ISO 3506 — property classes for corrosion-resistant stainless steel fasteners

Scope — and what it does not cover

ISO 3506 sets the mechanical properties of fasteners made from corrosion-resistant stainless steel — Part 1 bolts, screws and studs, Part 2 nuts, Part 3 set screws and similar parts not loaded in tension, Part 4 tapping screws. It defines the steel groups (austenitic A1–A5, martensitic C1, C3 and C4, ferritic F1) and the property classes that follow the hyphen. It does NOT cover carbon and alloy steel fasteners (ISO 898-1 and -2), corrosion resistance or service life in any particular environment, passivation (ISO 16048), thread tolerances (ISO 965), or the dimensional standard of the part.

Standards are copyrighted documents. We explain scope, intent and practical application in our own words; obtain the current edition from ISO, ASTM, DIN or your national standards body.

What actually bites in production

• 1The designation is two separate facts joined by a hyphen. The letter and digit identify the STEEL; the number after the hyphen is one tenth of the nominal tensile strength in MPa. A4-80 is a molybdenum-bearing austenitic steel at 800 MPa nominal, A2-70 is a 304-type steel at 700 MPa.
• 2A2 and A4 gain nothing from heat treatment. They are austenitic and stay austenitic through any normal cycle, so classes 70 and 80 are produced by cold working the wire — and a solution anneal makes the part softer, not stronger.
• 3The C classes are the opposite case. C1, C3 and C4 are martensitic stainless steels that really are quenched and tempered, which is why C1-110 exists and A2-110 does not.
• 4Acceptance is by tensile testing: tensile strength, 0.2 % proof stress and elongation. Hardness on austenitic classes is an indicative check, not the class criterion, so a hardness reading alone never demonstrates A2-70.
• 5The cold-worked classes are tabulated only up to a diameter limit. Above it the properties are agreed between buyer and manufacturer instead of read from the table — which is exactly the size range where large anchor bolts live.
• 6Austenitic stainless galls. A stainless nut running on a stainless bolt of the same group seizes far more readily than a plated steel pair, so lubricant or a different mating material is a design decision that belongs on the drawing.

What to write on the drawing or PO

• The full designation — steel group and class together, "ISO 3506-1 A4-80" — with the thread designation and pitch. "316 stainless bolt" is a material wish, not a specification.
• The dimensional standard separately (for example ISO 4014, ISO 4017, ISO 4032), because ISO 3506 fixes properties only.
• Which tests are required per lot and on what: tensile on the full-size fastener or on a machined specimen, proof load for nuts, elongation.
• Whether passivation is required after forming or machining, with its own standard (ISO 16048), and whether a free-iron check is expected.
• Any magnetic permeability limit. Cold-worked A2 becomes slightly magnetic; if that matters to the application it has to be written down, because the property class does not control it.

What goes wrong when it is cited loosely

• Writing "A2-70" and expecting corrosion performance from it. The class describes strength; corrosion behaviour comes from the steel group and the environment, and A2 is not a marine material.
• Asking a heat treater to harden A2 or A4 fasteners. There is no martensite transformation available, so the request means either the material or the class was specified wrongly.
• Mixing systems on one line. "A2-70 class 8.8" puts an ISO 3506 designation and an ISO 898-1 property class on the same part; only one of them can govern.
• Assuming an A4 nut on an A4 bolt is the safe pair. Same group on both sides is the classic galling combination, and it usually shows up during assembly, not in the lab.