ISO 3506 Mechanical Properties of Stainless Steel Fasteners

ISO 3506 covers corrosion-resistant stainless steel fasteners and is published in three parts: Part 1 bolts, screws and studs, Part 2 nuts, and Part 3 set screws. It is what makes a designation such as A2-70 or A4-80 a specification rather than a description of the material. The letter fixes the steel group and the number fixes the strength: A groups are austenitic (A2 is the 304 family, A4 the 316 family), while C covers martensitic and F ferritic stainless.

The number in the grade is tensile strength divided by ten, so A2-70 is 700 MPa and A4-80 is 800 MPa — the same written form as a carbon steel class, but a completely different scale. A4 resists chlorides better than A2, which is why it is the marine and chemical choice, but A4 is not stronger by virtue of being A4: at the same grade suffix the two are quoted to the same strength. There are also size limits. The 70 and 80 grades are normally only qualified up to M24, and A2-50 exists precisely because small austenitic screws cannot reach 700 MPa.

This standard fixes mechanical properties and test methods rather than dimensions, so there is no size table here. The head shapes and thread sizes it is applied to come from a separate product standard; the strength data itself — tensile, yield, hardness and the matching nut class — is on the class pages linked above.

Never read a stainless grade as an equal-strength substitute for a carbon steel class. A4-80 at 800 MPa only reaches the tensile strength of class 8.8, and A2-70 sits below it; nothing in this standard approaches the 1220 MPa of class 12.9. Stainless also fails differently. Austenitic grades cannot be hardened by heat treatment, only by cold work, so the strength actually achieved depends on how the part was formed and on its diameter. Note that the test methods reference ISO 898-1: the two standards share test procedures but define different property systems, and that shared reference is a common source of confusion.