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Duplex Stainless Steels

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Stainless steel has become indispensable in countless different industries over the past one hundred years.  Since its inception in the early 20th century, metallurgists and material scientists have continued to develop new stainless steel alloys that perform better than industry standards, as well as accomplish more demanding functions that can only be accomplished with high-performance materials.  This includes the family of steels known as duplex stainless steels.

While traditional carbon steel has been used for millennia, its major drawback is a susceptibility to corrosion.  Developing a form of steel that could withstand the elements and resist rusting became something of a Holy Grail.  Over the course of the 19th century, major advancements were made with the addition of chromium.  Finally, in the early 1900’s scientists finally achieved a stable, commercially viable form of stainless steel that was first marketed under the trade name of Staybrite.

Today, manufacturers and producers have many choices when it comes to stainless steel, including several popular duplex options.  The smartest option is to partner with an experienced metal supplier that is committed to helping its customers find the perfect material for every application.

What is duplex stainless steel?

Duplex stainless steels are a family of stainless steels that combine characteristics of both austenitic and ferritic grades. The metallurgical structure of duplex stainless steel consists of two distinct phases.  They have the face-centered cubic lattice of austenitic stainless and the body centered cubic lattice of ferritic stainless in equal proportion.

This provides duplex stainless steels with several benefits.  First, this family of stainless steel has superior corrosion resistance.  This is particularly true of chloride stress corrosion and chloride pitting corrosion.

At the same time, duplex stainless steels tend to have greater strength than traditional austenitic grades, such as 304 or 316.  To achieve these advancements, duplex steels have a higher chromium content, generally somewhere between 20% and 28%.  There’s also more molybdenum added, as much as 5%, with less nickel, usually below 9%.  As for nitrogen content, it usually is between 0.05% and 0.50%.

Duplex stainless steels are stronger than regular austenitic and ferritic stainless grades based on yield strength.  They have less nickel added and provide significant cost benefits over traditional stainless steel grades.  To take one example, type 304 stainless steel has a typical yield strength of 36 KSI.  By comparison, type 2205 standard duplex stainless steel has a yield strength at 74 KSI.  Meanwhile, a super duplex grade such as 2507 exhibits a yield strength of 84 KSI.

To better understand what sets duplex stainless steels apart, it’s necessary to point out that each of the classes of stainless steel, such as austenitic, ferritic, martensitic, and precipitation hardened, has a unique crystalline molecular structure that is responsible for its characteristics.  Today, there are over 150 unique grades that can be categorized in these five families, so no matter what the application is, manufacturers can rest assured there is a stainless steel grade that can do the job.

How was duplex stainless steel first developed?

The idea for duplex stainless steel has been around for a long time.  It was first conceived in the early 1930s.  The days saw a great deal of experimentation as scientists tried to figure out what was even possible with stainless steel, and which methods of alloying would prove viable in a commercial setting.  While the early duplex stainless steels still had a long way to go to match today’s varieties, it found uses in certain industries, such as papermaking and heat exchangers.

For the most part however, they hadn’t cracked the formula yet that would combine the advantages of both ferritic and austenitic steels, and so the early uses were limited. Several decades would pass before advances in decarburization made it possible to further reduce the amount of carbon while increasing the amount of chromium.  The late 1960’s would see the development of the modern day duplex stainless steels we are familiar with today.

What really drove the development of duplex stainless steels was a shortage of nickel that was increasing the price of austenitic stainless steel.  Duplex steels require less nickel and can be produced more economically.  There are additional cost savings associated with the increased strength that allows for thinner cross sections of material to bear the same load.

By the 1980’s, super and hyper duplex stainless steels were first developed.  These new grades were even stronger than their predecessors, while also being more corrosion resistant.  This combination of traits has made these alloys popular in several critical industries, such as the deep sea oil drilling, but unlike some of the more common duplex stainless steels, these high performance grades can still be quite costly.

What are the most popular duplex stainless steels?

For the above reasons, duplex stainless steel still accounts for a relatively small segment of the overall stainless steel market.  It is customarily divided into four categories of alloys. The first of these is known as lean duplex.

The lean duplex stainless steels contain little to no molybdenum and lower amounts of nickel.  These grades are widely found in storage tanks and similar applications, and a good example is type 2304.

Regular duplex has the standard amounts of nickel and molybdenum, usually 3-6% and 2-3%, respectively.  The most popular of the duplex stainless steels, 2205, falls into this category, and is common in the pharmaceutical industry.

Super duplex stainless steels are known for their high molybdenum (3.5-4%) and chromium (25%) content.  This makes them especially strong and corrosion resistant, and their superior performance makes them worth the cost in the oil, gas, and chemical industries.

Lastly, hyper duplex stainless steels have even greater amounts of molybdenum and chromium, 4.8% and 27% respectively. A good example is grade S32707, which can be found in the energy and oil industries, especially in deep sea applications.

Your Duplex Stainless Steel Supplier

For manufacturers and product designers that require a stainless steel that’s extremely strong, durable and corrosion resistant, duplex stainless steel is a great choice.  At Clinton Aluminum, our experienced and committed staff is standing by to answer even the most complicated questions.  Contact one of our friendly and knowledgeable customer representatives today to learn more.

 

 

 

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