When buyers ask for stainless steel bolts, nuts or screws, two grades come up again and again: A2 and A4. Choosing the wrong one can mean premature corrosion in the field — or paying more than your project needs. This guide explains the difference between A2 (304) and A4 (316) stainless steel fasteners and how to pick the right grade.
What do A2 and A4 mean?
“A2” and “A4” are stainless steel fastener designations under ISO 3506. The letter “A” stands for austenitic stainless steel; the number indicates the alloy group:
- A2 ≈ AISI 304 stainless steel (also called 18/8).
- A4 ≈ AISI 316 stainless steel, which adds molybdenum.
A2 (304) stainless steel fasteners
A2/304 contains roughly 18% chromium and 8% nickel. It offers good corrosion resistance in most indoor, urban and general outdoor environments, is non-magnetic, and is the most economical and widely stocked stainless grade. Typical uses: furniture, railings, machinery, construction, food equipment and general hardware.
A4 (316) stainless steel fasteners
A4/316 adds about 2–3% molybdenum, which dramatically improves resistance to chlorides and salt. This makes A4 the standard choice for marine, coastal, swimming-pool, chemical and high-corrosion environments where A2 would eventually pit and rust. It costs more, but it lasts far longer in aggressive conditions.
A2 vs A4: key differences
| Property | A2 (304) | A4 (316) |
|---|---|---|
| Main alloy | 18% Cr, 8% Ni | 16–18% Cr, 10–14% Ni, 2–3% Mo |
| Corrosion resistance | Good (general) | Excellent (salt / chemical) |
| Best for | Indoor / general outdoor | Marine / coastal / chemical |
| Magnetic | No (typically) | No (typically) |
| Relative cost | Lower | Higher |
When to choose A2 vs A4
- Choose A2 (304) for indoor projects, general construction, furniture and dry or mild outdoor conditions where cost matters.
- Choose A4 (316) within ~5 km of the sea, for boats and docks, swimming pools, chemical plants, or anywhere road salt and chlorides are present.
Property classes: 50, 70 and 80
Stainless fasteners also carry a strength class such as A2-70 or A4-80. The number is one-tenth of the tensile strength in MPa: class 70 ≈ 700 MPa (cold worked, the most common), class 80 ≈ 800 MPa (high strength), and class 50 (soft / annealed). Always confirm both the grade (A2/A4) and the class (50/70/80) when ordering.
Frequently asked questions
Is A4 always better than A2? Not necessarily — A4 resists salt far better, but for indoor or mild environments A2 performs well at a lower price.
Are A2 and A4 magnetic? In the annealed state both are essentially non-magnetic; cold forming can make them slightly magnetic, which is normal.
Can I mix A2 and A4 in one assembly? It is best to keep the same grade throughout to avoid galvanic and corrosion mismatches in harsh environments.
Need A2 or A4 stainless fasteners?
Chaoshuo Trading is a China-based fastener source factory supplying A2 (304) and A4 (316) stainless steel bolts, nuts, screws, washers and threaded rods in DIN/ISO standards, with factory-direct pricing and OEM customization. Contact us for a quote: info@chaoshuotrade.com | WhatsApp +86 158 0311 9164 | www.chaoshuotrade.com