Stainless fasteners are marked with a material code and a number: A2-70, A4-80. The number is widely read as a strength class by analogy with steel, and that reading is wrong in a way that matters.
What the number is
For stainless, the number after the dash is the minimum tensile strength in MPa divided by 100. So:
| Marking | Minimum tensile | Yield (approx) |
|---|---|---|
| A2-70 | 700 MPa | 450 MPa |
| A4-70 | 700 MPa | 450 MPa |
| A4-80 | 800 MPa | 600 MPa |
Compare those with steel: class 8.8 is 800 MPa tensile, class 10.9 is 1000 MPa. An A4-80 stainless bolt is barely at the level of a class 8.8 steel bolt, and A2-70 sits below it. There is no stainless equivalent of 12.9 by this route.
Stainless fasteners are also not heat-treated to achieve their strength. The properties come from cold working, which means strength varies with how much the part was worked, and it drops in the annealed zone if the fastener is subsequently welded or exposed to high temperature.
A2 or A4: what the difference is
The two grades are different austenitic alloys:
- A2 (304 / 1.4301) — the general-purpose grade. Good resistance to atmospheric corrosion, fresh water, and many food environments. Inadequate in chloride-rich conditions.
- A4 (316 / 1.4401) — contains molybdenum. Resists chlorides far better, which is why it is the default for marine, coastal, and chemical plant use, and for anything exposed to road salt.
Both are austenitic and therefore non-magnetic in the annealed condition, but cold working makes them slightly magnetic, which is why a magnet is not a reliable way to tell A2 from A4.
The failure mode specific to stainless
Stainless fasteners gall — they cold-weld to themselves under friction. A stainless nut run onto a stainless bolt can seize so completely that the fastener must be cut off. This is much more likely with fine threads and with high tightening speed.
Practical countermeasures:
- Use a lubricant intended for stainless on assembly. This is the single most effective step.
- Slow the assembly down and avoid power-driving at high speed where possible.
- Use a differing material pair where the application allows. A stainless nut on a stainless bolt is the worst case; stainless on steel is less prone.
Choosing between stainless and coated steel
| Requirement | Better choice |
|---|---|
| Chloride exposure, marine | A4 stainless |
| General outdoor, inland | A2 stainless |
| High strength, plated | Class 8.8 or 10.9 steel, zinc plated |
| High strength plus corrosion | Not achievable in stainless — consider a coated high-class steel with the baking controlled |
See materials for density, corrosion behaviour and typical grades of each fastener material, and grades for the full property class table.