What is the difference between duplex and super duplex?

Stainless steels are segregated into various classes, mainly due to the distinction in their microstructural phases. While each class has a distinct microstructural phase, stainless steels that belong to the duplex category have a microstructure that is similar to two other classes, i.e. – ferritic and austenitic stainless steel. Duplex stainless steels contain a mixed microstructure with austenite and ferrite. The ratio of both these crystalline structures is in an equivalent ratio, which means that duplex steels contain both austenite and ferrite in a 50% ratio. Duplex stainless steels may further be divided into three additional subcategories, one of which is a super duplex stainless steel. This is why super duplex grades also have a dual microstructure. Duplex grades have very high PREN or pitting resistance equivalent number values. These values range from 22 to 45.

From all the three duplex categories, it is the super duplex kind of stainless steel that offers high end resistance to corrosion. The PREN of super duplex grades ranges between 38 – 45. The chemistry of such stainless steel grades is highly alloyed, more so than any conventional stainless steel alloy, for instance, grade 304 or even 316. Because they are highly alloyed, the resistance to corrosion they offer tends to be of a superior degree. Their mechanical strength is also very high. Although these stainless steel metals derive their strength and corrosion resistance from being highly alloyed, this is also the reason why super duplex steel grades tend to be more difficult to process. Furthermore, because of the higher contents of elements such as chromium, nickel, molybdenum, nitrogen, and even tungsten, the formation of intermetallic phases, becomes much easier. Due to these intermetallic phases, the impact resistance of the super duplex stainless steel variety leads to a drastic reduction. This is why buyers need to deal with manufacturers or suppliers, who are not only experienced but will also deliver high quality products. In instances where either stainless steel grade has been manufactured using faulty methods, the end product will yield very poor performance.

The price difference between duplex and super duplex

While both grades have corrosion resistance properties better than conventional austenitic stainless steel alloys, the resistance to pitting, in particular, is a quality that is higher in super duplex stainless steel grades. Resistance to pitting is an important characteristic, which is achieved by increasing the content of chromium to about 27%, as compared to the pre-existing 22% in standard duplex grades. However, by increasing the content of chromium, the dual microstructure of the alloy undergoes changes. And so, the chemistry of super duplex stainless steels will contain additions of nickel along with the other elements. Nickel, which is a traded commodity metal is expensive, as the cost is volatile. On the other hand, molybdenum being a rare element is expensive. By increasing the content of nickel and molybdenum in the alloy, the cost of super duplex alloys tends to increase.

Welding duplex and super duplex stainless steels

Unlike conventional austenitic stainless steel alloys, where the base metals are affected during welding, the HAZ in these alloys experiences issues. For instance, the major issues noted in the HAZ or heat affected weld zone, during welding include loss in toughness, post-weld cracking as well as a decrease in the alloys corrosion resistance properties. This is why experienced welders are required to weld either grade using SAW, TIG, and MIG techniques, with the alloys being given a proper post welding processing.

Duplex and  corrosion resistance

As explained above, the addition of chromium, nickel, molybdenum, nitrogen, and tungsten in their alloys improves their corrosion resistance behavior. The resistance to pitting could be valued as high as 45. Moreover, they can be used at elevated temperatures without the occurrence of oxidation corrosion. These alloys also have superior resistance to several alkalines, acidic and neutral solutions.

Duplex & super duplex stainless steel cutting speed

Because these alloys have a tendency to work hard, they may be machined keeping some parameters in mind. And so keeping in mind the alloy’s high strength and low thermal conductivity properties, manufacturers may make use of machines that are both stronger and rigid, in combination with high performance inserts. The machining may also require the use of a high quantity of coolant. Lastly, the use of correct speeds and feeds aids in the efficient machining of the alloys of duplex and super duplex stainless steels.

Duplex and Super Duplex Chemical Composition

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Duplex and Super Duplex Stainless Steel Equivalent Grades