Duplex stainless steels, which combine many of the beneficial properties of hydraulic tubes, were originally developed in the early 1930s. The primary duplex grades provided good performance characteristics, but had limitations from the as-welded condition. The metallurgical processes during that time were not suited to producing grades together with the right austenite-ferrite balance. Also, these early duplex grades were relatively loaded with carbon content since efficient process approaches for decarburization were not available back then. Consequently, fabrications utilizing these materials tended to get mainly cast productions and were limited by just a few specific applications.
Throughout the late 1960s and early 1970s, there was several factors that led to the expansion of duplex steels. First, the creation of vacuum and argon oxygen decarburization (VOD and AOD) processes opened the doorway to produce modern duplex grades. These developments made it possible to achieve low carbon content along with high chromium content, high nickel content, plus a favorable balance of ferrite and austenite. This ended in materials with excellent properties. The alloy content provides good effectiveness against local and uniform corrosion. The duplex microstructure plays a part in high effectiveness against chloride stress corrosion cracking under many conditions and high strengthii. Modern duplex steels also provide good weldability.
These modern duplexes appeared simultaneously period of increased activity within the offshore industry. This industry required a stainless steel that could handle aggressive environments. While austenitic steels can also resist these aggressive environments, a nickel shortage at the time drove up their prices. Many of these factors combined to encourage the offshore oil industry to adopt a close examine Duplex Steels.
Duplex 2205 (UNS S31803/32205) was the first “second generation” duplex steel to be developed commercially. It was actually developed and designed by the German steel Krupp producer inside the mid-1970siii. It is still the most popular duplex grade today and is currently considered the job horse of your Duplex familyiv. Duplex 2205 provides corrosion resistance in several environments that may be superior to 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 interesting to keep in mind 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 potential to create detrimental intermetallic phases at elevated temperatures. To get optimum corrosion resistance and to avoid these intermetallic phases, the chromium, molybdenum and nickel levels should be saved in the bigger 50 % of the ranges for S31803. This modified 2205 is called S32205 which is typical of today’s commercial creation of Duplex 2205iv.
While Duplex 2205 will continue to gain momentum in several industries over time, in some instances the extraordinary corrosion resistance is greater 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 are intended for applications where they can replace the 304 and even 316 grades. By way of example, lean duplex alloys are used in many architectural applications because of the high strength, good corrosion resistance, and minimize overall cost when compared to popular steel grade 316i.
Also, starting within the 1980s, the oil industry was one of the many drivers for the growth of even higher alloyed duplex materials, known as super duplex and hyper duplex. These higher alloyed duplex grades are made to handle extreme environments, such as 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 measure of resistance to pitting corrosion, also called PRE or PREN) beyond 40. Hyper duplex grades have got a PRE number which 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). 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.
Whilst the seamless stainless steel tubes is certainly a small number of the general steel volumes, the duplex sector is actually a growing industry with strong prospects for continued growth. Research from your International Stainless Steel Forum, ISSF, reveals that duplex production soared from 6,000 metric tons on a monthly basis in 2004 to ten thousand 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. As well as potential immediate material saving money, duplex usage in many situations also can lead to longer life cycles and minimize maintenance costs.