Toprecision — Precision Fastening & Connection Solutions

A2-70 reads like a strength spec, and it is — but only partly. Understanding ISO 3506 prevents both overpayment and surprise failures.

Bottom line: for most Precision Screws applications the decision comes down to load, environment and assembly method — the Precision Screws guide walks through the selection.

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.

Key specifications at a glance

Need help with fastener standards and specs?

Toprecision manufactures to ISO, DIN, ASME and GB standards, and provides full RoHS/REACH documentation for export. Send the standard reference and drawing — we will confirm the spec and supply a cross-reference table. Request standards support.