A2-50, A2-70, A2-80 — What Is the Difference?
What stainless property classes mean, and how to choose. Why the same STS304 can have different strengths.
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.
| Property class | Min. tensile strength | 0.2% proof stress | Notes |
|---|---|---|---|
| A2-50 | 500 N/mm² min. | 210 N/mm² min. | Soft (not cold worked). Unmarked distributor stock is often this class |
| A2-70 | 700 N/mm² min. | 450 N/mm² min. | Cold work hardened. Our main line |
| A2-80 | 800 N/mm² min. | 600 N/mm² min. | Developed in house |
Basis: ISO 3506-1 (mechanical properties of corrosion-resistant stainless steel fasteners), for nominal diameters up to M24
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.
Why A2-80 is difficult
The intrinsic tensile strength of STS304 is roughly 515–740 N/mm². A2-80 (800 N/mm²) sits above the upper bound of the material's own strength, so work hardening has to be pushed to its limit. Three consequences follow.
- Reduced ductility — as strength rises, elongation falls and the risk of brittle fracture increases
- Tooling and press load — die breakage and thread cracking during rolling become more likely
- Magnetism — strain-induced martensite makes non-magnetic STS304 magnetic (details)
A2 versus A4
If the number is strength, the letter is the steel group.
| Group | Steel type | Character |
|---|---|---|
| A2 | STS304 family (AISI 304 / SUS304) | Austenitic. Adequate corrosion resistance in general environments |
| A4 | STS316 family (AISI 316 / SUS316) | Molybdenum added for pitting resistance. Better in chloride and marine service |
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 or XM7 on a specification. It is ordinary STS304 with a small copper addition, a cold-heading grade.
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.
Where we stand
A2-70 and A2-80 are in regular production. For other requirements — the A4 (316) family, or controlled-element specifications — tell us the controlled items and acceptance levels and we will review them with you.
Selection guide
| Situation | Recommended |
|---|---|
| General fastening, low strength demand | A2-50 |
| Structural joints, vibration or cyclic load | A2-70 |
| Weight reduction (size down) | A2-70 / A2-80 |
| Marine or chloride environment | A4 family (316) |
| High corrosion resistance and high strength | A4-80 — please enquire |
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
Bolted joints are sized on proof stress. On that basis, an A2-70 M3 screw carries about 22% more load per fastener than an A2-50 M4 screw. That means fewer fastening points, or a smaller size, saving space and weight. Reviewing the property class early in design pays off.