A2-50, A2-70, A2-80 — What Is the Difference?

The same STS304 (A2) material is divided into A2-50, A2-70 and A2-80. The trailing number is the minimum tensile strength divided by ten.

Basis: ISO 3506-1:2020 (mechanical properties of corrosion-resistant stainless steel fasteners), M1.6–M39. Proof stress is the stress at 0.2% non-proportional elongation (R_pf); d is the nominal diameter

An unmarked screw cannot be assigned a property class from its marking alone. If you need a given strength, confirm it with a test certificate.

Why the same material gives different strengths

Austenitic stainless steel cannot be strengthened by heat treatment. Unlike carbon steel, quenching does not work. Strength comes instead from work hardening during cold forming. As the head is upset and the thread is rolled, the metal structure is plastically deformed and hardens.

A2-70 and above is therefore a strength created by material and process together. It starts with material selection: the wire itself must have the strength and chemistry (work-hardening response) to reach the target class, and the forging and thread-rolling conditions must then be matched to it. Neither alone is sufficient.

Wire of the same specification also varies from lot to lot in composition and strength, which shifts the final result. We therefore verify by tensile testing on every lot and issue a lot test certificate on request.

Head geometry matters too. Countersunk and low-head screws may carry less load at the head than in the shank, so meeting the property class in the material does not guarantee the same tensile load for every shape and length. ISO 3506-1 treats such parts separately as fasteners with reduced loadability.

Why A2-80 is difficult

In the solution-annealed (soft) condition, STS304 is relatively weak; for reference, the plate standard ASTM A240 sets a minimum tensile strength of 515 N/mm² for type 304. Cold-heading wire for fasteners is covered by different standards (such as ASTM A493), and the final strength depends on the wire's delivery condition and the forming process. Even so, A2-80 (800 N/mm²) is far above the annealed level, so cold work hardening has to be pushed to its limit. Three consequences follow.

• Reduced ductility — as strength rises, elongation falls and the risk of cracking and fracture during forming increases
• Tooling and press load — die breakage and thread cracking during rolling become more likely
• Possible magnetism — if strain-induced martensite forms during cold working, normally non-magnetic STS304 can become magnetic. The degree depends on composition and the amount of cold work (details)

A2 versus A4

If the number after the hyphen is the property class, A2 and A4 denote the steel group. A2 broadly corresponds to the 304 family and A4 to the 316 family, but the standard's composition ranges are not identical to any single material designation.

A4-80 therefore means "made from a 316-family material, minimum tensile strength 800 N/mm²" — for applications demanding high corrosion resistance and high strength at the same time.

Why copper (Cu) is added to fastener-grade stainless

You may have seen STS304Cu on a specification. It is ordinary STS304 with a small copper addition, a cold-heading grade, and the material of our A2 products. XM7 (STS XM7) is also a copper-bearing cold-heading stainless, but it is a separate grade with a different composition specification, so do not treat the two designations as the same on drawings or certificates.

Screws are not machined; they are cold forged and thread rolled. Austenitic stainless, however, hardens rapidly as it is deformed, which loads the tooling during forming and makes thread cracking likely.

Adding copper slows that work-hardening rate and makes forming easier. This is why the grade is so widely used in the fastener industry.

But copper is a problem in one sector

Lithium-ion batteries. Copper entering a cell can dissolve and re-deposit during charge and discharge, potentially causing an internal short. Copper contamination is therefore treated as a controlled item for parts around the cell.

This creates demand for copper-free (Cu Free) material. As explained above, removing copper raises forming difficulty considerably — more so at higher strength classes, and more so again in the 316 family.

Cu Free requirements vary with the location in the product and the customer's own standard. Copper present as an alloying element and copper picked up on the surface during manufacture or handling are different issues, so it is best to confirm the allowable Cu content of the material and the surface contamination control criteria separately.

Where we stand

A2-70, A2-80, A4-70 and A4-80 are supplied made to order. For other requirements, such as controlled-element specifications, tell us the controlled items and acceptance levels and we will review them with you.

Selection guide

Joints under vibration or cyclic load cannot be specified by property class alone. ISO 3506-1 does not specify fatigue strength or loosening behaviour, so a separate joint design review covering preload, locking and fatigue is needed.

How we supply

Catalogue stock items are A2-50. A2-70, A2-80, A4-70 and A4-80 are supplied made to order; tell us the application, quantity and required date and we will review it with you. Request a quote