The short answer: galling is cold welding. Tightening pressure breaks the thin oxide film that makes stainless "stainless," the bare metal high points shear and lock together, and the nut can weld itself to the bolt before the joint ever reaches its designed preload. Four controls prevent most of it: lubricate the threads, slow the driver down, keep threads clean and undamaged, and put a hardness difference between bolt and nut. "Just use 316" is not on the list — the A2-bolt-with-A4-nut folklore is weaker than its reputation.
Galling is the one failure mode where the parts pass every dimensional and material inspection and still scrap themselves in assembly. It deserves a line in the assembly specification, not just a hope.
What actually happens between the threads
Stainless steel survives by its passive chromium-oxide film. Under the contact pressure and sliding of thread engagement, that film is wiped off at the high points, and clean, ductile metal meets clean, ductile metal. The British Stainless Steel Association describes what follows as severe adhesive wear — a solid-phase welding process where load and relative motion transfer material between the surfaces until the interface locks.[1] The Industrial Fasteners Institute's guidance reaches the same conclusion from the fastener side: galling is most prevalent in stainless steel, aluminum and titanium precisely because these alloys self-generate protective oxide films that are then broken during tightening.[2]
Two details explain why the problem shows up unpredictably:
- Austenitic stainless is ductile and work-hardens at the surface. The smeared metal does not flake away cleanly; it builds up, raising friction with every turn until the joint seizes.[1] [3]
- The nut thread is usually the rougher surface. Bolt threads are rolled — burnished, with the grain following the thread form. Tapped and nut threads are cut. When a pair galls, the internal thread is the more common aggressor, which is why galling can appear and disappear between nominally identical lots.[2] [4]
What a galled joint looks like
The signature is a torque reading that climbs sharply mid-tightening while preload does not. The installer sees the wrench approach target and believes the joint is tight; in reality the torque is being consumed by a welding interface.[5] Keep turning and the bolt twists off or the threads strip; stop in time and the fastener may still come apart with visibly torn, smeared flanks and transferred metal.
That separates galling from a plain over-torque failure: an over-torqued bolt turns freely until it fractures in tension or torsion, with clean thread flanks. A galled bolt fights back long before its strength limit — and a half-galled joint can pass final inspection while carrying almost no preload, then fail later in service.[5] [6]
Removal is the expensive part. Severely seized pairs rarely come apart; the maintenance reality is cutting the nut or stud off and replacing both.[6] Threads with any galling history should be replaced, not cleaned up and reused.
The four controls that work
1. Lubricate
ISO 3506-2 puts it in the standard text itself: lubrication of stainless steel nuts is strongly recommended to avoid galling during tightening, with thread damage, high preload and high tightening speed named as the aggravating factors.[7] Effective choices are anti-seize compounds based on nickel, copper, molybdenum disulfide or PTFE, or a factory-applied wax with a declared friction coefficient.[3] [8]
Three qualifications matter:
- Lubrication changes the torque math. Fastenal's torque-tension guidance for stainless bolting puts the nut factor at roughly 0.35 dry and 0.16 lubricated — lubrication cuts the torque needed for the same preload by more than half.[9] A torque value copied from a dry-stainless chart onto a lubricated joint can overload the fastener. Take the torque from the lubricant or fastener manufacturer's data for the actual condition, and verify critical joints with a torque-tension test rather than a universal percentage reduction.[6] [9]
- Match the compound to the service. Graphite-bearing lubricants are a galvanic and high-temperature corrosion risk on stainless; sulfur-containing compounds such as MoS2 are restricted in some high-integrity bolting because of stress-corrosion-cracking concerns; food and cleanroom service rules out most anti-seize pastes entirely.[3] [8] [10]
- Lubricant is a specification item, not an assembler's preference. If the PO is silent, each batch may arrive with a different friction condition and the torque table becomes fiction.
2. Slow down
Friction heat rises with installation speed, and heat accelerates galling. High-RPM drivers, impact tools and pulse tools are the usual cause when an assembly line suddenly starts seizing fasteners that hand tools installed without trouble.[2] [8] [11] The fix is a slow, continuously applied tightening — and it costs nothing.
3. Control the threads
Coarse threads in a standard 2A/2B (or 6g/6H) fit tolerate handling damage far better than fine threads or tight fits.[11] Dirt, blasting grit and dents from bulk handling are all galling starters, so threads should be clean and inspected before assembly — a quick field check from ASSDA: if a cloth snags when wiped along the thread, the surface is rough enough to gall.[3] Rolled external threads outperform cut ones; specify rolled threads and reject burred lots rather than fighting them at the bench.[4]
4. Put a hardness difference in the pair
Adhesive wear thrives between identical surfaces. ASSDA's guidance, backed by ASTM G98 galling-resistance testing, puts the working threshold at roughly a 50 Brinell hardness difference between the mating threads.[3] [12] The standards have absorbed this: EN 1090-2, the European execution standard for steel structures, names dissimilar grade pairings — its example is an A4-50/A4-80 bolt-nut combination — as an approved anti-galling method, with proprietary high-work-hardening alloys for severe cases.[13]
This is also where the "A2 bolt, A4 nut" advice belongs. Mixing 304 and 316 sometimes helps and sometimes does not, because the two grades overlap in hardness; what protects the joint is the hardness gap, not the grade labels.[3] [6] [11] The two grades differ in corrosion resistance, not in galling resistance — see 304 vs 316 Stainless Steel Fasteners for what the upgrade actually buys. A class-80 bolt against a softer class-50 nut of the same grade delivers the gap deliberately — and even here, reports on class-80 pairs are mixed, so lubrication stays the primary control.[1] [3]
For severe or repeated service — instrument fittings, aerospace, vacuum — the established answer is surface engineering: silver-plated nut threads are standard practice on stainless tube fittings precisely to prevent galling, and PTFE-based dry-film coatings achieve the same effect with a controlled, low friction coefficient.[11] [14]
Setting torque for stainless
Stainless torque tables are lower than alloy-steel tables for the same diameter, and they should be: class 70 proof stress is 450 MPa against 640 MPa for class 8.8 (see A2-70 and A4-80: How to Read Stainless Fastener Markings). Two more points belong in the assembly spec:
| Condition | Nut factor K | Typical clamp-load basis |
|---|---|---|
| Dry stainless | ≈ 0.35 | Deliberately derated, ~40% of yield, because dry assembly risks galling |
| Lubricated stainless | ≈ 0.16 | Up to ~75% of yield |
Both sets of figures are from Fastenal's stainless torque-tension guide.[9] The gap between the rows is the point: whether the threads are dry, waxed or pasted changes the joint more than any other variable on the table. Never hold torque constant and change the lubricant.
The same logic governs disassembly. Back a stainless nut off slowly and steadily; a galled joint forced with an impact gun will usually take the stud with it.
When it still happens
If torque spikes before target, stop. Forcing past the spike converts a cleanable thread into a twisted-off stud. Cut and replace the pair, then fix the cause — lubricant, speed, thread condition or hardness pairing — before the next one. A joint that galls repeatedly is a specification problem, not an operator problem.
What to put on the order or assembly spec
- Bolt and nut grade and class, with the hardness pairing if it is deliberate (e.g. a class-80 bolt with a class-50 nut).
- Thread series and fit: coarse where possible, standard tolerance class, rolled external threads.
- Lubricant: compound type, or "factory-waxed with declared friction coefficient," and the torque values that go with that condition.
- Tooling: maximum driver speed; no impact or pulse tools.
- Thread condition: clean, undamaged, protected from grit in storage and handling.
- Reuse policy: replace any pair with a galling history.
Fighting seized threads on a current job? Send the size, grade, finish and assembly method. We will quote the fasteners with a lubricant condition and torque guidance that match the joint.
References
- British Stainless Steel Association. Galling resistance of stainless steels. Mechanism definition, ductility and work-hardening factors, class-80 pairing behavior.
- Industrial Fasteners Institute, Standards Book (6th ed., p. B-28), as reproduced by Greenslade & Co. Thread Galling. Oxide-film mechanism, installation-speed effect, internal-thread aggressor.
- Australian Stainless Steel Development Association. Technical FAQ 5: Galling and its Control. Hardness-difference threshold, surface-condition checks, lubricant selection and caveats, class-70/80 reports.
- Euro Inox / Nickel Institute. Erection and Installation of Stainless Steel Components. Rolled versus machined threads and grain direction.
- AISC. Design Guide 27: Structural Stainless Steel. Galling in structural bolting, including friction masking preload at the wrench.
- Fabory. The Seizing of Stainless Steel Fasteners. Assembly behavior, mixed-grade assessment, seizure-removal and torque-verification advice.
- ISO. ISO 3506-2:2020 — Stainless steel nuts with specified grades and property classes, clause 7.3. Standard text recommending lubrication; named risk parameters.
- Bossard. Prevent Galling While Tightening Stainless Steel Fasteners. Lubricant chemistry limits, including graphite at temperature; driver tooling.
- Fastenal Engineering. Torque-Tension Relationship for Stainless Steel ASTM A193/A193M B8, B8M Class 1. Nut factors dry vs lubricated, derated clamp-load basis, slow continuous torque.
- U.S. Nuclear Regulatory Commission. ML15352A084. MoS2 and sulfur-containing lubricants restricted on high-strength bolting due to stress-corrosion-cracking concern.
- Fastenal Engineering. Galling. Coarse-thread and 2A/2B guidance, driver-speed and locknut risk, PTFE-based coatings.
- ASTM International. ASTM G98-17 — Standard Test Method for Galling Resistance of Materials. The threshold-stress test behind the hardness-difference rule.
- BSI. BS EN 1090-2:2018 — Execution of steel structures and aluminium structures, Part 2, clause 8.9. Dissimilar-grade pairing and high-work-hardening alloys as anti-galling methods.
- Swagelok. Tube Fitter's Manual (MS-13-03). Silver-plated female nut threads as standard anti-galling practice.
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