Duplex stainless steels, which combine a lot of the beneficial properties of stainless steel hydraulic tubing, were originally developed in the early 1930s. The initial duplex grades provided good performance characteristics, but had limitations inside the as-welded condition. The metallurgical processes at that time were not suitable for producing grades using the right austenite-ferrite balance. Also, these early duplex grades were relatively high in carbon content since efficient process methods for decarburization were not available at the time. Consequently, fabrications with such materials tended to be mainly cast productions and were confined to just a few specific applications.
Through the late 1960s and early 1970s, there are several factors that triggered the advancement of duplex steels. First, the roll-out of vacuum and argon oxygen decarburization (VOD and AOD) processes opened the doorway to generate modern duplex grades. These developments made it possible to obtain low carbon content together with high chromium content, high nickel content, along with a favorable balance of ferrite and austenite. This ended in materials with very good properties. The alloy content provides good resistance to local and uniform corrosion. The duplex microstructure leads to high effectiveness against chloride stress corrosion cracking under many conditions and high strengthii. Modern duplex steels also have good weldability.
These modern duplexes appeared concurrently time of increased activity in the offshore industry. This industry required a steel that can handle aggressive environments. While austenitic steels could also stand up to these aggressive environments, a nickel shortage at the time drove up their prices. Most of these factors combined to encourage the offshore oil industry to take a close take a look at Duplex Steels.
Duplex 2205 (UNS S31803/32205) was the 1st “second generation” duplex steel being developed commercially. It was actually developed and introduced by the German steel Krupp producer inside the mid-1970siii. It remains the most frequent duplex grade today and is currently considered the work horse of the Duplex familyiv. Duplex 2205 provides corrosion resistance in lots of environments that may be superior to types 304 (UNS S30400), 316 (UNS S31600) and 317 (UNS S31700) austenitic steels. Also, the yield strength is all about double that relating to u bend pipes.
It can be interesting to remember that this composition range which had been originally looking for 2205 (S31803) was later going to be too broad. Depending on the original composition specifications, Duplex 2205 had the opportunity to produce detrimental intermetallic phases at elevated temperatures. To get optimum corrosion resistance as well as to avoid these intermetallic phases, the chromium, molybdenum and nickel levels have to be held in the higher 50 % of the ranges for S31803. This modified 2205 is referred to as S32205 which is typical of today’s commercial creation of Duplex 2205iv.
While Duplex 2205 continues to gain momentum in several industries over time, in some instances the extraordinary corrosion resistance has become higher than needed. It has triggered the creation of numerous lean duplex grades, including 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 so are designed for applications in which they may replace the 304 and even 316 grades. By way of example, lean duplex alloys are used in many architectural applications as a result of high strength, good corrosion resistance, minimizing overall cost compared to the widely used stainless-steel grade 316i.
Also, starting in the 1980s, the oil industry was one of the many drivers for the development of even higher alloyed duplex materials, referred to as super duplex and hyper duplex. These higher alloyed duplex grades are made to handle extreme environments, like the highly corrosive conditions and pressures encountered 39dexhpky great depths in the newer oil and gas fields[v]. Super duplex grades use a pitting resistance equivalent (a measure of effectiveness against pitting corrosion, also known as PRE or PREN) beyond 40. Hyper duplex grades use a PRE number that is certainly 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). Extremely 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.
Even though the duplex pipes is certainly a small percentage of the complete 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 monthly 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 beginning to take into consideration overall life cycle costsvi. Together with potential immediate material cost benefits, duplex usage in many situations can also lead to longer life cycles and minimize maintenance costs.