Guide
What causes stainless steel galling on threads, and how do you prevent it?
Stainless steel galling is adhesive wear in which two stainless surfaces sliding under load tear through their thin oxide films, cold-weld at the high spots and drag metal from one part onto the other. On a thread it shows up as a nut that suddenly locks partway down a bolt, a screw that will not come back out of a tapped hole, or torn, smeared flanks when the joint is finally cut apart.
By Saola Manufacturing engineering team · Published · Updated
Galling is not a defect of one part. It is a property of a material couple under a particular load, speed, surface condition and lubricant, which is why the same bolt can assemble cleanly on the bench and seize on the production line. The good news for a design engineer is that almost every lever that controls it can be written on a drawing or a purchase order: the grade and condition of each mating part, the thread class and how the thread is made, the lubricant or coating, and the installation method. This guide walks through each lever, shows a callout, and lists what to put on the RFQ so the supplier quotes the part you actually need.
On this page
Why does stainless steel galling happen on threads?
Stainless steel relies on a very thin passive oxide film, and when thread flanks slide under high contact pressure that film is wiped away and the bare metal welds together. Austenitic grades such as 304 and 316 are the worst offenders because they are soft, ductile and work-harden quickly, so a small weld grows instead of shearing off cleanly.
The welding institute TWI describes galling as two surfaces in contact seizing as a result of cold welding, and notes that it is most commonly seen when tightening bolts of stainless steel, aluminum or titanium, which depend on a passive oxide layer for corrosion resistance (TWI, what is galling). A thread is close to the worst geometry you could design for it. Contact is concentrated on the flanks of the first few engaged threads, the sliding is continuous and one-directional, and friction heat has nowhere to go in a small fastener, so the local temperature climbs as the nut is run down.
The sequence is usually the same. A burr, a chip or a high spot on one flank breaks through the oxide; the exposed metal adheres to the mating flank; the adhered lump work-hardens and plows a groove; friction rises, which raises the heat and the contact pressure; and within a fraction of a turn the joint locks. Once that happens the fastener is usually scrap, and on a tapped hole in an expensive housing the housing may be scrap too, which is why galling is an engineering problem and not just an assembly nuisance.
It also produces misleading failure reports. A NASA review of mechanical hardware recovered from the Long Duration Exposure Facility found that seizures first blamed on cold welding in space were in fact installation galling damage or improper removal, and stressed the need for effective thread lubrication on stainless fasteners (NASA NTRS, LDEF mechanical systems). If a stainless joint will not come apart in service, check how it was assembled before blaming the environment.
Which stainless pairings gall, and which are safer?
Two parts in the same annealed austenitic grade are the highest-risk couple. Risk falls when the parts differ in hardness, when one of them is a galling-resistant alloy, or when the surfaces are separated by a lubricant or a dissimilar coating.
Industry guidance from the Australian Stainless Steel Development Association puts the hardness difference needed for effective prevention at about 50 Brinell between nut and bolt (ASSDA FAQ 5, galling and its control). The same source warns that pairing 304 bolts with 316 nuts, often quoted as a cure, does not necessarily give enough hardness difference, even with a cold rolled, harder 304 thread against an annealed, softer 316 nut. The condition of each part matters more than the grade name, but even a cold worked austenitic part against an annealed one may not reach that gap. ASTM F593 bolts and ASTM F594 nuts are each specified in several conditions, from annealed to cold worked and strain hardened, so state the condition and check the hardness on the certificates rather than assuming it from the alloy group.
Where the hardness gap cannot come from the austenitic family, look at a different family for one side of the joint. A precipitation-hardening grade such as 630 (17-4 PH) or a hardened martensitic grade against an austenitic mate gives a large hardness difference. The trade-off is corrosion resistance: martensitic grades sit below the austenitic grades, and 630 is generally compared with 304 rather than 316, so check the hardened part against the service environment. Nitrogen-strengthened austenitic stainless UNS S21800, sold under the trade name Nitronic 60, was developed for galling and wear resistance and is named by ASSDA among galling-resistant grades, and bar is covered by ASTM A276; using it for the nut or a bushing alone often solves the problem without changing the bolt.
When two candidate couples need to be ranked with data rather than reputation, ASTM G196 ranks the galling resistance of material couples quantitatively, and ASTM G98 is the older screening method that it is reported to improve on for repeatability. Both standards test dry sliding, because galling usually does not occur under lubricated conditions in those rigs, so their results tell you about the bare couple and not about your lubricated assembly.
| Couple (external / internal thread) | Relative galling risk | First fix to consider |
|---|---|---|
| 304 or 316 annealed on 304 or 316 annealed | High | Anti-seize, slow installation, or change one part’s condition or alloy |
| 316 annealed on 304 annealed (“mixed grades”) | High | Grade mix alone rarely creates enough hardness gap; add lubricant |
| Austenitic cold worked or strain hardened on austenitic annealed | Moderate | Check the hardness of both parts on the certs; lubricate |
| Austenitic on hardened 630 (17-4 PH) or martensitic | Lower | Confirm corrosion of the hardened part is acceptable in service |
| Austenitic on UNS S21800 nut or bushing | Lower | Specify S21800 on the mating part only |
| Stainless screw into aluminum tapped hole | Moderate | Lubricate; consider a thread insert for repeated assembly |
| Any couple with a silver-plated or dry-film-lubricated nut | Lower | Specify the coating on one part, not both |
How do thread design and fit reduce galling?
Give the thread room and a clean, smooth flank. A class 2A/2B fit with its built-in allowance, rolled rather than cut external threads, and coarse rather than fine pitch all lower the risk before any lubricant is added.
Thread class. Under ASME B1.1, classes 1A and 2A external threads carry an allowance, a deliberate gap between the largest bolt and the smallest nut, while class 3A has none. Internal classes 1B, 2B and 3B all start at basic size, so the allowance lives on the external thread, and the choice that removes it is 3A, not 3B. On stainless that allowance is where a lubricant film lives and where a small burr can pass without seizing. Specifying 3A on a stainless bolt “for quality” removes it and can turn otherwise sound parts into a galling problem. A 3B internal thread tightens the tolerance on the hole; see the 2B vs 3B guide for when that is worth it. In metric threads the same logic applies: a 6g external thread carries a fundamental deviation, while an h position such as 6h has none.
Rolled versus cut threads. A rolled external thread has a burnished, work-hardened flank with no tearing, while a cut or ground thread can carry feed marks and micro burrs. Industry guidance from ASSDA notes that cold-rolled surfaces perform better than machined ones, and that both very highly polished and very rough flanks gall more readily, so the preferred flank sits in a moderate roughness window between a mirror polish and a coarse machined finish (ASSDA FAQ 5). Tapped holes cannot be rolled in the same way, but a sharp tap, the correct drill size and a full deburr of the hole entry remove most of the starting points.
Pitch and length of engagement. Coarse threads tolerate small damage and contamination better than fine threads and carry the load on deeper flanks, so fine threads on stainless should be a deliberate choice. Engagement longer than the joint needs only adds sliding distance, so do not lengthen a tapped hole to “be safe” on a stainless part.
Cleanliness and protection. Chips, grit and blast media act as the first high spot. Specify that threads be free of chips and burrs, and where parts are bulk-packed, protect threads from nicks in transit. A dented first thread is a reliable way to start a seizure.
Which lubricants and coatings prevent galling?
A thread lubricant or anti-seize applied at assembly is the single most effective and cheapest fix. A coating such as silver plating or a dry-film lubricant on one of the two parts is the fix when assembly lubrication cannot be trusted or is not allowed.
Under reducing friction, ASSDA points to the wide range of lubricants containing nickel, molybdenum, copper, silver or graphite, or combinations of these (ASSDA FAQ 5). Choose by service, not by habit. Nickel-based anti-seize suits high-temperature stainless joints. Molybdenum disulfide pastes suit general assembly. Copper-based compounds are widely used but are a poor choice where copper contamination or galvanic attack matters. Food, pharmaceutical and drinking-water equipment usually restrict the lubricant to an approved grade, so name that requirement rather than leaving the choice to the assembler.
Coatings change the couple instead of separating it with a paste. Silver plating on stainless nuts is a long-standing practice in high-temperature and aerospace joints, and dry-film lubricants are bonded to one thread so the assembly stays clean. Put the coating on one part only, usually the internal thread or the nut, and remember that any coating has thickness that eats the thread allowance; the guide on threads after plating explains how to dimension for it. Passivation, by contrast, removes free iron and restores the passive film; it improves corrosion resistance but is not an anti-galling treatment, as the passivation guide explains.
One trap catches many assemblies: torque. A lubricated thread converts far more of the applied torque into clamp load than a dry one, so a dry torque value applied to a joint with anti-seize can overload or yield the bolt. Whenever a lubricant is specified, the torque on the drawing or work instruction must be the value for that lubricated condition, or the joint should be controlled by clamp load or turn angle instead.
How should stainless threads be installed to avoid seizing?
Slowly, by hand or with a controlled-speed driver, and never with an impact wrench. If a stainless nut starts to bind, stop, back it off and find out why instead of forcing it.
Friction heat is a large part of the galling mechanism, so speed matters. TWI notes that stainless bolts need slower tightening than carbon steel equivalents to limit friction heating and seizure (TWI). Power drivers on an assembly line should have a speed limit written into the work instruction, and impact tools should be excluded for stainless fasteners. A nut that binds partway is a warning; adding force at that point turns a recoverable joint into a cut-off job.
Start every thread clean and straight. Cross-starting is more damaging on stainless than on carbon steel because the first damaged flank becomes the weld site. For field service, where joints are opened and closed repeatedly, ship a small supply of the specified lubricant with the spares or specify a coated nut, so the protection does not depend on a technician’s memory.
Finally, keep the assembly trial honest. A first article that assembles once on the bench with extra lubricant proves little; trial the joint with the production lubricant, production tool and production speed, and include a disassembly and reassembly if the part will be serviced.
What should the drawing and RFQ say for galling-prone stainless threads?
Name the grade and condition of both mating parts, the thread class and how the thread is made, the lubricant or coating, and the assembly torque in its lubricated condition. A note that says only “stainless” leaves every one of those choices to someone else.
- Material and condition. For example, “Bolt: ASTM F593 Group 2 (316), condition CW. Nut: machined from UNS S21800 bar per ASTM A276.” State the condition, not only the grade.
- Thread and class. “3/8-16 UNC-2A, rolled thread” for the external thread, and “3/8-16 UNC-2B” for the tapped hole, written as covered in the thread callout guide.
- Thread condition. “Threads free of chips, burrs and nicks; deburr hole entry.”
- Lubricant or coating. “Assemble with nickel anti-seize or approved equal,” or name the coating on the nut by its specification.
- Torque basis. State the torque value for the lubricated condition and the maximum driver speed, or control by clamp load.
- Verification. Ask for material certificates for both parts and a go/no-go thread gauge check of each lot.
On the supplier side, ask whether the stainless grade is a material the shop runs routinely, how threads are made (rolled, cut, tapped or milled) and how they are gauged. At Saola, turned parts are cut from bar and tube on CNC lathes at our lead CNC plant, and thread fit is checked with go/no-go gauges before a lot ships, as described on the CNC turning page. Stainless bar is not on that page’s published bar list today, so for a stainless turned part, say so on the RFQ and ask before assuming. The surface finishing menu is a fixed list of eight finishes; passivation, silver plating, nitriding and dry-film lubricants are not on it, so those would have to be named and arranged separately. Material certification is available on request, and ISO 9001 certification is in progress, not held. Send the drawing with the thread and lubricant notes through the quote form.
Frequently asked
Does using a 304 nut on a 316 bolt stop galling?
Usually not by itself. ASSDA notes the hardness difference is not necessarily enough, even with a cold rolled 304 thread against an annealed 316 nut. It is the hardness difference, not the grade name, that does the work. Change the alloy of one part, or add a lubricant.
Can a galled stainless bolt be reused?
No. Galled flanks carry torn and transferred metal that will seize again and give unpredictable clamp load. Replace the fastener, and inspect a tapped hole with a go/no-go gauge before trusting it again.
Is galling the same as thread seizing from corrosion?
No. Galling happens during assembly or disassembly as the threads slide; corrosion seizure builds up over time in service. They can look alike once a joint is stuck, so check when the problem started and whether there is corrosion product in the thread.
Does electropolishing or passivation make threads gall less?
Neither is an anti-galling treatment. Passivation restores the passive film for corrosion resistance, and a very smooth, highly polished flank can actually gall more readily. Use a lubricant, a coating or a different couple for galling.
Are fine threads worse than coarse threads for galling?
Generally yes. Fine threads have shallower flanks and less room for a burr or debris, and a damaged flank is more likely to start a seizure. Use fine pitch on stainless only where the design needs it, and lubricate it.
Sources
- FAQ 5: Galling and its Control, Australian Stainless Steel Development Association, accessed October 2026
- What is galling?, TWI, accessed October 2026
- LDEF Mechanical Systems (Spear and Dursch), NASA Technical Reports Server, 1992
- ASTM G196-24, Standard Test Method for Galling Resistance of Material Couples (scope), ASTM International, 2024
- ASTM G98-23, Standard Test Method for Galling Resistance of Materials (scope), ASTM International, 2023
- ASTM F593-22, Standard Specification for Stainless Steel Bolts, Hex Cap Screws, and Studs (scope), ASTM International, 2022
- ASTM F594-22, Standard Specification for Stainless Steel Nuts (scope), ASTM International, 2022
- ASTM A276/A276M-25, Standard Specification for Stainless Steel Bars and Shapes (scope), ASTM International, 2025
- ASME B1.1-2024, Unified Inch Screw Threads (UN, UNR and UNJ Thread Forms), ASME, 2024