaged threads. Always inspect your bolts and nuts before assembly – look for dirt, corrosion, or dings on the threads. Debris or rust can dramatically increase friction and lead to galling. Clean the fasteners with a wire brush or solvent if needed, and chase any deformed threads with a die or tap. If a nut does not run down easily by hand on a bolt, do not force it; that resistance means something is wrong (burrs or misthreading) which could induce galling if you apply high torque. Using new nuts on new bolts is ideal for important joints, as reused fasteners may have picked up damage. In critical applications, some engineers even specify rolled threads for bolts (instead of cut threads) because rolled threads have a smoother surface finish and harden the surface, which helps to minimize friction and galling propensity.
Optimize Fastener Material and Design: If galling is a concern, make smart choices in fastener selection. Material matters – for example, pairing a stainless steel bolt with a hardened, plated steel nut can avoid galling (the hard zinc-plated nut is much less likely to seize on the stainless bolt). In bolted joints that must use stainless steel for corrosion resistance, consider using different grades of stainless for the bolt and nut. Galling is worst when the two mating metals are the same type. Using dissimilar alloys (e.g., a 316 stainless bolt with a 410 stainless or Nitronic 60 nut) can reduce the tendency to gall – though be mindful of any strength or corrosion trade-offs. Thread pitch and fit are another design consideration: coarse threads (e.g., UNC) are more forgiving than fine threads in galling-prone situations. A looser fit (e.g., 2A/2B class) gives a bit more clearance for lubricant and less rubbing of flanks, whereas tight fine threads maximize contact and friction. If your application permits, opt for coarse threads and avoid overly snug fits when using materials like stainless steel. Finally, coatings and surface treatments can be a savior. Applying a low-friction coating or plating to fasteners can dramatically improve galling. Common solutions include PTFE or Xylan® coatings, silver plating (often used on aerospace fasteners to prevent galling), or even a simple wax finish on stainless nuts. These provide a lubricious surface and barrier that keeps the metal from binding. The bottom line: by selecting the right nut/bolt material combination and thread parameters, you can greatly mitigate galling before the wrench even comes out.
In summary, thread galling occurs when the combination of high pressure, friction, and adhesion causes two fastener components to fuse together. It’s a problem most often seen with stainless steel or similar alloy fasteners under heavy load. Fortunately, engineers have a toolbox of solutions to combat galling – from using proper lubrication and installation techniques to choosing better materials and thread designs. The Velocity Washer, in particular, stands out as an innovative solution that tackles galling at its root by removing the harmful friction under load. By implementing these strategies, you can ensure your threaded fasteners tighten smoothly and come apart when they’re supposed to, saving time, money, and headaches in the long run.
_____
Keywords:
galling, thread galling, galling prevention, stainless steel galling, titanium fastener galling, threaded fastener damage, galling causes, galling solutions, anti-galling washer, Velocity Washer, bolting reliability, ASME PCC-1 galling, bolted joint galling, fastener seizure, cold welding threads, flange joint galling, galling mitigation, galling resistant materials, stainless steel bolt seizure, bolt lubrication galling, anti-seize galling prevention, galling in high torque, galling failure analysis, galling in stainless fasteners, bolting best practices, thread lubricant galling prevention, Velocity Bolting galling solution, galling-resistant washers
References:
Bolt Depot. “Thread Galling.” Fastener Materials and Grades – Bolt Depot Fastener Information. (Accessed 2025) boltdepot.com
Fastenal Engineering Team. “Thread Galling.” The Blue Print (Fastenal Engineering Blog), July 14, 2025.blueprint.fastenal.com
Aztech Locknut Company. “Thread Galling.” Tech Article, citing Fastenal and others, 2016.aztechlocknut.com
ASME PCC-1–2019. “Guidelines for Pressure Boundary Bolted Flange Joint Assembly.” (Emphasis on thread lubrication in bolted joint procedures)velocitybolting.com
Velocity Bolting Inc. “The Galling Phenomenon: What exactly is galling?” Blog post by Ryder Britton, July 14, 2025.velocitybolting.com
Velocity Bolting Inc. “Velocity Washer – Prevent Galling.” Product information page, 2025.velocitybolting.com
Disclaimer:
Portions of this article were generated with the assistance of ChatGPT, a large language model developed by OpenAI. The content is provided for informational purposes only and does not constitute professional, legal, financial, or academic advice. The views expressed do not necessarily reflect those of the author, and readers are encouraged to independently verify any information presented.
The AI-generated content has been reviewed and edited for clarity and accuracy where appropriate.
