Duplex stainless steels, which combine many of the beneficial properties of hydraulic pipes, were originally developed in early 1930s. The original duplex grades provided good performance characteristics, but had limitations within 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 rich in carbon content since efficient process procedures for decarburization were unavailable at that time. Consequently, fabrications using these materials tended to get mainly cast productions and were limited to only some specific applications.
In the late 1960s and early 1970s, there have been several factors that triggered the continuing development of duplex steels. First, the introduction of vacuum and argon oxygen decarburization (VOD and AOD) processes opened the entrance to create modern duplex grades. These developments made it feasible to obtain low carbon content in combination with high chromium content, high nickel content, along with a favorable balance of ferrite and austenite. This led to materials with really good properties. The alloy content provides good potential to deal with local and uniform corrosion. The duplex microstructure contributes to high resistance to chloride stress corrosion cracking under many conditions and strengthii. Modern duplex steels also have 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 that could handle aggressive environments. While austenitic steels might also resist 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 take a detailed have a look at Duplex Steels.
Duplex 2205 (UNS S31803/32205) was the first “second generation” duplex steel to be developed commercially. It had been developed and designed by the German steel Krupp producer in the mid-1970siii. It continues to be the most common duplex grade today and is currently considered the project horse of the Duplex familyiv. Duplex 2205 provides corrosion resistance in lots of 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 those of u bend pipes.
It is interesting to notice the composition range that was originally looking for 2205 (S31803) was later going to be too broad. Depending on the original composition specifications, Duplex 2205 had the possible to make detrimental intermetallic phases at elevated temperatures. To experience optimum corrosion resistance and also to avoid these intermetallic phases, the chromium, molybdenum and nickel levels need to be held in the greater one half of the ranges for S31803. This modified 2205 is called S32205 and it is typical of today’s commercial manufacture of Duplex 2205iv.
While Duplex 2205 is constantly gain momentum in a variety of industries after a while, occasionally the extraordinary corrosion resistance continues to be higher than needed. It has resulted in the growth 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 they are designed for applications in which they are able to replace the 304 and even 316 grades. For instance, lean duplex alloys are employed in several architectural applications due to high strength, good corrosion resistance, and lower overall cost when compared to widely used stainless grade 316i.
Also, starting inside the 1980s, the oil industry was one of the primary 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, including the highly corrosive conditions and pressures encountered 39dexhpky great depths inside the newer oil and gas fields[v]. Super duplex grades have got a pitting resistance equivalent (a measure of effectiveness against pitting corrosion, also called PRE or PREN) higher than 40. Hyper duplex grades possess a PRE number that may be 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 normally have corrosion properties on par with austenitic 6MO (UNS NO8367) grades in certain applications.
As the heat exchanger tubes is a very small portion of the complete stainless-steel volumes, the duplex sector can be a growing industry with strong prospects for continued growth. Research in the International Stainless Steel 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 gain in popularity as various industries are beginning to take into consideration overall life cycle costsvi. Together with potential immediate material saving money, duplex usage in many situations also can result in longer life cycles minimizing maintenance costs.