AISI 329 and AISI 318 are both duplex stainless steels — their structure is half austenite, half ferrite. The difference between them is a generational one: 329 is a first-generation duplex, 318 is the nitrogen-alloyed duplex that is the standard today.
What did nitrogen change?
First-generation duplexes caused trouble in welding. The International Molybdenum Association’s fabrication guide says so plainly: in these grades the heat-affected zone of welds had low toughness because of excessive ferrite, and significantly lower corrosion resistance than the base metal.
Nitrogen was the answer. According to IMOA, nitrogen alloying brings heat-affected-zone toughness and corrosion resistance close to those of the base metal in the as-welded condition, and slows the formation of detrimental intermetallic phases. Penn Stainless makes the same point: the higher nitrogen content of today’s duplexes helps avoid excessive ferrite.
The practical consequence: AISI 318 is a duplex that can be welded as supplied; AISI 329 is not, in the same sense.
The difference in composition
329 (1.4460) has more chromium but less molybdenum. 318 (1.4462) gives up a little chromium, nearly doubles the molybdenum and brings nitrogen into the specification. That is what decides pitting resistance.
Compared in figures
The values are EN 10088-3 cast analysis limits and solution-annealed (+AT) bar.
| PROPERTY | AISI 329 (1.4460) | AISI 318 (1.4462) |
|---|---|---|
| Carbon (C) | 0.05 % max | 0.030 % max |
| Silicon (Si) | 1.00 % max | 1.00 % max |
| Manganese (Mn) | 2.00 % max | 2.00 % max |
| Phosphorus (P) | 0.035 % max | — |
| Chromium (Cr) | 25.00 – 28.00 % | 21.00 – 23.00 % |
| Nickel (Ni) | 4.50 – 6.50 % | 4.50 – 6.50 % |
| Molybdenum (Mo) | 1.30 – 2.00 % | 2.50 – 3.50 % |
| Proof strength (Rp0.2) | ≥ 450 MPa | ≥ 450 MPa |
| Elongation | ≥ 20 % | ≥ 25 % |
| Hardness | ≤ 260 HB | — |
PREN: the pitting resistance indicator
The indicator commonly used for duplexes is PREN = %Cr + 3.3 × %Mo + 16 × %N. Published values for AISI 318 range from 31 to 35 — and that spread is a real specification difference, not sloppiness (see below).
IMOA and Outokumpu both give the same warning: PREN is only an indicator, not a design value. The coefficient on nitrogen is variously taken as 16, 22 or 30 depending on the source. Ask which formula was used when comparing quotations.
The upper temperature limit duplexes are forgotten for
Duplexes are NOT high-temperature steels. The published upper service temperature for AISI 318 falls in the band of roughly 280 to 316 °C (Alleima 280 °C, Industeel 280 °C, Outokumpu’s ASME II-D approval 316 °C, Macsteel 300 °C). The reason is metallurgical rather than mechanical: alpha-prime precipitation in the ferrite phase, and sigma phase higher up, reduce toughness.
What to watch when ordering
The names of these two materials are the most confused corner of the industry.
“AISI 318” is not in fact an AISI designation. There is no Type 318 in ASTM A240. “318” and “318LN” are the European market’s nicknames for 1.4462; the real designations are UNS S31803 / S32205 and EN 1.4462. Do not have “AISI 318” written on a certificate.
Five organisations give five different answers for the UNS equivalent of 1.4460: Ugitech says S31200, Rodacciai says S32950, and METALCOR, Langley Alloys and Sverdrup Steel say S32900. The compositions are not the same either — EN 1.4460 is nitrogen-alloyed and has higher nickel, while ASTM S32900 has no nitrogen requirement. Base your order and certificate on the material number (1.4460).
S31803 and S32205 are not the same thing. S32205 targets the upper end of the chromium, molybdenum and nitrogen ranges. As Langley Alloys puts it: material made to S32205 fully meets S31803, but the reverse is not true. IMOA adds that a 0.14 % nitrogen minimum is what makes reliable welded fabrication possible.
Welding
AISI 318 welds well: no preheat is needed and post-weld heat treatment is not normally required. However, a duplex filler must be used (2209 / E2209 / ER2209) — an austenitic filler does not give the right result. Interpass temperature is kept below 150 °C.
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 329 | AISI 318 |
|---|---|---|
| AISI / trade name | AISI 329 | AISI 318 |
| EN material no. (W.Nr) | 1.4460 | 1.4462 |
| UNS number | S32900 | S31803 |
| AMS specifications | — | — |
| Product page | AISI 329 technical page | AISI 318 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.