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It is important to note that there are several key differences between Duplex Stainless Steels and Super Duplex Stainless Steels. In general, super duplex stainless steels are based on chromium alloys added at a concentration of 25%, whereas duplex stainless steels are based on chromium alloys added at a concentration of 22%. As chromium is added to a material, the level of pitting corrosion resistance will also be increased. However, there are other variables to consider as well.

Duplex Stainless Steel

Stainless steels that have the designation Duplex are named such because they are made up of two duel phases, austenitic (face-covered cubic lattice), and ferrite (body-centered cubic lattice). In comparison with traditional austenitic stainless steels, super duplex ss have a high chrome content that ranges from 20 – 28%, a molybdenum content that can reach 5%, a lower nickel content that stands at 9%, and a nitrogen content ranging from 0.5% to 0.05 – 0.5%. 

The purpose of this type of stainless steel is to produce alloys in which the microstructure contains both ferrite and austenite and it is intended to produce alloys containing 50/50 mixtures. The mix can be 40/60, respectively. A stainless steel with such a microstructure is capable of exceeding austenitic stainless steel in terms of tensile strength, as well as demonstrating enhanced corrosion resistance, including crevice corrosion, pitting, and stress corrosion cracking, when compared to austenitic stainless steels.

Duplex Stainless Steel pipes

The main characteristics of Duplex stainless steel are that it is primarily characterized by molybdenum content of up to 5%, chromium content of up to 28%, as well as a very low level of nickel content, in comparison with austenitic stainless steel. It has been found that 2205 is the most widely used duplex stainless steel, which includes 22% chrome and 5% nickel, while 2507 is duplex stainless steel with 7% nickel and 25% chromium, and because of the greater corrosion resistance of the two, it is also known as a super duplex.

This type of steel has significantly higher mechanical characteristics than particularly austenitic steel, so it is very resistant to corrosion, and at a low temperature it can transform into a ductile or we can say brittle steel at around 50 degrees Celsius.

Super Duplex Stainless Steel

PREN stands for Pitting Resistance Equivalent Number, calculated from %Cr + 3.3x%Mo + 16x%N, in which Cr = chrome, Mo = molybdenum, and N = nitrogen, where Cr = molybdenum. When chromium content is increased by 34 to >40, the PREN increases, suggesting a greater ability to resist pitting corrosion in a wide range of environments. As a result of their long lifespan in seawater, super duplex grades are often the choice for subsea and marine applications. 

Stainless steels such as super duplex stainless steel are another popular type of stainless steel, and they are known for their cracking resistance, stress corrosion, and mechanical properties that outperform all other stainless steels. The alloy is composed of an austenitic-ferrite nickel and chrome alloy with molybdenum added. Compared to other conventional austenitic stainless steels, super duplex stainless steels have an excellent tensile strength at moderate temperatures as well as excellent resistance to pitting.

Super Duplex Stainless Steel pipes

Because of its high corrosion resistance, super duplex stainless steel type is an ideal material for both offshore and onshore environments in different applications due to its high corrosion resistance.

As with all stainless steel types, duplex and super duplex have their own properties and have been considered to be the best choice for various industrial and domestic applications. As a matter of fact, almost every manufacturer around the world uses these types of stainless steel for manufacturing applications and industrial equipment. The stainless steels used in this sector have been tested and inspected according to the standards and regulations of the industry, making it easy for you to use both stainless steels for industrial applications. 

Cost-Efficient Use Of Alloying Additions

To maintain the desired balance between austenitic and ferritic microstructures, it would normally be necessary to add alloying elements such as nickel to enhance chromium content. In super duplex stainless steels, nitrogen is utilized as a primary alloying element, which results in an attractive combination of strength, excellent corrosion resistance, and competitive pricing that is competitive against a number of corrosion-resistant grades which contain far higher alloyed contents.

As a result of its dual-phase microstructure, super duplex stainless steels have a combination of austenitic and ferritic grains, making them more attractive than austenitic stainless steels. They are generally twice as strong as austenitic stainless steel. Their toughness and ductility are somewhere between austenitic and ferritic stainless steel grades. A further advantage over austenitic stainless steels is their resistance to cracking due to stress corrosion.

This is the first time that hyperduplex stainless steels have been developed, consisting of a composition of 27 and 29 percent chromium, which increases the corrosion resistance even further. However, they are more difficult to manufacture, are only available in tube form, and are usually not available from stock.

How Does The Duplex And Super Duplex Differ From Each Other?

A stainless steel is classified into a number of classes based on its microstructural phases which are primarily different from the other classes. While every class of stainless steel has a distinct microstructural phase, stainless steels belonging to the duplex category have a microstructure that is very similar to the microstructure of two other classes, namely the ferritic and austenitic stainless steels. It is known that duplex stainless steel contains a mixture of austenite and ferrite as the microstructure of the metal. Having both austenite and ferrite present in duplex steel in a 50% ratio is equivalent to the ratio of both these crystalline structures. 

This type of stainless steel is called a super duplex stainless steel. It can be further divided into three subcategories, one of which is called a super duplex stainless steel. Due to the dual microstructure of super duplex grades, they also have a very high pitting resistance equivalent number or PREN (pitting resistance equivalent numbers), with values ranging from 22 to 45. These numbers indicate the extent to which super duplex grades resist pitting attacks.

differences between Duplex Stainless Steel and Super Duplex Stainless Steel

In addition to the three duplex types, the super duplex type of stainless steel offers high corrosion resistance. Its PREN ranges from 38 to 45. 

As a result of the incredible alloying of such stainless steel grades, such stainless steel grades have much higher chemistry than conventional stainless steel grades like grades 304 and 316. 

Due to the fact that they are highly alloyed, their corrosion resistance is of high quality, and their mechanical strength is also very high. It should be noted, however, that stainless steels such as super duplex steel are very tough due to the high alloying they possess, and this is also the reason why super duplex steel grades can be more difficult to process. Moreover, the higher content of elements such as chromium, nickel, molybdenum, nitrogen, and even tungsten in these materials, makes it remarkably easy to form intermetallic phases. This intermetallic phase also results in a drastic reduction in the impact resistance of the super duplex stainless steel variety due to the presence of intermetallic phases within the material. Therefore, buyers must deal with manufacturers or suppliers who are not only experienced but will also produce high-quality products. If either stainless steel grade has been manufactured using faulty methods, the end product may yield a very poor performance if manufacturing process used to manufacture it is faulty.

A Comparison Of The Price Difference Between Duplexes And Super Duplexes

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 more evident in super duplex stainless steel grades, compared to conventional austenitic stainless steel alloys. As compared to standard duplex grades, which contain about 22% chromium, it is possible to achieve a high level of pitting resistance by increasing the chromium content to about 27%. A greater quantity of chromium changes the dual microstructure of super duplex stainless steel, which is why if the alloy is made of nickel, nickel will be added to its chemistry along with other elements in super duplex stainless steels. As a commodity metal, nickel is an expensive metal because its price fluctuates. Molybdenum, on the other hand, being a rare element, is also expensive. Increasing the concentrations of nickel and molybdenum in the alloy tends to lead to a higher price for super duplex steels.

The Welding Of Duplex And Super Duplex Stainless Steels

A significant difference between conventional austenitic stainless steel alloys and these alloys is that, unlike conventional austenitic stainless steel alloys, the HAZ in these alloys is affected by welding. During welding, there are several issues are noted in the HAZ or heat-affected weld zone, such as loss of toughness, cracks after welding, and a decrease in the corrosion resistance of the alloy. 

Welders with experience in SAW, TIG, and MIG welding processes will be able to weld either grade using SAW, TIG, or MIG welding techniques, with alloys being subjected to proper post-welding treatment.

Corrosion Resistance Of Duplex And Superduplex

As explained above, the addition of chromium, nickel, molybdenum, nitrogen, and tungsten to their alloys improves their corrosion resistance behavior. Their pitting resistance could reach 45 percent, and they have superior oxidation corrosion resistance. Furthermore, they can be used at elevated temperatures without oxidation corrosion. Additionally, they are resistant to many alkalines, acids, and neutrals.

The Cutting Speed Of Duplex And Superduplex Stainless Steel

It has been known that these alloys tend to work hard, so it is likely to be wise to machine them in accordance with some parameters in order to ensure they will perform as intended. By taking into consideration the alloy's strong thermal conductivity and its low thermal conductivity properties, manufacturers will have the option of making use of machines that are both rigid and stronger, and combining them with high-performance inserts. As well as using a lot of coolant in the machining process, there will also be a lot of preparation needed during the process. The use of specific speeds and feeds during the machining of duplex and super duplex stainless steel alloys has proven to be a very effective and efficient method for ensuring an efficient process of machining these alloys.