The Helix pedestrian bridge in Singapore was constructed using Outokumpu tubes and bars

Duplex families and applications: Part 3 reviewed

Joanne McIntyre - 15 October 2015

The final article in this fascinating series by Jacques Charles looks at the lean duplex grades and presents some conclusions.

About the author

Mrs Joanne McIntyre
Joanne McIntyre is the Editor in Chief of Stainless Steel World magazine, and Conference Coordinator for the Duplex Seminar & Summit.
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While lean duplex stainless steel (DSS) is considered to be a modern grade by some, in fact a lean duplex grade - i.e. DSS grade without Mo and restricted Cr, Ni additions – was already developed in France before the 1960s (URF1 – 21Cr%1,5Cu). Shortly afterwards Ugine developed grade AF20, a 20% Cr DSS with almost no Mo and Cu. The grade enjoyed limited commercial success.

By the late 1980s, Sandvik AB had obtained a lean duplex patent. Extensive marketing of the S 32304 lean duplex, mainly for seamless pipes, was undertaken. Industeel and the former Avesta started the production of plates a few years later.

In Europe the major stainless steel producer proposed the S32101 (LDX2101 from Outokumpu) and S32202 (UR2202 from Industeel / ArcerlorMittal and Aperam). The latest grade was designed to keep corrosion resistance at the level of the 316 stainless steel grade with reduced amounts of Mn and 23% Cr. Other grades were developed, such as S32001 (Nitronic 19D from Acerinox / AK steel), the lowest alloying duplex grade of the lean duplex family.

Recently the former 2304 grade has been ‘redesigned’ (UNS S 32304) / 2304Mod), mainly by enhancing N additions which make it possible to further increase the mechanical properties of the grade. The latest grade introduced among the lean duplexes is FDX 25 from Outokumpu, a grade designed to take advantage of the TRIP effect (austenite transforms into martensite during cold deformation).

The multitude of grades available causes some confusion to end-users and standardisation seems impossible. Volume markets are still not there and Ni cost has again deceased … which reduces some of the benefits expected by the use of lean duplex grades. Furthermore the grades appear more difficult to manufacture and transform than expected since they are very sensitive to nitride precipitations. The anticipated expected bright future has not yet materialized.

The new DSS, leaner in alloying than the standard 2205 DSS, present attractive advantages compared to 304/316 austenitic grades when considering their mechanical properties or alloying element total costs. Nowadays duplexes still only represents 1% of stainless steel consumption! Will they be a success story with significant market shares? The next decade, will provide the answer!

Read Jacque Charles’ full article here.

Photo: The Helix pedestrian bridge in Singapore was constructed using Outokumpu duplex tubes and bars (2205, 650 tons). Photo copyright Outokumpu.

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