Stainless steels are divided into different categories mainly due to their different microstructure phases. Although each grade of stainless steel has a different microstructure phase, the microstructure of duplex stainless steel is similar to the other two classes of stainless steel, named ferrite stainless steel and austenitic stainless steel. Duplex stainless steel has a mixed structure of austenite and ferrite. The ratio of the two crystal structures is equal, which means that 50% of the duplex steel contains both austenite and ferrite. Duplex stainless steel can be further divided into three subcategories, one of which is super duplex stainless steel. That's why the super duplex also has a double microstructure. The duplex stage has a very high PREN or pitting resistance equivalent. The value ranges from 22 to 45.
From all three duplex categories, it is super duplex type stainless steel that offers high-end corrosion resistance. The PREN value of super duplex ranges from 38 to 45. The chemical compositions of this stainless steel are highly alloyed, higher than any conventional stainless steel alloy, such as grade 304 or even 316. Because they are highly alloyed, they tend to provide corrosion resistance to a superior degree. They also have high mechanical strength. While the strength and corrosion resistance of these stainless steel metals comes from high alloying, this is why super duplex grades are often more difficult to work with. In addition, the formation of intermetallic phases is made more easily by the high content of elements such as chromium, nickel, molybdenum, nitrogen and even tungsten. Because of these intermetallic phases, the impact resistance of super duplex stainless steel decreases dramatically. That's why buyers need to deal with manufacturers or suppliers who not only have experience, but also will provide high quality products. In the case of stainless steel grades fabricated using the wrong method, the end product will produce very poor performance.