EN 
ZH 
| Attribute | 304 (A2) | 316 (A4) |
|---|---|---|
| EN / UNS | 1.4301 / S30400 | 1.4401 / S31600 |
| Chromium | 17.5–19.5% | 16.5–18.5% |
| Nickel | 8.0–10.5% | 10.0–13.0% |
| Molybdenum | ≤0.5% (effectively 0) | 2.0–3.0% |
| PREN | ~18–20 | ~23–28 |
| Tensile, class 70 | 700 MPa | 700 MPa |
| Typical cost | baseline | +25–35% |
| Best for | indoor, dry, mild | coastal, marine, chemical |
304 stainless steel fasteners use the classic 18-8 mix of about 18% chromium and 8% nickel, carbon capped at 0.08%. Under ISO 3506 they carry the A2 grade in classes A2-50, A2-70, and A2-80. The AISI number 304 and the fastener grade A2 are the same alloy; the dash number is the strength class. 304 is the default for indoor equipment, architectural trim, and food contact where the atmosphere stays dry or mildly humid, and its low cost and easy forming explain why it covers most fastener demand.
316 stainless steel fasteners take the 18-8 base and add 2–3% molybdenum, EN 1.4401. Under ISO 3506 the grade is A4 in classes A4-50, A4-70, and A4-80. That molybdenum is the whole story: it stabilises the passive chromium oxide layer and sharply improves pitting and crevice resistance in chloride bearing environments. This is why 316 earns the “marine grade” label and is the standard choice for coastal structures, shipbuilding, offshore platforms, and process plants using chlorinated cleaning or brines. The trade off is a modest material premium and lower machinability, both acceptable given the service life it delivers.
Plenty of buyers trip over the naming system, so here is the crosswalk in one place.
| AISI (USA) | EN (Europe) | ISO fastener grade | Typical property classes |
|---|---|---|---|
| 304 | 1.4301 | A2 | A2-50, A2-70, A2-80 |
| 316 | 1.4401 | A4 |
| Element (max unless range) | 304 (A2, 1.4301) | 316 (A4, 1.4401) |
|---|---|---|
| Carbon, C | 0.08% | 0.08% |
| Chromium, Cr | 17.5–19.5% | 16.5–18.5% |
| Nickel, Ni | 8.0–10.5% | 10.0–13.0% |
| Molybdenum, Mo | ≤0.5% | 2.0–3.0% |
| Manganese, Mn | 2.0% | 2.0% |
| Silicon, Si | 1.0% | 1.0% |
| Nitrogen, N | 0.10% | 0.10% |
| Property class | Min tensile Rm (MPa) | Min yield Rp0.2 (MPa) | Applies to A2 and A4 |
|---|---|---|---|
| -50 | 500 | 210 | A2-50, A4-50 |
| -70 | 700 | 450 | A2-70, A4-70 |
| -80 | 800 | 600 | A2-80, A4-80 |
PREN rolls the corrosion relevant elements into a single score using the formula:
PREN = %Cr + 3.3×%Mo + 16×%N
Chromium counts once, molybdenum counts 3.3 times because of its outsized effect on pitting, and nitrogen counts 16 times. Plug in typical values and 304 lands at about 18–20, while 316 lands at about 23–28. The higher the PREN, the better the resistance to initiation of pitting. A jump from 20 to 26 is the difference between a fastener that pits in two seasons and one that runs for a decade in the same seawater splash zone.
304 begins to pit in chloride at roughly 200 ppm under warm conditions, and the threshold drops with temperature. Real coastal air is never a clean lab value, because salt aerosol and crevices under bolt heads concentrate chlorides well above ambient. rates corrosivity from C1 (dry indoor) to CX (offshore). For the grade system behind these ratings, see our . 304 is comfortable through C3, but from C4 (coastal) upward it pits within 18–24 months, matching the boardwalk failure in our opening example. 316, PREN near 26, holds through C5 and CX for the design life.
For 316 over 304, the material and conversion premium runs about 15–40% across fastener forms, with most standard bolts, nuts, and screws in the 25–35% band. Larger or cold worked items sit higher; high volume parts sit lower. Molybdenum is pricier and more volatile than chromium or nickel, so the gap widens when molybdenum spikes. A typical M10 A2-70 versus A4-70 hex bolt costs roughly a third more, which is cheap against the cost of a failure.
That premium is a rounding error next to replacement labour, access equipment, and production downtime. On the boardwalk example, the 30% first cost saving became a 340% net loss after one replacement cycle. In marine and coastal service, the 316 premium pays back at the first scheduled maintenance that 304 would have missed. In dry indoor or controlled environments, where 304 will never see chlorides, paying for 316 is pure waste, and that is where
| ISO 3506-1:2020 | ASTM F593 | ASTM A193 | Property description |
|---|---|---|---|
| A2 (304) | Group 1 (304) | B8 (304) | general purpose austenitic |
| A4 (316) | Group 2 (316) | B8M (316) | molybdenum marine grade |
| A2-70 / A4-70 | F593 304 / 316 Condition A | B8 / B8M Class 1 | 700 MPa, annealed |
| A2-80 / A4-80 | F593 304 / 316 Condition CW | B8 / B8M Class 2 | 800 MPa, cold worked |
ISO 3506-1:2020 is the global reference for stainless fastener properties and grades, covering A2 and A4 across the -50, -70, and -80 classes. ASTM F593 governs stainless bolts, hex cap screws, and studs in the United States and groups 304 and 316 as Groups 1 and 2. ASTM A193 B8 and B8M cover stainless bolting for high temperature and high pressure service such as pressure vessels, valves, and flanges, where the class (1 annealed, 2 strain hardened) sets strength. Reference the sources directly: the , the , and the . For the alloy family background, the
| Environment | Recommended grade | Reason |
|---|---|---|
| Indoor dry or clean (C1) | 304 (A2-70) | no chloride, best value |
| Mild outdoor, low pollution (C2–C3) | 304 (A2-70) | low corrosion risk |
| Coastal, urban polluted (C4) | 316 (A4-70) | chloride pitting threat |
| Marine, offshore (C5 / CX) | 316 (A4-70 or -80) | high chloride, long life |
| Chemical process, chlorides | 316 (A4) or higher | depends on exact media |
| Food, pharma, washdown | 316 (A4) | chlorinated cleaning, hygiene |
| High load structural | A2-80 / A4-80 | property class drives strength |
But the grade is only half the specification. Pair it with the correct for your assembly and driving method, and review drives if installation torque or cam out is a concern. Where weight or a different corrosion profile matters,
316 is stronger than 304. False at equal property class. A2-70 and A4-70 share the same 700 MPa tensile. Strength comes from the class, not the alloy.
Stainless steel never rusts. False. Stainless resists rust through its passive film. Break that film in a tight crevice with trapped chlorides and even 316 will pit. Design for drainage and avoid crevices.
316 is a universal corrosion cure. False. A refinery paid for that assumption: an engineer specified A4 for all external bolting, assuming marine grade meant universal protection. In a unit with sulfuric acid mist at 40°C, the 316 bolts suffered uniform corrosion where a higher alloy such as 904L or alloy 20 was required. The result was a three day unplanned shutdown. 316 is the chloride answer, not the chemical answer.
Magnetism means fake stainless. False. Cold worked A2-80 and A4-80 develop measurable magnetism from deformation, not from being counterfeit. Use a PMI gun or MTC, not a fridge magnet, to verify grade.
A: For any chloride exposure, yes. The 25–35% material premium for 316 (A4) is small against the cost of premature failure, replacement labour, and downtime. In dry or controlled indoor environments, 304 (A2) is the better value and 316 is unnecessary.
A: In mild, non coastal outdoor air rated ISO 9223 C2 to C3, 304 performs well for years. Near the coast or in any chloride rich environment, 304 pits within 18–24 months. Specify 316 (A4) for coastal, marine, and de icing salt exposure.
A: A2 is the ISO designation for 304 stainless, A4 for 316. The -70 denotes a property class with 700 MPa minimum tensile strength and 450 MPa yield. At the same class, A2-70 and A4-70 have identical strength; only the alloy and corrosion resistance differ.
A: No. At equal property class, for example A2-70 versus A4-70, the tensile and yield strengths are the same. 316 is not stronger than 304; it is more corrosion resistant because of its 2–3% molybdenum. Strength comes from the property class, not the grade.
A:
, and how these two sit among carbon, alloy, and , use our material guides, and compare value against
