SS 329: a duplex stainless steel, not a martensitic one

Because it is magnetic. A duplex steel is made with a balanced mix of two structures: austenite, the non-magnetic structure in 304 and 316, and ferrite, the magnetic structure in 430. In 329 a large share of the steel is ferrite, so a magnet pulls on it firmly.

That is where the similarity ends. Martensitic grades like 410 and 420 harden on quenching. 329 does not. Ferritic grades like 430 have no nickel and only modest toughness and corrosion resistance. 329 has nickel and molybdenum, and corrosion resistance in many environments closer to or better than 316. If you have been given 329 as a substitute for 410, or the other way round, stop and check the application.

Mixing austenite and ferrite combines the strengths of both. ASTM A240 sets 329's minimum yield at 485 MPa, more than twice the 205 MPa minimum for 304 and 316. That allows thinner walls for the same pressure, or smaller sections for the same load.

The ferrite also gives much better resistance to chloride stress corrosion cracking than the standard austenitic grades. That is the failure where 304 or 316 vessels and tubes crack in hot chloride water. With 23–28% chromium and 1–2% molybdenum, 329's resistance to pitting is also good, though it depends on where in the composition range a particular heat falls.

329 is one of the original duplex grades. Its main weakness was welding. Without a deliberate nitrogen addition, the heat-affected zone of a weld tends to end up with too much ferrite, which lowers toughness and corrosion resistance at the weld. For welded equipment that was a serious drawback.

Second-generation duplex steels fixed this by adding nitrogen, which helps austenite re-form as the weld cools. The most widely used is 2205 (UNS S31803 / S32205), with 22–23% chromium, 3.0–3.5% molybdenum and 0.14–0.20% nitrogen in the S32205 version. 2205 has better weldability, higher pitting resistance and is far more widely produced. For new welded equipment, most designers now specify 2205 rather than 329. See our duplex steel page for 2205 and related grades.

329 still has a place in bar and machined components, replacement parts built to older drawings, and heat-exchanger and process parts where it has a service history.

329 is used for heat exchanger tubing and parts in chloride-bearing cooling water, chemical process equipment where stress corrosion cracking is a risk, pump and valve parts, shafts, and components in pulp and paper and oil and gas processing. Our chemical process plants page covers where duplex grades fit among other corrosion-resistant materials.

Like all duplex steels, 329 should not be used for long periods above roughly 300 °C, because its ferrite is prone to 475 °C embrittlement and, at higher temperatures, sigma phase. It is a strength and corrosion grade for ambient to moderate temperatures, not a heat-resisting grade.

Its higher strength means more force is needed for forming and machining than with 316. Welding needs controlled heat input and suitable duplex filler; for critical welded work, discuss 2205 with your designer first.

Because 329's composition range is wide, two heats can behave quite differently. A heat near the top of the chromium and molybdenum ranges resists pitting much better than one near the bottom. When comparing offers, look at the actual chromium, nickel and molybdenum values on the test certificate, not only the grade name. A ferrite content measurement is sometimes requested for duplex material, and we can discuss this at enquiry. Our guide on reading a test certificate explains the layout.

329 is not produced in the volumes it once was. We supply it in bar and other forms subject to availability, and will confirm size and certification at enquiry. Where 329 is not available in your size, we can tell you whether 2205 is a suitable alternative for your application. Every lot is supplied with a test certificate.