Scope
ISO 3506 specifies mechanical properties — tensile, yield, hardness, ductility — for stainless steel fasteners (bolts, screws, studs, and nuts) in metric sizes. It is structured in multiple parts:
- ISO 3506-1 — Bolts, screws, studs (the part most often referenced)
- ISO 3506-2 — Nuts
- ISO 3506-3 — Set screws and similar non-tensioned fasteners
- ISO 3506-4 — Tapping screws
The standard does not specify dimensions; pair it with a dimensional spec (DIN 931, DIN 933, DIN 934, etc.) for the geometry.
How to read a stainless callout
A typical stainless metric callout looks like:
ISO 3506-1 - M16 × 80 - A4-80This decodes as:
- ISO 3506-1 — bolt-grade stainless steel mechanical properties
- M16 × 80 — 16 mm diameter × 80 mm length
- A4 — austenitic stainless, molybdenum-bearing (≈ 316)
- 80 — minimum tensile strength = 800 MPa (≈ 116 ksi)
The letter-number pair after the size is what matters. Letter = alloy group; number × 100 = min tensile in MPa.
Alloy groups
| Group | Material family | Approximate inch equivalent | Notes |
|---|---|---|---|
| A1 | Free-machining austenitic | 303 | Improved machining; less corrosion resistance than A2 |
| A2 | Standard austenitic | 304 / 18-8 | The everyday stainless; F593 Group 1 counterpart |
| A3 | Stabilized austenitic | 321 / 347 | Niobium- or titanium-stabilized for elevated-temperature service |
| A4 | Molybdenum-bearing austenitic | 316 | Marine and chloride service; F593 Group 2 counterpart |
| A5 | Stabilized molybdenum austenitic | 316Ti / 316Nb | A4 with stabilization for above 800°F |
| C1 | Martensitic | 410 | Hardenable; better strength but lower corrosion resistance |
| C3 | Martensitic | 431 | Higher chromium martensitic |
| C4 | Martensitic free-machining | 416 | Improved machining |
| F1 | Ferritic | 430 | Magnetic; moderate corrosion resistance |
Most North American work uses A2 and A4 — they cover marine, food and pharma, water, and most chemical processing applications.
Strength conditions (the number after the dash)
Within each alloy group, the strength is set by the cold-working condition:
| Condition | Code | A2 / A4 tensile range |
|---|---|---|
| Soft / annealed | -50 | ≥ 500 MPa (≈ 73 ksi) |
| Cold-worked | -70 | ≥ 700 MPa (≈ 102 ksi) — the most common stock |
| Strain-hardened (high-strength) | -80 | ≥ 800 MPa (≈ 116 ksi) — A4 only in most stocks |
| Special | -100, -110 | Higher classes for specialized applications |
A2-70 and A4-70 are the dominant grades for general-purpose stainless work. A4-80 is what you reach for when the joint needs both 316-grade chloride resistance and high tensile.
When to use each grade
| Service condition | Recommended grade |
|---|---|
| General indoor / outdoor, low corrosion | A2-70 (304-class) |
| Marine, coastal, chloride exposure | A4-70 (316-class) |
| Food, pharma, chemical with chlorides | A4-70 or A4-80 |
| High-strength + chloride resistance | A4-80 |
| Sustained high temperature (>800°F) | A5 (stabilized A4) |
| Strength-critical, mild corrosion only | C1 / C3 (martensitic) — note: less corrosion-resistant than A-class |
Related specifications
- ASTM F593 — Inch-series stainless bolt counterpart (Groups 1, 2, 3 align loosely with A2, A4, A5)
- ASTM F594 — Inch-series stainless nut counterpart
- ISO 898-1 — Carbon and alloy steel mechanical properties (for non-stainless metric)
- ISO 3506-2 — Companion nut specification