⚡ Key takeaways
- What happens: under high pressure, metal atoms on stainless thread flanks interdiffuse and cold-weld, so the joint tightens until it shears.
- Where it strikes: A2-70 or A4-80 bolts with a nut of the same grade, no lubrication, driven fast.
- Why: the passive film is only nanometres thick, thermal conductivity is poor, and friction heat causes local melting and adhesion.
- The fix: slow driving, MoS₂ or copper paste, a PTFE coating, and pairing dissimilar grades.
- The standard: GB/T 3098.6-2014, mechanical properties of stainless steel fasteners.
📑 Contents
1. What galling is
2. Why stainless is the worst offender
3. Prevention
4. What to do once it has seized
5. FAQ
1. What galling is
2. Why stainless is the worst offender
3. Prevention
4. What to do once it has seized
5. FAQ
1. What galling is
Galling, or seizing, is a sudden stall during tightening: torque spikes, and further force either shears the bolt or tears the thread. On removal the internal and external threads are found welded together.
It is not hydrogen embrittlement:
- Hydrogen causes delayed fracture after assembly.
- Galling happens during assembly.
- The fracture surface shows obvious metal transfer and a blue tint.
2. Why stainless is the worst offender
GB/T 3098.6 covers the A1, A2, A3, A4, A5, F1, F2 and F3 austenitic grades. Their characteristics:
- Thin passive film: a Cr₂O₃ layer only a few nanometres thick, which ruptures during tightening.
- Poor thermal conductivity: about one third that of carbon steel, so friction heat cannot escape.
- Rapid work hardening: the sheared surface hardens and adheres more readily.
- Same-grade pairing: an A2 bolt with an A2 nut gives the worst interdiffusion.
High-risk conditions: stainless bolts above M10 in fine pitch, no lubrication, high-speed power driving, rough thread flanks, high altitude or high temperature.
3. Prevention
| Measure | Effect | Notes |
|---|---|---|
| Drive slowly | High | ≤ 15 rpm to avoid friction heat |
| Thread lubricant | High | MoS₂, copper paste, nickel anti-seize |
| PTFE coating | High | Isolates the metal surfaces |
| Dissimilar pairing | Moderate | A4 bolt with a zinc-plated carbon steel nut |
| Better finish | Moderate | Thread Ra ≤ 1.6 |
| Coarse instead of fine pitch | Moderate | Less contact area, less friction |
4. What to do once it has seized
- Do not force it out; it will shear.
- Apply penetrating oil (WD-40 type) and leave for 30 minutes.
- Heat the nut, not the bolt, with a hot-air gun so differential expansion loosens the contact.
- If that fails, drill the bolt out and re-tap.
5. FAQ
Q. Does A4-80 gall more than A2-70? Yes. Higher strength means more work hardening at the surface.
Q. Does threadlocker prevent galling? No. Chemical adhesive increases friction rather than reducing it.
Q. Must a stainless bolt use a stainless nut? No. An A4 bolt with a class 8 carbon steel nut is common and cheaper.
🛠️ Engineering support: Toprecision supplies stainless bolts with PTFE coating or pre-applied MoS₂.
👉 Engineering services ｜ 👉 Request a technical quote
👉 Engineering services ｜ 👉 Request a technical quote