AISI 303 and AISI 304 come from the same 18-8 austenitic family; their chromium and nickel contents are almost identical. The one deliberate difference is sulphur — and that single difference makes the two steels non-interchangeable.
What does the sulphur do?
0.15 to 0.35 % sulphur is added on purpose to AISI 303 (1.4305). The sulphur combines with manganese to form manganese sulphide inclusions in the steel. Those inclusions break the chip and reduce friction between the tool and the workpiece. The result is a steel that machines far faster and more cleanly on automatic lathes.
One point deserves emphasis: in the specification the sulphur is a MINIMUM, not a maximum. Reading “S 0.15–0.35” as a ceiling and accepting a certificate showing 0.02 % sulphur as 303 is a common mistake in the field. In AISI 304 sulphur is an impurity and does have a ceiling.
The price: welding and corrosion
The machinability the sulphur buys is paid for in two places.
Welding. Aalco and thyssenkrupp both rate 303’s weldability as “poor”; Outokumpu states plainly that it is “not recommended for applications requiring welding”; Rodacciai marks 1.4305 as NOT resistant to intergranular corrosion, both as delivered and as welded. AISI 304, by contrast, is the standard choice for welded fabrication and normally needs no post-weld heat treatment.
Corrosion. The sulphide inclusions that give the machinability are also pit initiation sites. Aalco and thyssenkrupp describe 303 as at risk in chloride environments above 60 °C and unsuitable for marine service; Outokumpu restricts it to “mildly corrosive environments”; Carpenter advises against using it in vessels containing gases or liquids under high pressure.
Compared in figures
The values below are EN 10088-3 cast analysis limits.
| PROPERTY | AISI 303 (1.4305) | AISI 304 (1.4301) |
|---|---|---|
| Carbon (C) | 0.10 % max | 0.07 % max |
| Silicon (Si) | 1.00 % max | 1.00 % max |
| Manganese (Mn) | 2.00 % max | 2.00 % max |
| Sulphur (S) | 0.15 – 0.35 % (minimum) | 0.015 – 0.030 % max (see note) |
| Chromium (Cr) | 17.00 – 19.00 % | 17.50 – 19.50 % |
| Nickel (Ni) | 8.00 – 10.00 % | 8.00 – 10.50 % |
| Copper (Cu) | 1.00 % max | not specified |
| Molybdenum (Mo) | not specified | not specified |
| Elongation (bar, +AT) | ≥ 35 % | ≥ 45 % |
| Machinability | excellent | good |
| Weldability | poor — not recommended | excellent |
Note on sulphur: for 1.4301, EN 10088-3:2005 and Rodacciai give 0.030 % max, while EN 10088-1:2014, Ugitech and thyssenkrupp give 0.015 % max. Both figures come from different editions of the standard; state which edition you are working to when ordering.
What to watch when ordering
AISI 303 is not covered by ASTM A276.
S30300 does not appear in the ASTM A240 or A276 grade lists. 303 bar falls under ASTM A582 (free-machining steels). A request for “303 to A276” has no technical basis.
The ASTM and EN versions are also not the same steel: ASTM A582 allows carbon up to 0.15 % and phosphorus up to 0.20 %, while EN 1.4305 limits carbon to 0.10 % and phosphorus to 0.045 %. A bar that passes A582 can fail EN 10088-3.
Which one is right for your job?
If the part is produced in volume on a machine tool and the environment is dry or only mildly corrosive, 303 is the right choice: shafts, bushes, fittings and fasteners fall into this class.
If the part will be welded, will contact food, dairy or pharmaceutical products, or will see chlorides or seawater, 303 is the wrong choice and 304 should be used. If you need both machinability and corrosion resistance, the answer is not 303 but a 304 grade with optimised sulphur.
Designations, standards and AMS equivalents
The same steel is called different things depending on which system you are speaking in. Most ordering and certification confusion starts here, so a short summary:
- AISI — the American Iron and Steel Institute’s grade naming (410, 316L and so on). It is the most widely used name in everyday use, but on its own it is not a purchasing specification: composition and mechanical requirements are defined in the ASTM standards.
- UNS — the Unified Numbering System operated jointly by ASTM and SAE (S41000 and so on). It is the number used for ordering and certification in North America, and it is more precise than the AISI name.
- W.Nr / EN — the European material number (1.4006 and so on) and EN name (X12Cr13 and so on). This is the governing designation in Europe. It is usually only an approximate equivalent of the AISI grade; carbon, sulphur or molybdenum limits frequently differ.
- AMS — SAE’s Aerospace Material Specifications. An AMS specification binds not only the composition but also the melting practice, the product form, the heat treatment condition and the inspection requirements. In aerospace and defence orders this is the designation that governs.
| DESIGNATION | AISI 303 | AISI 304 |
|---|---|---|
| AISI / trade name | AISI 303 | AISI 304 |
| EN material no. (W.Nr) | 1.4305 | 1.4301 |
| UNS number | S30300 | S30400 |
| AMS specifications | AMS 5635 · AMS 5638 | AMS 5511 · AMS 5513 · AMS 5560 · AMS 5639 |
| Product page | AISI 303 technical page | AISI 304 technical page |
Two reminders. First: an AMS number is specific to a product form and condition. The same grade may have separate AMS specifications for bar, sheet and forgings, so establish which product form you need before calling up an AMS number.
Second: the AISI name, material number, UNS and AMS data in the table above are the designations shown on Defence Metal’s product pages. If you are working to a different edition of a standard or to a different product form, state the material number and the specification edition explicitly on your order.