How stainless grades are named
A stainless bolt marked A4-80 tells you two things. The letter and digit give the steel group: A = austenitic, D = duplex, C = martensitic, F = ferritic. The number after the dash is the strength class: tensile strength in MPa ÷ 10, so 80 means 800 MPa minimum. India follows the same system in IS 1367 (Part 14).
Chemical composition (ISO 3506-1)
| Grade | Typical steels (AISI / EN) | C max | Mn max | Cr | Ni | Mo | Other |
|---|---|---|---|---|---|---|---|
| A1 | 303 / 1.4305 (free-machining) | 0.12 | 6.5 | 16–19 | 5–10 | ≤0.7 | S 0.15–0.35, Cu 1.75–2.25 |
| A2 | 304, 304L, 304Cu / 1.4301, 1.4307, 1.4567 | 0.10 | 2.0 | 15–20 | 8–19 | – | Cu ≤4 |
| A3 | 321 / 1.4541 (Ti stabilised) | 0.08 | 2.0 | 17–19 | 9–12 | – | Ti or Nb stabilised |
| A4 | 316, 316L / 1.4401, 1.4404 | 0.08 | 2.0 | 16–18.5 | 10–15 | 2–3 | Cu ≤4 |
| A5 | 316Ti / 1.4571 (Ti stabilised) | 0.08 | 2.0 | 16–18.5 | 10.5–14 | 2–3 | Ti or Nb stabilised |
| A8 | 6Mo super-austenitic / 1.4547 (254 SMO type) | 0.03 | 2.0 | 19–22 | 17.5–26 | 6–7 | Cu ≤1.5 |
| D2 | Lean duplex / 1.4162 (2101 type) | 0.03 | 6.0 | 19–24 | 1.5–5.5 | 0.1–1 | N 0.05–0.20 |
| D4 | Duplex / 1.4362 (2304 type) | 0.04 | 6.0 | 21–25 | 1–5.5 | 0.1–2 | N 0.05–0.30 |
| D6 | Duplex 2205 / 1.4462 | 0.03 | 2.0 | 21–23 | 4.5–6.5 | 2.5–3.5 | N 0.08–0.35 |
| D8 | Super duplex 2507 / 1.4410 | 0.03 | 6.0 | 24–26 | 6–8 | 3–4.5 | N 0.20–0.35 |
| C1 | 410 / 1.4006 (martensitic) | 0.09–0.15 | 1.0 | 11.5–14 | ≤1 | – | hardenable |
| C3 | 431 / 1.4057 type (martensitic) | 0.17 | 1.0 | 16–18 | 1.5–2.5 | – | hardenable |
| C4 | 416 / 1.4005 (free-machining martensitic) | 0.17 | 1.5 | 12–14 | ≤1 | ≤0.6 | S 0.15–0.35 |
| F1 | 430 / 1.4016 (ferritic) | 0.08 | 1.0 | 15–18.5 | ≤1 | – | magnetic, not hardenable |
Values in % (maximum unless a range is given). Si is 1.0% max for all grades; P and S are low except in the free-machining grades A1 and C4. Grades A8 and D2–D8 were added in the 2020 edition of ISO 3506-1. Check the standard for full limits.
Mechanical properties by class
| Group | Property class | Tensile Rm min (MPa) | 0.2% proof Rp0.2 min (MPa) | Elongation min | Hardness |
|---|---|---|---|---|---|
| Austenitic A1–A8 and duplex | 50 | 500 | 210 | 0.6 d | – |
| Austenitic A1–A8 and duplex | 70 | 700 | 450 | 0.4 d | – |
| Austenitic A1–A8 and duplex | 80 | 800 | 600 | 0.3 d | – |
| Austenitic A2–A8 and duplex | 100 | 1000 | 800 | 0.2 d | – |
| Martensitic C1 | 50 / 70 / 110 | 500 / 700 / 1100 | 250 / 410 / 820 | – | about 155–220 / 220–330 / 350–440 HV |
| Martensitic C3 | 80 | 800 | 640 | – | about 240–340 HV |
| Martensitic C4 | 50 / 70 | 500 / 700 | 250 / 410 | – | about 155–220 / 220–330 HV |
| Ferritic F1 | 45 / 60 | 450 / 600 | 250 / 410 | – | about 135–220 / 180–285 HV |
Elongation is given as a length in terms of the nominal diameter d (full-size test). Classes apply up to M39 (class 50) and up to M24–M39 for the higher classes, depending on edition and grade. Class 50 is not available in A8. Hardness values for martensitic and ferritic grades are approximate.
Matching nuts (ISO 3506-2) use the same groups and classes; for example an A4-80 bolt takes an A4-80 nut. Thin nuts carry half the class, such as 040.
How stainless fasteners are made
- Raw material: solution-annealed stainless wire rod or bar. Cold-heading grades such as 304Cu (1.4567) have extra copper to reduce work hardening, so they form more easily.
- Forming: cold heading at lower speeds than carbon steel, because austenitic stainless work hardens quickly and is hard on dies. Large sizes are hot forged or machined from bar.
- Strength: austenitic grades cannot be hardened by heat treatment. Class 50 is lightly worked or machined, class 70 is normal cold-formed, and classes 80 and 100 are heavily cold worked.
- Threads: rolled. Stainless threads are prone to galling, so an anti-seize compound is recommended for assembly.
- Finishing: cleaning and passivation to remove free iron and restore the chromium oxide layer. Martensitic grades (C1, C3, C4) are hardened and tempered like carbon steel.
Which grade where
| Environment | Suggested grade |
|---|---|
| Indoors, general industry, food machinery (no chlorides) | A2-70 (304) |
| Outdoors, coastal air, water treatment, chemical plants, swimming pools (indirect) | A4-70 / A4-80 (316) |
| Welded assemblies or service at elevated temperature | A3 / A5 (321, 316Ti), or low-carbon 304L / 316L |
| Seawater, desalination, offshore, high chloride | D6 (2205), D8 (2507) or A8 (6Mo) |
| Wear parts, pump shafts, valve parts needing hardness | C1 or C3 (martensitic), with lower corrosion resistance |
| Decorative, light duty, magnetic needed | F1 (430) |
Stainless fasteners in India
A2 and A4 fasteners in India are sold as "SS 304" and "SS 316". Indian stainless producers in Maharashtra and Gujarat make wire and bar for fasteners, and A2 and A4 are widely stocked. Duplex and super duplex fasteners are mostly supplied against project orders for refineries, desalination and offshore work. Always ask for the class marking (A2-70, A4-80) and a test certificate, because unmarked "SS" fasteners are often class 50 or lower-alloy material.
Figures are typical values for guidance. For design, inspection and purchase, always follow the current edition of the standard on your drawing.