Duplex stainless steels, which combine most of the beneficial properties of hydraulic pipes, were originally developed in early 1930s. The primary duplex grades provided good performance characteristics, but had limitations in the as-welded condition. The metallurgical processes during that time were not suitable for producing grades with all the right austenite-ferrite balance. Also, these early duplex grades were relatively full of carbon content since efficient process approaches for decarburization were unavailable at that time. Consequently, fabrications with such materials tended to become mainly cast productions and were restricted to just one or two specific applications.
In the late 1960s and early 1970s, there was several factors that resulted in the growth of duplex steels. First, the introduction of vacuum and argon oxygen decarburization (VOD and AOD) processes opened the entrance to make modern duplex grades. These developments made it possible to obtain low carbon content along with high chromium content, high nickel content, as well as a favorable balance of ferrite and austenite. This contributed to materials with excellent properties. The alloy content provides good potential to deal with local and uniform corrosion. The duplex microstructure leads to high resistance to chloride stress corrosion cracking under many conditions and strengthii. Modern duplex steels also provide good weldability.
These modern duplexes appeared concurrently time period of increased activity in the offshore industry. This industry required a stainless that may handle aggressive environments. While austenitic steels might also endure these aggressive environments, a nickel shortage back then drove up their prices. Many of these factors combined to encourage the offshore oil industry to consider a detailed look at Duplex Steels.
Duplex 2205 (UNS S31803/32205) was the very first “second generation” duplex steel to get developed commercially. It had been developed and created by the German steel Krupp producer inside the mid-1970siii. It is still the most typical duplex grade today and it is currently considered the project horse of the Duplex familyiv. Duplex 2205 provides corrosion resistance in lots of environments that is certainly better than types 304 (UNS S30400), 316 (UNS S31600) and 317 (UNS S31700) austenitic steels. Also, the yield strength is around double that relating to u bend pipes.
It is actually interesting to notice that the composition range which was originally set for 2205 (S31803) was later determined to be too broad. Based on the original composition specifications, Duplex 2205 had the possible to create detrimental intermetallic phases at elevated temperatures. To get optimum corrosion resistance as well as avoid these intermetallic phases, the chromium, molybdenum and nickel levels should be held in the higher one half of the ranges for S31803. This modified 2205 is known as S32205 and it is typical of today’s commercial creation of Duplex 2205iv.
While Duplex 2205 continues to gain momentum in various industries after a while, in some cases the extraordinary corrosion resistance has become more 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 suitable for applications in which they can replace the 304 as well as 316 grades. For instance, lean duplex alloys are used in several architectural applications due to high strength, good corrosion resistance, and reduce overall cost when compared to the commonly used stainless steel grade 316i.
Also, starting within the 1980s, the oil industry was one of the many drivers for the introduction of even higher alloyed duplex materials, called super duplex and hyper duplex. These higher alloyed duplex grades are created to handle extreme environments, including the highly corrosive conditions and pressures encountered 39dexhpky great depths in the newer oil and gas fields[v]. Super duplex grades have got a pitting resistance equivalent (a way of measuring resistance to pitting corrosion, also called PRE or PREN) beyond 40. Hyper duplex grades have got 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 usually have corrosion properties comparable to austenitic 6MO (UNS NO8367) grades in certain applications.
While the seamless stainless steel tubes is an extremely small amount of the overall steel volumes, the duplex sector is a growing industry with strong prospects for continued growth. Research through the International Stainless Forum, ISSF, reveals that duplex production soared from 6,000 metric tons on a monthly basis in 2004 to 10,000 metric tons by 2005 and reached 22,000 metric tons in 2008v. Duplex steels still grow in popularity as various industries are starting to consider overall life cycle costsvi. Together with potential immediate material cost savings, duplex usage in lots of situations could also cause longer life cycles and minimize maintenance costs.