Stainless Steel Fasteners: 304 vs 316 vs 410 — When to Use Each
"Stainless" is not a single material. A 304 bolt rusts under salt spray within months; a 316 bolt survives coastal infrastructure for decades; a 410 bolt is hard enough to drill but pits badly in chlorides. Picking the wrong stainless grade is the most common fastener-selection mistake we see in returns — and it is invisible until the bolt starts weeping orange.
The short answer: 304 (A2-70) for indoor and atmospheric use, 316 (A4-80) for marine, coastal and chloride exposure, 410 only where you need a heat-treatable, magnetic, higher-strength stainless in dry service.
The three stainless fastener grades at a glance
| Grade (AISI / EN) | Class | Tensile (MPa) | Chloride resistance | Magnetic | Cost index |
|---|---|---|---|---|---|
| 304 / A2-70 | Austenitic 18-8 | 700 | Poor (pits in salt) | Slightly | 1.0x |
| 316 / A4-80 | Austenitic + Mo 2% | 800 | Good (marine OK) | Slightly | 1.6x |
| 410 | Martensitic | 1100–1400 (HRC 35) | Poor (pits rapidly) | Yes (strong) | 0.9x |
| 321 | Austenitic Ti-stabilized | 700 | Good (high temp) | Slightly | 1.8x |
What the grade names actually mean
A2-70 (304): austenitic group 2, cold-worked to 700 MPa minimum tensile. This is the default stainless bolt you buy off the shelf. It handles atmospheric corrosion, food contact and most indoor environments — but it is not marine hardware.
A4-80 (316): group 4, molybdenum-bearing, 800 MPa. The 2% Mo is the difference between surviving coastal air and pitting within a year. Use it for dock hardware, pool equipment, coastal solar mounts, and anywhere de-icing salt hits the joint.
410: martensitic, heat-treatable to HRC 35–40. It is the "stainless that can be hardened" — used for blades, valve parts and high-wear fasteners — but its corrosion resistance is only marginally better than plain steel. Never put it outdoors.
The three mistakes that cause returns
1. Using 304 by the ocean. Salt spray + 304 = rust streaks within 6–12 months. The premium from 304 to 316 is about 60% on the fastener, but it is the difference between a 2-year failure and a 20-year service life.
2. Confusing "magnetic" with "not stainless." A2 and A4 austenitic stainless are slightly magnetic after cold heading — that is normal and not a sign of fake material. Only 410/430 martensitic grades are strongly magnetic. Do not reject 304 bolts because a magnet sticks.
3. Using 410 in wet service. 410 is hard, cheap and magnetic — and it pits. If the joint sees moisture, switch to 304 or go full 12.9 alloy-steel with zinc-aluminum coating instead.
Toprecision supplies A2-70, A4-80 and precision-machined 410 stainless fasteners, including micro M0.6–M4 screws and custom turned parts. Send the service environment — coastal, industrial, food, high-temperature — and we will recommend the grade, not just the catalog option. Request a stainless-grade consult with salt-spray and hardness test reports.
