Technical guide
A2 or A4 stainless – and when duplex?
A2 (type 304) is fine indoors and in mildly corrosive conditions. Choose A4 (type 316, with 2–3 % molybdenum) wherever chlorides are present: coast, splash zones, chemicals and cleaning agents. For permanent immersion in seawater or hot chlorides, move up to duplex or super duplex.
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Short answer
- A2 – 18 % Cr, 8 % Ni. Indoor, dry, food-grade without chlorides.
- A4 – adds molybdenum. Coastal, chemical, cleanroom and offshore topsides.
- Duplex / super duplex – for submerged and hot-chloride service.
- Strength: property class 50, 70 or 80 – the number is the tensile strength ÷ 10 MPa.
Comparison
A2 versus A4.
| A2 | A4 | |
|---|---|---|
| Comparable steel | Type 304 | Type 316 |
| Chromium | 18–20 % | 16–18 % |
| Nickel | 8–11 % | 10–14 % |
| Molybdenum | – | 2–3 % |
| PREN (Cr + 3.3 × Mo, from composition) | approx. 18–20 | approx. 23–28 |
| Chlorides (sea air, salt, de-icing, chemicals) | Not recommended – pitting | Suitable for most exposures |
| Typical use | Indoor, machinery, dry outdoor | Coastal, offshore topsides, chemical, pharma, cleanroom |
Strength
Property classes to ISO 3506-1.
The same for A2 and A4. Classes 70 and 80 are cold worked and available in smaller diameters; ask us for large sizes.
| Property class | Min. tensile strength Rm | Min. 0.2 % proof stress Rp0.2 | Condition |
|---|---|---|---|
| 50 | 500 MPa | 210 MPa | Soft – as machined or hot formed |
| 70 | 700 MPa | 450 MPa | Cold worked – the standard for stainless bolts |
| 80 | 800 MPa | 600 MPa | Highly cold worked – highest strength |
Beyond A4
When 316 is not enough.
- 01Duplex (UNS S31803, F51)Much higher strength than 316 and better resistance to pitting and stress corrosion cracking. For offshore, seawater-cooled and chloride-rich process service.
- 02Super duplex (F53 / UNS S32750, F55 / UNS S32760)Higher chromium, molybdenum and nitrogen again – the usual choice for submerged seawater service.
- 03Nickel alloys (625, 825, 925)For hot acids, sour wells and the most aggressive media. We supply header plugs and specials in these alloys.
- 04A1 – free-machiningContains sulphur for machinability, which lowers corrosion resistance. Not offered with EPK cleanroom treatment.
FAQ
Questions we get every week.
- Is A4 the same as 316?
- A4 is the ISO 3506 steel group that covers 316-type stainless (molybdenum-alloyed austenitic steel). In daily use the names are interchangeable; the certificate shows the actual grade.
- Are stainless bolts magnetic?
- Annealed A2 and A4 are practically non-magnetic. Cold working, as for class 70 and 80, can make them slightly magnetic. That says nothing about the quality.
- Can I put stainless bolts in galvanized steel?
- Yes in most atmospheres: the stainless bolt is small compared to the zinc surface, so contact corrosion stays limited. In wet or immersed conditions, insulate.
- Why do my stainless bolts seize?
- Galling – stainless on stainless cold-welds under load. Use an anti-seize lubricant, tighten more slowly, or choose a coated nut.
- Which stainless for cleanrooms?
- A2 or A4 with EPK treatment: electropolished, Kolsterised, cleaned to Grade 2 or 4 and vacuum-packed. See our cleanroom page.
- What does A4-80 mean?
- Steel group A4 (316-type), property class 80: at least 800 MPa tensile strength.
Next step
Configure, download or ask.
Reviewed October 2026 by HBS Solutions. Based on ISO 3506-1, ASTM A193 (B8, B8M) and published composition data. Values are minimums from the standard; always check the edition and options named in your project specification.
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