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Stainless steel screw galling: causes, prevention and on-site fixes for seized threads

2026-09-16

Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.

Why stainless steel is especially prone to galling

Austenitic stainless steels (304/A2, 316/A4) conduct heat at roughly one third the rate of carbon steel and work-harden readily. During tightening, the thread flanks see localised high pressure, micro-slip and momentary heat, producing cold welding between mating surfaces — the phenomenon known as thread galling. ISO 3506 governs material and mechanical properties but says nothing about installation friction, which is why projects so often report 'the material certificate was fine, yet the bolt would not go in'. The risk actually rises with longer thread engagement, larger sizes (M10 and up) and perfectly clean, oil-free threads. The reason is that austenitic stainless has no protective oxide layer that reforms instantly under sliding contact, and its low thermal conductivity means the heat generated at a thread flank cannot dissipate quickly; the contact zone therefore reaches a temperature high enough for the asperity tips to weld, and once a weld forms it tears material from one flank and deposits it on the other. That deposited material makes the thread tighter, which raises friction, which raises temperature, which accelerates the process — a self-reinforcing loop that explains why a fastener that turns freely for five turns can seize solid on the sixth. Some practical patterns are worth knowing. First, galling is more likely on larger diameters, because the same clamping force is spread over a longer thread flank and the sliding distance per turn is greater. Second, it is more likely on long engagement, which is why a stud threaded 3d deep into a tapped hole is a classic problem and a short 1d engagement rarely seizes. Third, it is more likely on a bolt and nut of identical material from identical stock, because identical crystal structure and hardness means there is no hardness differential to keep the surfaces apart. Fourth, it is more likely with power tools at high speed, and least likely with a slow hand wrench.

Three keys to prevention: slow speed, lubrication, sensible pairing

(1) Control the speed: keep electric-tool run-in below 100 rpm — high-speed friction is the primary trigger. (2) Apply anti-seize compound: molybdenum-disulphide, PTFE or copper-based pastes cut the friction coefficient by roughly 30-50% and suppress micro-slip adhesion at very low cost, making them the most cost-effective measure of all. (3) Pair sensibly: match nuts and bolts from the same batch and material, avoid overly tight thread fits, and lubricate throughout when engagement exceeds 2d. Before batch installation, trial-fit 3-5 sets to confirm a smooth, free-running feel with no binding. The anti-seize options are not interchangeable, so it helps to know what each is for. Dry, unlubricated A2 running against A2 gives a friction coefficient of roughly 0.20-0.35 with a wide scatter and the highest galling risk; the same pair with a molybdenum-disulphide paste falls to about 0.10-0.15 and seizes very rarely; a PTFE-based paste gives similar friction with the cleanest appearance and is the usual choice where the joint will be visible or where a molybdenum residue would be objectionable; a copper-based paste is intended for high-temperature joints rather than for the friction reduction itself, and should be avoided in food, potable-water and marine splash applications because copper accelerates galvanic attack on aluminium and on nearby steel. A wax-based or silicone anti-seize is adequate for light-duty electrical and instrument work where contamination must be minimal. Since lubrication lowers friction, it also raises the preload achieved for the same applied torque — often by 20-30% — so any torque figure taken from a dry-thread table should be reduced when anti-seize is applied, or the joint may be over-tightened into yield. That is a point worth putting on the method statement, because a fitter who adds lubricant to solve a seizing problem can inadvertently create a preload problem. Where the design permits it, an even more robust solution is to eliminate the stainless-on-stainless couple: a 316 bolt into a 304 nut, or a coated stainless fastener, or a stainless bolt with a bronze or nylon-insert nut, all reduce the tendency to cold-weld without relying on the fitter to remember the lubricant.

How to recover a joint that has already seized

The moment torque resistance spikes, stop — forcing it widens the cold-welded area, and the usual outcome is a broken bolt stuck in the hole. The right response: apply penetrating lubricant and let it soak for 10-15 minutes, then work the fastener slowly back and forth two or three times with hand tools to clear debris before continuing. A fully seized small part can be removed with a screw extractor; precision or expensive substrates may need wire EDM or spark erosion, which costs far more than prevention. Always replace with brand-new fasteners and re-apply anti-seize on reassembly. Consider what a seizure actually costs on a Hong Kong site. A seized M10 stainless bolt on a facade bracket is not a fastener problem, it is an access problem: the bracket often sits behind a finished panel, so recovery means scaffolding or a gondola, a technician with a drill and extractor set, and a risk of damage to the adjacent anodised aluminium. Realistically a single seized fixing can consume two to three hours of a two-person crew plus access costs, which dwarfs the price of the bolt many times over. That is the economic case for spending a few minutes and a small amount of anti-seize at the assembly stage. There are also a few recovery techniques worth avoiding. Do not apply heat: thermal expansion will not free a cold-welded thread, and on stainless it can sensitise the material and promote corrosion. Do not use an impact wrench to shock it free, as this usually shears the shank below the head and leaves a harder problem. Do not hammer the head, which deforms the bearing surface and may crack the substrate. Do not use a carbon-steel extractor and leave fragments in the hole, because a later corrosion cell forms around the remains. Where the fastener must be removed from a machined or hardened component, wire EDM is the least damaging option even though it costs more per hour, because it removes material without side loads. After any recovery, inspect the tapped hole for damaged threads, chase it with a tap if necessary, and treat the assembly as a fresh installation with lubricant applied.

Common pitfalls and acceptance points for stainless assemblies

Pitfall one is assuming stainless fasteners need no assembly preparation. Galling is not a material defect and it will not appear in a certificate; it is entirely a function of how the parts are installed. Pitfall two is using a power driver because it is faster. A high-speed driver is the single largest cause of galling in the field, and switching to a low-speed driver or a hand wrench for the final turns is the simplest fix available. Pitfall three is lubricating the bolt but not the nut or the tapped hole. The thread flank that matters most is the engaged one, and if only the bolt is coated the nut may still seize. Apply the compound to the engaged threads, not to the shank. Pitfall four is ignoring the substrate material. A stainless bolt into an aluminium or a galvanised steel tapped hole introduces a galvanic couple, and the bolt may seize with corrosion products rather than by cold welding; use an isolating washer and a suitable lubricant, or change one of the materials. Pitfall five is reusing a fastener that has already been partly seized. The flank geometry has been altered and the second installation will be tighter. At acceptance, the practical checks are: confirm the specified grade and that the fasteners are a matched set; confirm the anti-seize product is the correct type for the service (not copper in a marine or food environment); verify the torque value was adjusted for the lubricated condition; and inspect a sample of completed joints for smooth thread run-in, full nut engagement and no visible thread galling or pick-up. Where the assembly will be inspected periodically, release a few sample joints and record the breakaway torque, so a trend of increasing friction can be spotted before it becomes a seizure. Need anti-galling treated stainless screws or installation advice? Contact JIALI Industrial (164.hk) with your application for selection and samples.

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