The first cast of duplex was made at Avesta in Sweden in 1930 when the idea of duplex stainless steels were subjected to discuss within industries. During last 30 years the duplex steels have been remarkably evolved and used in a significant way. The evolution in steel making techniques affected mainly to this take off.
Duplex stainless steels is a combination of many of the beneficial properties of ferritic and austenitic steels. First duplex steel grades had a great performance characteristics as well as limitations. The metallurgical processes at that time were not suitable for producing grades with the right austenite-ferrite balance. These early duplex steels were also high in carbon content because of the unavailability of efficient decarburization techniques. Because of these limitations the products using this materials were limited to specific applications. (Alvares-Armas, 2008)
Advancement of processing techniques in late 1960s and early 1970s led to the fabrication of modern duplex steels. Introduction of vacuum and argon oxygen decarburization (VOD and AOD) was the main factor that led to this massive development in duplex steels during this period. By the advancement of these techniques it was possible to achieve steels with low carbon content mixing with high nitrogen and chromium contents in balance with austenite and ferrite. This eliminated lot of shortcomings in old duplex steels. The alloy content provided a resistant to corrosion while duplex microstructure contributed to prevent chloride stress cracking under severe conditions. And these duplex steels possessed good weld ability also.
Modern duplexes were used for offshore and near shore industries where conditions are severe and aggressive. Even though austenitic steels could also be used for aggressive environment the prices of austenitic steels were high during the time due to shortage of nickel. Hence the usage of these duplex alloys were increased. (Gagnepain, 2008)
The need of even higher alloyed duplex was risen with massive development of oil industry. In 1980s the oil industries needed alloys that can handle extreme environments such as highly corrosive, and extreme pressure exerted environments like sea beds and great depths which duplex steels at that time could not handle. As a result of this super duplex steels and hyper duplex steels were crafted. The pitting resistance of super duplex steels are higher than 40 while in hyper duplex steels the number is greater than 48. Pitting resistance is a measure of resistance to pitting corrosion, also referred to as PRE or PREN.
Super duplex SAF 2507 SD (UNS S32750) and hyper duplex grades SAF 3207 HD (UNS S33207) and SAF 2707 HD (UNS S32707) are some of the main grades in production today.
Processing and manufacturing
Processing and manufacturing of super duplex is as same as the austenitic steels. With the difference in microstructure and metallurgy there are certain...