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Early Duplex Development:
Duplex stainless steels, which combine manyof the beneficial properties of ferritic and austenitic steels, were originallydeveloped in the early 1930′s. The initial duplex grades provided good performance characteristics, but hadlimitations in the as-welded condition. The metallurgical processes at that time were not suitable for producing grades
with the right austenite-ferritebalance.Also, these early duplex grades were relatively high in carbon
contentsince efficient process techniques for decarburization were not available at thetime. Consequently, fabrications using these materials tended to be mainly castproductions and were limited to only a few specific applicationsi.
Modern Duplex Development:
During the late 1960s and early 1970s, therewere several factors that led to the advancement of duplex steels. First, theintroduction of vacuum and argon oxygen decarburization (VOD and AOD) processesopened the door to produce modern duplex grades. These developments made it Possible to achieve low carbon content in combination with high chromium content, highnitrogen content, and a favorable balance of ferrite and austenite. Thisresulted in materials with very good properties. The alloy content providesgood resistance to local and uniform corrosion. The duplex microstructure contributesto high resistance to chloride stress corrosion cracking under many conditions andhigh strengthii. Modern duplex steels also have good weldability.These modernduplexes appeared at the same time period of increased activity in the offshoreindustry.
This industry required a stainless steel that could handle aggressiveenvironments. While austenitic steels could also stand up to these aggressive environments, a nickel shortage at the time drove up their prices. All of thesefactors combined to encourage the offshore oil industry to take a close look atDuplex Steelsi.
Duplex 2205 – The work horse of the DuplexFamily:
Duplex 2205 (UNS S31803/32205) was the first “second generation” duplexsteel to be developed commercially. It was developed and introduced by theGerman steel Krupp producer in the mid-1970s iii.It is still the most common duplexgrade today
and is currently considered the work horse of the Duplex familyiv.Duplex 2205 provides corrosion resistance in many environments that is superiorto types 304 (UNS S30400), 316 (UNS S31600) and 317 (UNS S31700) austeniticsteels. Also, the yield strength is about double that of austenitic steels.Itis interesting to note that the composition range that was originally set for 2205 (S31803) was later determined to be toobroad. Based on the original composition specifications, Duplex 2205 had the potentialto form detrimental intermetallic phases at elevated temperatures. In order toachieve optimum corrosion resistance and to avoid these intermetallic phases,the chromium, molybdenum and nitrogen levels need to be kept in the higher halfof the ranges for S31803. This modified 2205 is referred to as S32205 and is typicalof today’s commercial production of Duplex 2205iv.
While Duplex 2205 continues to gain momentumin various industries over time, in some cases the extraordinary corrosionresistance has been higher than needed. This has led to the development ofnumerous lean duplex grades, such as LDX 2101 (S32101), ATI 2003 (UNS 32003)and Duplex 2304 (UNS S32304). These new lean duplex stainless steels containless alloying elements than 2205 and are intended for applications in Which they can replace the 304 and even 316 grades. For example, lean duplex alloysare being used in many architectural applications due to the high strength, goodcorrosion resistance, and lower overall cost compared to the commonly usedstainless steel grade 316i.
Super Duplex and Hyper Duplex:
Also, starting in the 1980s, the oilindustry was one of the main drivers for the development of even higher alloyedduplex materials, referred to as super duplex and hyper duplex. These higheralloyed duplex grades are designed to handle extreme environments, such as the Highly corrosive conditions and pressures encountered at great depths in the newer oiland gas fields[v]. Super duplex grades have a pitting resistance equivalent (ameasure of resistance to pitting corrosion, also referred to as PRE or PREN) higherthan 40. Hyper duplex grades have a PRE number that is 48 or higher[v]. Currentgrades in production today include super duplex SAF 2507 SD (UNS S32750) andhyper duplex grades SAF 3207 HD (UNS S33207) and SAF 2707 HD (UNS S32707).These high alloy duplex materials have higher strength than Duplex 2205 and tendto have corrosion properties on par with austenitic 6MO (UNS NO8367) grades insome applications.
Duplex Usage Today:
While the current duplex stainless steelmarket is a very small percentage of the overall stainless steel volumes, theduplex sector is a growing industry with strong prospects for continued growth.Research from the International Stainless Steel Forum, ISSF, reveals thatduplex production soared from 6,000 metric tons a month in 2004 to 10,000 metrictons by 2005 and reached 22,000 metric tons in 2008v. Duplex steels continue to gain in popularity as various industries are
starting to take into consideration overalllife cycle costsvi. In addition to potential immediate material cost savings,duplex usage in many situations can also lead to longer life cycles and lower