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Stainless steel fasteners come in several grades, and the right choice depends on where they will be used. This guide compares the three grades most used in India: SS 304, SS 316 and SS 202.
| SS 202 | SS 304 | SS 316 | |
|---|---|---|---|
| Chromium | 17–19% | 17.5–20% | 16–18% |
| Nickel | 4–6% | 8–10.5% | 10–14% |
| Manganese | 7.5–10% | up to 2% | up to 2% |
| Molybdenum | none | none | 2–3% |
| Rust resistance | Fair: indoor and dry | Good: general outdoor | Excellent: marine and chemical |
| Fastener marking | Not in ISO 3506 | A2 | A4 |
| Relative cost | Lowest | Medium | Highest |
SS 304, also called 18/8 stainless, is the most used stainless steel for bolts, nuts and washers. It resists rust in air, water and most everyday conditions, and keeps a clean look for years. Fasteners in SS 304 are marked A2, for example A2-70.
Use it for: outdoor structures, building fixtures, food and dairy equipment, water tanks, electrical enclosures and general engineering.
SS 316 adds 2–3% molybdenum, which greatly improves resistance to chlorides in sea water, salty air, swimming pools and many chemicals. It costs more than SS 304 but lasts far longer in harsh places. Fasteners in SS 316 are marked A4, for example A4-70.
Use it for: coastal and marine sites, chemical and pharma plants, effluent treatment, swimming pools, and food plants that use strong cleaning chemicals.
SS 202 replaces part of the nickel with manganese, which makes it cheaper. Its rust resistance is lower than SS 304, so it suits indoor and dry conditions, furniture and decorative work where cost matters.
Use it for: indoor fittings, furniture, racks and light fabrication in dry areas.
Stainless steel bolts to ISO 3506-1 in class 70 (A2-70 and A4-70) have a tensile strength of 700 MPa, a little below a high tensile 8.8 bolt at 800 MPa. Class 80 bolts reach 800 MPa. Where very high strength is needed, high tensile steel bolts with a protective coating are normally used instead. See our bolt grade chart.
SS 304 and SS 316 are austenitic steels and are almost non-magnetic when annealed. Cold heading and thread rolling can make them slightly magnetic, which is normal and does not reduce rust resistance. A magnet test alone cannot tell you the grade, so ask for a material test certificate.
Use nuts, bolts and washers of the same grade. When stainless steel touches zinc plated or galvanized steel in wet conditions, galvanic corrosion can attack the zinc. Stainless steel threads can also gall (seize) if they are tightened too fast; a little anti-seize compound and steady tightening help.
JMIKS Fasteners supplies bolts and machine screws, nuts, washers and studs in SS 304, SS 316 and SS 202, from M3 to M36, from Ahmedabad. Send your requirement for a quotation.