7/11/13 Lean Dupl ex Stai nl ess Steel :: KEY to METALS Ar ti cl e www.keytometals.com/page.aspx?ID=Check Article&site=kts&NM=364 1/3 Lean Duplex Stainless Steel Abstract: The early grades w ere alloy s of chromium , nickel and molybdenum. Thermodynamically, because the austenite is forming from the ferrite, it is impossible for the alloy to go pas t the equilibrium level of aus tenite. Lean Duplex Stainless Steels hav e many end uses in oil and gas due to t heir high strength and good corrosion properties. They have been us ed for subs ea applications including flexible flowlines, umbilical tubing, subsea manifolds, water injection lines and downhole chemical injection tubing. Historical Evolution Duplex stainless steels (DSSs), meaning those with a mixed microstructure of approx im ately equal p roportions o f austenite and ferrit e, hav e existed for mo re than 80 years [1]. T he early grades were alloys of chromium , nickel and molybdenum. Thermody nami cally , because the austenite is forme d from the f errite, it is im poss ible for the alloy t o go pas t the equilibrium lev el of aus tenite. The main problem with Duplex is that it v ery easi ly forms bri tt le intermetalic phas es, su ch as Sigm a, Chi, R and A lpha Prime . Pr olonged heating in the range 350°C to 550°C can c aus e 475°C temper em brittlement. The first wrought duplex stainless steels were produced in Sweden in 1930 and were used in the s ulfite paper indus try . These grades were developed to reduce the intergranular corrosion problems in the early high-carbon austenitic stainless steels. Duplex castings were produced in Finland in 19 30, and a patent was granted in France in 1936 for the forerunner of what would ev entually be known as Uranus 50. It bec ame obv ious that a balance of f errite and austenite had better resistance to chloride stress -corrosion cracking than a fully austenitic mi crostructure. Engineers hav e exploited this advantages of dupl ex ov er aus tenitic steels ev er s ince. In Franc e, the UR 50 grade wi th 20-35% ferrite (UNS S32404) was m arketed in various product forms, including forging, for such industries as oil refinement, food processing, pulp and paper, and pharmaceutical. These steels were produced in high frequency induction furnaces usin g precisely weighed alloying additions. Partial v acuum ensu red carbon rem ov als , rudimentary de-ox idation and restricted nitrogen ingress . Nev ertheless , plate product s remained sensi tiv e to edge crac ks. During the late 1960s and early 1970s, there were two m ain factors which advanced the dev elopm ent and us e of duplex alloy s. First, there was a nickel shortage that push ed up the price of austenitic steels , in combination wi th increased activ ity in the offshore oil indus try which dem anded a s tainless steel m aterial to handle aggress iv e env ironments. Second, steel production techniques improved dramatically with the introduction of the vacuum and argon oxygen decarburization (V OD and A OD) pract ices. These techniques ma de it poss ible to produce m uch cleaner steels with a v ery low carbon le v el and w ell controlled ni trogen content . In t he 70’s , t he introduct ion of continuous casting in s tainless steel production has contribut ed to lower production costs an d higher quali ty . F rom 1970 onwards , t he addition of nitrogen and the lowering of carbon content improv ed corrosion resis tance and high temperature stability of the duplex structure, e.g. the HAZ, by stabilizing the austenite. Development of new steels inevitably brings new problems in manufacturing and joining. This is particularly true f or welding w here the desired m aterial properties, carefully produced by the steel m aker, can be radically c hanged by a process that locally melts and recasts part of the micros truct ure. Because the cooling rate determines the amount of ferrit e that can transform to austenite,