ISO 3506 Decoded: What A2-70 and A4-80 Actually Promise
A2-70 reads like a strength spec, and it is — but only partly. Understanding ISO 3506 prevents both overpayment and surprise failures.
Breaking down the code
The first character group is the alloy family: A2 is austenitic 304-class chromium-nickel steel; A4 is 316-class with molybdenum for chloride resistance. The number is the tensile strength in decanewtons per square millimeter ×10: -70 means 700 MPa Rm, -80 means 800 MPa. So A4-80 is stronger than A2-70, and also more corrosion-resistant — the two axes are independent, not nested.
What the standard does not promise
- Yield strength: -70 class guarantees Rp0.2 of 450 MPa; design margin must respect it, not just Rm.
- Corrosion life: ISO 3506 grades alloy, not environment — salt-spray hours are not in the standard.
- Magnetic behavior: cold work makes A2 partly magnetic, surprising inspectors who use magnet tests for sorting.
Specifying smartly
Choose A4 wherever chlorides appear; pay for -80 only when the load case needs it — higher-strength austenitic stock is less ductile. Topprecision supplies A2-70, A2-80, A4-70 and A4-80 across its stainless range with 3.1 certificates naming the actual grade and lot.
