What is Duplex Stainless Steel and what are its Properties?

what is duplex steel and its properties

Over the last century, stainless steel has become indispensable in a wide range of industries. Since its inception in the early twentieth century, metallurgists and material scientists have continued to develop new stainless steel alloys that outperform industry standards and perform more demanding functions that can only be achieved with high-performance materials. This includes the duplex stainless steel family of steels.

While traditional carbon steel has been used for millennia, its main disadvantage is corrosion. Creating steel that could withstand the elements and resist rust became a sort of Holy Grail. With the addition of chromium, major advances were made throughout the nineteenth century. Finally, in the early 1900s, scientists developed a stable, commercially viable form of stainless steel, which was initially marketed under the brand name Staybrite.

When it comes to stainless steel, today’s manufacturers and producers have many options, including several popular duplex options. The best option is to work with an experienced metal supplier who is dedicated to assisting its customers in finding the best material for each application.

What is Duplex Stainless Steel?

The duplex stainless steel grade is a type of stainless steel that combines austenitic and ferritic properties. Duplex stainless steel has two distinct phases in its metallurgical structure. They have an equal proportion of austenitic stainless face-centered cubic lattices and ferritic stainless body-centered lattices.

This provides several advantages to duplex stainless steel. For starters, this stainless steel family has superior corrosion resistance. This is especially true for chloride pitting corrosion and chloride stress corrosion.

At the same time, duplex stainless steels have higher strength than traditional austenitic grades like 304 or 316. Duplex steels have a higher chromium content to achieve these advancements, typically ranging between 20% and 28%. There is also more molybdenum, up to 5%, and less nickel, typically less than 9%. Nitrogen content is usually between 0.05% and 0.50%.

In terms of yield strength, duplex stainless steels outperform standard austenitic and ferritic stainless grades. They have less nickel added and offer significant cost savings over traditional stainless steel grades. For example, type 304 stainless steel has a typical yield strength of 36 KSI. In comparison, the yield strength of type 2205 standard duplex stainless steel is 74 KSI. Meanwhile, a super duplex grade such as 2507 has a yield strength of 84 KSI.

Duplex Stainless Steel Properties:

Outstanding corrosion resistance:

The increased addition of chromium and molybdenum increases the corrosion resistance of Duplex Stainless Steel. Molybdenum naturally repels corrosive media, whereas chromium forms a passivation layer that protects Duplex Stainless Steel components.

Does not succumb to crack corrosion:

Several grades of stainless steel frequently fail in the presence of concentrated and prolonged exposure to chloride-containing environments. Duplex Stainless Steel, on the other hand, is extremely dependable in situations where other grades of SS fail to perform well. Even when exposed to chloride environments continuously, stainless steel remains strong and does not corrode.

Strength & Reliability:

Duplex Stainless Steel has superior mechanical properties when compared to regular austenitic grades of stainless steel. Duplex Stainless Steel retains its strength and physical properties even when beaten into thin and flat sheets, delivering consistent high-performance.

Weldability:

Duplex Stainless Steel is more difficult to weld and machine than austenitic grades such as Stainless Steel 316 Seamless Pipes. To avoid precipitation formation, post-weld annealing and rapid cooling are required.

Which duplex stainless steel is the most popular?

Due to the aforementioned reasons, duplex stainless steel remains a relatively small segment of the overall stainless steel market. Alloys are typically classified into four types. The first of these is referred to as a lean duplex.

The lean duplex stainless steels have little to no molybdenum and less nickel. Type 2304 is a good example of a grade that is commonly used in storage tanks and similar applications.

Nickel and molybdenum levels in the regular duplex are typically 3-6% and 2-3%, respectively. 2205, the most widely used duplex stainless steel, falls into this category and is widely used in the pharmaceutical industry.

Super duplex stainless steels are distinguished by their high molybdenum (3.5-4%) and chromium (25%). This makes them particularly strong and corrosion resistant, and their superior performance justifies the investment in the oil, gas, and chemical industries.

Finally, hyper duplex stainless steels contain even more molybdenum and chromium, at 4.8% and 27%, respectively. Grade S32707 is a good example, which can be found in the energy and oil industries, particularly in deep-sea applications.  

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