Duplex stainless steels, which combine many of the beneficial properties of hydraulic tubes, were originally developed in the early 1930s. The initial 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 the right austenite-ferrite balance. Also, these early duplex grades were relatively loaded with carbon content since efficient process procedures for decarburization were unavailable at the time. Consequently, fabrications utilizing these materials tended to be mainly cast productions and were restricted to just one or two specific applications.
In the late 1960s and early 1970s, there were several factors that generated the expansion of duplex steels. First, the introduction of vacuum and argon oxygen decarburization (VOD and AOD) processes opened the door to generate modern duplex grades. These developments made it possible to accomplish low carbon content along with high chromium content, high nickel content, along with 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 contributes to high effectiveness against chloride stress corrosion cracking under many conditions and strengthii. Modern duplex steels likewise have good weldability.
These modern duplexes appeared at the same time period of increased activity in the offshore industry. This industry required a stainless steel that could handle aggressive environments. While austenitic steels may also stand up to these aggressive environments, a nickel shortage back then drove up their prices. Many of these factors combined to let the offshore oil industry to take a detailed examine Duplex Steels.
Duplex 2205 (UNS S31803/32205) was the 1st “second generation” duplex steel being developed commercially. It had been developed and designed by the German steel Krupp producer in the mid-1970siii. It continues to be most frequent duplex grade today and is also currently considered the work horse in the Duplex familyiv. Duplex 2205 provides corrosion resistance in several environments that is certainly preferable over types 304 (UNS S30400), 316 (UNS S31600) and 317 (UNS S31700) austenitic steels. Also, the yield strength is around double that relating to stainless steel seamless pipes.
It is actually interesting to keep in mind that the composition range which had been originally looking for 2205 (S31803) was later determined to be too broad. In line with the original composition specifications, Duplex 2205 had the opportunity to make 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 held in the larger 1 / 2 of the ranges for S31803. This modified 2205 is called S32205 and it is typical of today’s commercial manufacturing of Duplex 2205iv.
While Duplex 2205 is constantly gain momentum in different industries after a while, sometimes the extraordinary corrosion resistance has been greater than needed. It has generated the development of numerous lean duplex grades, such as 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 suitable for applications through which they could replace the 304 and also 316 grades. By way of example, lean duplex alloys are being used in lots of architectural applications due to the high strength, good corrosion resistance, and lower overall cost when compared to frequently used stainless steel grade 316i.
Also, starting in the 1980s, the oil industry was one of 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, such as the highly corrosive conditions and pressures encountered 39dexhpky great depths within the newer oil and gas fields[v]. Super duplex grades use a pitting resistance equivalent (a measure of resistance to pitting corrosion, also known as PRE or PREN) more 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). These 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 certain applications.
Whilst the duplex pipes is certainly a small percentage of the overall stainless volumes, the duplex sector is 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 a month 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 beginning to think about overall life cycle costsvi. As well as potential immediate material financial savings, duplex usage in many situations can also cause longer life cycles and reduce maintenance costs.