Stainless Steels: Duplex As their name implies, duplex stainless steels are stainless steels containing two primary phases, f.c.c austenite and b.c.c. ferrite. These alloys were first observed in the 1920s as an outgrowth of studies on austenitic stainless steels. Their balance of good corrosion and mechanical properties has made them an important part of the stainless steel family of alloys.
1. Development of Microstructure The development of a duplex microstructure is a natural outgrowth of the metallurgy of the Fe–Cr–Ni system, which is the basis of all stainless steels. This is illustrated in Fig. 1, which shows the 70 wt.% Fe isopleth for the Fe–Cr–Ni system, which is approximately the amount of iron present in most stainless steels. The austenite is referred to as the γ phase and
the ferrite as δ. (In wrought ferritic stainless steels, the ferrite is typically referred to as α, and this nomenclature is also often used in the duplex stainless steel literature.) The more chromium in the alloy, the more ferrite will be present, and the more nickel, the more austenite will be present. Figure 2 shows a typical microstructure for a duplex stainless steel. Here the ferrite is etched darker than the austenite and is denoted as α. Most duplex stainless steels initially solidify as δ, with the γ phase developing on cooling or during working in the δjγ phase field. The amount and morphology of austenite and ferrite depends upon the exact composition of the alloy, the cooling rate after solidification, the annealing temperature, and the processing conditions. Alloying elements in addition to iron, chromium, and nickel also play a role in determining the amount of ferrite that can be developed. The exact effectiveness of any element in stabilizing ferrite or austenite varies from study to study, depending upon the exact alloy composition, processing, and heat treatment. Molybdenum, silicon, niobium, aluminum, and titanium stabilize ferrite, and manganese, copper, carbon, and nitrogen stabilize austenite. Carbon and nitrogen are particularly potent austenite stabilizers, being 15–35 times more effective than nickel (on a wt.% basis). The weighing factors for the other elements, for austenite or ferrite stabilization, are about 0.25–3.0 (also on a wt.% basis relative to nickel or chromium). Duplex stainless steels typically contain 20–70 vol.% ferrite. They can exist in either wrought or cast forms. In addition, castings which are nominally considered as being austenitic often contain 5–40 vol.% ferrite, and are thus strictly speaking duplex stainless steels, although they are generally not referred to as such. Austenitic stainless steel weld metal is also almost always duplex, typically containing 5–20 vol.% ferrite. In the case of castings and weld metal, the ferrite is present primarily to prevent hot cracking during solidification. In the case of those alloys that are termed duplex, the ferrite is present because of its influence on corrosion and mechanical properties.
2. Development of Additional Phases
Figure 1 Pseudobinary diagram showing the 70% isoplethal section of the Fe–Cr–Ni phase diagram (reproduced by permission of EDP Sciences from ‘Stainless Steels,’ 1993, pp. 613–59).
Duplex stainless steels can contain many phases in addition to austenite and ferrite. The development of these phases, and the way various alloying elements influence this development, is illustrated in Fig. 3. For the most part, the phases shown in Fig. 3 embrittle the alloy without strengthening it and should therefore be avoided. This is particularly true for σ and χ phases. The exception is the αh phase, which is a chromiumrich b.c.c. phase that strengthens the ferrite as well as reduces its ductility. As such, duplex stainless steels are sometimes deliberately heat treated to produce some 1
Stainless Steels: Duplex annealing is also required to prevent undesirable precipitates from developing. 3. Alloy Properties and Uses
Figure 2 Typical duplex microstructure (reproduced by permission of ASM International from ‘Duplex Stainless Steels,’ 1983, pp. 693–756).
Figure 3 Schematic representation of the possible precipitates in duplex stainless steels (reproduced by permission of EDP Sciences from ‘Duplex Stainless Steels’ 91, ’ 1991, pp. 3–48).
αh. This phase can develop by either nucleation and growth or by spinodal decomposition, depending upon the composition of the alloy. The spinodal decomposition into chromium- and iron-rich regions also strengthens the ferrite. Many of the phases illustrated in Fig. 3 develop in the ferrite or at the ferrite\austenite boundaries, so the amount and morphology of the ferrite determine the degree to which the alloy properties are degraded. Figure 3 shows the temperature ranges at which these phases form, and this sets limits on the temperature ranges at which these alloys can be fabricated or used. Rapid cooling from high-temperature processing or 2
The chief reasons for using duplex stainless steels are their good resistance to oxidation, corrosion, and stress corrosion—all this while maintaining superior mechanical properties. These alloys generally exhibit the same, or lower, general corrosion rates as austenitic stainless steels in dilute sulfuric acid. This is also true in dilute hydrochloric acid, and in the caustic solutions encountered in the pulp and paper industry. Some of these alloys are also applicable for exposure to organic acids. Many of these alloys possess superior pitting and crevice corrosion resistance, compared with austenitic stainless steels. Duplex stainless steels possess superior resistance to both transgranular and intergranular stress corrosion cracking. The exact degree to which any of these alloys is resistant to any form of corrosion or stress corrosion depends upon its composition, microstructure, and the exact nature of the solution and solution temperature to which it is exposed. Duplex stainless steels also possess excellent mechanical properties, and are consequently sometimes utilized instead of austenitic or ferritic stainless steels. Duplex steels generally possess higher yield and ultimate tensile strengths than most austenitic or ferritic stainless steels. The degree to which this is so depends not only on the alloy composition, but also on the way that it is processed. This improved strength is generally achieved without compromising the toughness of the alloy, providing that the alloy does not contain any of the deleterious phases shown in Fig. 3. The toughness of ferrite is decreased with decreasing temperatures but, being present only as a constituent of a two-phase alloy, this not does not produce as sharp a ductile-to-brittle transition as in ferritic stainless steels. The higher the ferrite content, the sharper the ductile-to-brittle transition. Ferrite has a smaller expansion coefficient and larger thermal conductivity than austenite, so the more ferrite that is present, the lower will be the coefficient of expansion and the greater the thermal conductivity of the duplex stainless steel. Duplex stainless steels are generally very weldable; in fact, austenitic stainless steels utilize duplex weld metal. See also: Stainless Steels: Martensitic; Ferritic Stainless Steels; Austenitic Stainless Steels; Stainless Steels: Cast Bibliography Charles J, Bernhardsson S (eds.) 1991 Duplex Stainless Steels ’91. Les Editions de Physique, Les Ulis, France Charles J 1991 The duplex stainless steels: materials to meet your
Stainless Steels: Duplex needs. In: Charles J, Bernhardsson S (eds.) Duplex Stainless Steels’ 91. Les Editions de Physique, Les Ulis, France, pp. 3–48 Desestret A, Charles J 1993 The duplex stainless steels. In: Lacombe P, Baroux B, Beranger G (eds.) Stainless Steels. Les Editions de Physique, Les Ulis, France, pp. 613–59 Duplex Stainless Steels ’94 1994 TWI, Glasgow, UK Duplex Stainless Steels ’86 1986 Nederlands Instituut voor Lastechniek, The Hague
Gunn R N 1997 Duplex Stainless Steels. Abington Publishing, Cambridge, UK Lula R A (ed.) 1983 Duplex Stainless Steels. American Society for Metals, Metals Park, OH Solomon H D, Devine T M Jr. 1983 Duplex stainless steels—a tale of two phases. In: Lula R A (ed.) Duplex Stainless Steels. American Society for Metals, Metals Park, OH, pp. 693–756
H. D. Solomon
Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 8802–8804 3