The History of Duplex Stainless Steel

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　　Early Duplex Development:

　　Duplex stainless steels, which combine many of the beneficial properties of ferritic and austenitic steels, were originally developed in the early 1930′s. The initial duplex grades provided good performance characteristics, but had limitations in the as-welded condition. The metallurgical processes at that time were not suitable for producing grades with the right austenite-ferrite balance. Also, these early duplex grades were relatively high in carbon content since efficient process techniques for decarburization were not available at the time. Consequently, fabrications using these materials tended to be mainly cast productions and were limited to only a few specific applicationsi.

　　Modern Duplex Development:

　　During the late 1960s and early 1970s, there were several factors that led to the advancement of duplex steels. First, the introduction of vacuum and argon oxygen decarburization (VOD and AOD) processes opened the door to produce modern duplex grades. These developments made it possible to achieve low carbon content in combination with high chromium content, high nitrogen content, and a favorable balance of ferrite and austenite. This resulted in materials with very good properties. The alloy content provides good resistance to local and uniform corrosion. The duplex microstructure contributes to high resistance to chloride stress corrosion cracking under many conditions and high strengthii. Modern duplex steels also have good weldability.

　　These modern duplexes appeared at the same time period of increased activity in the offshore industry. This industry required a stainless steel that could handle aggressive environments. While austenitic steels could also stand up to these aggressive environments, a nickel shortage at the time drove up their prices. All of these factors combined to encourage the offshore oil industry to take a close look at Duplex Steelsi.

　　Duplex 2205 – The work horse of the Duplex Family:

　　Duplex 2205 (UNS S31803/32205) was the first “second generation” duplex steel to be developed commercially. It was developed and introduced by the German steel Krupp producer in the mid-1970siii. It is still the most common duplex grade today and is currently considered the work horse of the Duplex familyiv. Duplex 2205 provides corrosion resistance in many environments that is superior to types 304 (UNS S30400), 316 (UNS S31600) and 317 (UNS S31700) austenitic steels. Also, the yield strength is about double that of austenitic steels.

　　It is interesting to note that the composition range that was originally set for 2205 (S31803) was later determined to be too broad. Based on the original composition specifications, Duplex 2205 had the potential to form detrimental intermetallic phases at elevated temperatures. In order to achieve optimum corrosion resistance and to avoid these intermetallic phases, the chromium, molybdenum and nitrogen levels need to be kept in the higher half of the ranges for S31803. This modified 2205 is referred to as S32205 and is typical of today’s commercial production of Duplex 2205iv.

　　Lean Duplex:

　　While Duplex 2205 continues to gain momentum in various industries over time, in some cases the extraordinary corrosion resistance has been higher than needed. This has led to the development of numerous lean duplex grades, such as LDX 2101 (S32101), ATI 2003 (UNS 32003) and Duplex 2304 (UNS S32304). These new lean duplex stainless steels contain less alloying elements than 2205 and are intended for applications in which they can replace the 304 and even 316 grades. For example, lean duplex alloys are being used in many architectural applications due to the high strength, good corrosion resistance, and lower overall cost compared to the commonly used stainless steel grade 316i.

　　Super Duplex and Hyper Duplex:

　　Also, starting in the 1980s, the oil industry was one of the main drivers for the development of even higher alloyed duplex materials, referred to as super duplex and hyper duplex. These higher alloyed duplex grades are designed to handle extreme environments, such as the highly corrosive conditions and pressures encountered at great depths in the newer oil and gas fields[v]. Super duplex grades have a pitting resistance equivalent (a measure of resistance to pitting corrosion, also referred to as PRE or PREN) higher than 40. Hyper duplex grades have a PRE number that is 48 or higher[v]. Current grades in production today include super duplex SAF 2507 SD (UNS S32750) and hyper 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 tend to have corrosion properties on par with austenitic 6MO (UNS NO8367) grades in some applications.

　　Duplex Usage Today:

　　While the current duplex stainless steel market is a very small percentage of the overall stainless steel volumes, the duplex sector is a growing industry with strong prospects for continued growth. Research from the International Stainless Steel Forum, ISSF, reveals that duplex production soared from 6,000 metric tons a month in 2004 to 10,000 metric tons 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 overall life cycle costsvi. In addition to potential immediate material cost savings, duplex usage in many situations can also lead to longer life cycles and lower maintenance costs.