Duplex stainless steels, which combine lots of the beneficial properties of hydraulic pipes, were originally developed in early 1930s. The initial duplex grades provided good performance characteristics, but had limitations from the as-welded condition. The metallurgical processes during those times were not suitable for producing grades together with the right austenite-ferrite balance. Also, these early duplex grades were relatively full of carbon content since efficient process methods for decarburization were unavailable at that time. Consequently, fabrications utilizing these materials tended being mainly cast productions and were limited to only some specific applications.
During the late 1960s and early 1970s, there were several factors that resulted in the growth of duplex steels. First, the creation of vacuum and argon oxygen decarburization (VOD and AOD) processes opened the entranceway to generate modern duplex grades. These developments made it feasible to obtain low carbon content in combination with high chromium content, high nickel content, and a favorable balance of ferrite and austenite. This ended in materials with great properties. The alloy content provides good potential to deal with 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 provide good weldability.
These modern duplexes appeared at the same time time period of increased activity in the offshore industry. This industry required a stainless-steel which could handle aggressive environments. While austenitic steels might also withstand these aggressive environments, a nickel shortage at that time drove up their prices. Most of these factors combined to encourage the offshore oil industry to consider a close have a look at Duplex Steels.
Duplex 2205 (UNS S31803/32205) was the initial “second generation” duplex steel to get developed commercially. It was developed and designed by the German steel Krupp producer in the mid-1970siii. It is still the most frequent duplex grade today which is currently considered the project horse of your Duplex familyiv. Duplex 2205 provides corrosion resistance in several environments that is certainly better than types 304 (UNS S30400), 316 (UNS S31600) and 317 (UNS S31700) austenitic steels. Also, the yield strength is about double those of stainless steel u tubes.
It is interesting to remember how the composition range which was originally set for 2205 (S31803) was later going to be too broad. In accordance with the original composition specifications, Duplex 2205 had the opportunity to produce detrimental intermetallic phases at elevated temperatures. In order to achieve optimum corrosion resistance as well as avoid these intermetallic phases, the chromium, molybdenum and nickel levels need to be stored in the higher one half of the ranges for S31803. This modified 2205 is referred to as S32205 and it is typical of today’s commercial creation of Duplex 2205iv.
While Duplex 2205 will continue to gain momentum in different industries as time passes, in some instances the extraordinary corrosion resistance has been greater than needed. This has generated the development 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 therefore are meant for applications in which they are able to replace the 304 and also 316 grades. By way of example, lean duplex alloys are employed in several architectural applications due to high strength, good corrosion resistance, and minimize overall cost in comparison to the frequently used stainless grade 316i.
Also, starting inside the 1980s, the oil industry was one of the many drivers for the development of even higher alloyed duplex materials, called 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 within the newer oil and gas fields[v]. Super duplex grades have got a pitting resistance equivalent (a way of measuring effectiveness against pitting corrosion, also known as PRE or PREN) beyond 40. Hyper duplex grades have 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). These high alloy duplex materials have higher strength than Duplex 2205 and normally have corrosion properties comparable to austenitic 6MO (UNS NO8367) grades in a few applications.
As the seamless stainless steel tubes is a very small number of the complete stainless volumes, the duplex sector can be a growing industry with strong prospects for continued growth. Research in the International Stainless 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 still gain in popularity as various industries are beginning to consider overall life cycle costsvi. Along with potential immediate material cost savings, duplex usage in numerous situations also can result in longer life cycles minimizing maintenance costs.