The first two grades that come to mind when stainless steel is mentioned are AISI 304 and AISI 316. Both are austenitic, both are non-magnetic, and both are welded and formed the same way. The difference between them comes down to the presence of a single element: molybdenum.
What molybdenum does
There is no molybdenum in the composition of AISI 304. AISI 316 contains 2.0–3.0% molybdenum. In exchange its chromium is slightly lower (17.50–19.50% in 304 against 16.0–18.0% in 316) and its nickel is higher (from 8.00–10.50% to 10.0–14.0%).
Molybdenum provides resistance to pitting corrosion. What attacks the protective chromium oxide layer on the surface of stainless steel most severely is chloride ions — seawater, salt-laden air, chlorinated cleaning chemicals, some food processing environments. Where 304 begins to form pits on its surface in such an environment, 316 lasts far longer thanks to its molybdenum.
Their strength values at room temperature are close to one another; in other words the reason for choosing 316 is not strength but corrosion resistance.
Comparison in numbers
| PROPERTY | AISI 304 | AISI 316 |
|---|---|---|
| Carbon (C) | max 0.07% | max 0.08% |
| Chromium (Cr) | 17.50 – 19.50% | 16.0 – 18.0% |
| Nickel (Ni) | 8.00 – 10.50% | 10.0 – 14.0% |
| Molybdenum (Mo) | — | 2.0 – 3.0% |
| Tensile strength Rm | 500 – 700 MPa | 515 MPa |
| Yield strength Rp0.2 | min 190 MPa | 205 MPa |
| Elongation A50 | min 45% | 40% |
| Hardness | max 215 HB | max 217 HB |
| Thermal conductivity | 16.2 W/m·K | 16.3 W/m·K |
| Melting range | 1450 °C | 1370 – 1450 °C |
| UNS | S30400 · S30403 | S31600 |
| W.Nr (DIN/EN) | 1.4301 · 1.4307 | 1.4401 · 1.4404 |
| AMS | 5511 · 5513 · 5560 · 5565 · 5639 · 5647 | 5648 (bar, forging) · 5524 (plate, sheet) · 5573 (tube) |
| ASTM | A276 · A479 · A484 | A276 · A479 · A484 · A240 |
Values are taken from our product datasheets; section thickness and the applied heat treatment regime change the result.
Which one, and when?
AISI 304 is sufficient — kitchen and food equipment, architectural applications, indoor fasteners, general machine parts, water lines. In any environment without a chloride load, 304 is the correct and economical choice.
AISI 316 is required — marine and coastal environments, ship and offshore equipment, chemical and pharmaceutical plants, chlorinated water lines, salty food production, medical equipment. If an aerospace specification calls out AMS 5648, AMS 5524 or AMS 5573, the grade being asked for is 316.
The low-carbon versions
Both grades have a reduced-carbon “L” version: AISI 304L and AISI 316L. We explained why they are preferred for welded parts in our AISI 316 – AISI 316L comparison. If chloride resistance has to be taken beyond 316 as well, AISI 904L and duplex steels come into play.
Technical datasheets
AISI 304 — chemical composition, mechanical values, standard equivalents
AISI 316 — chemical composition, mechanical values, standard equivalents
Both grades are supplied to order in round bar, flat bar, plate, sheet, pipe and forged forms. Contact us for a quotation.