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DUPLEX STAINLESS STEEL

all you need to know about duplex stainless steel in this article

What is a Duplex Stainless Steel?

The microstructure of Duplex stainless steel consists of 50% austenitic and 50% ferritic steel grains with different ratios being employed as needed.

As a combination of two types of steel, Duplex took the best features of both while eliminating some of their prominent flaws.

This is why Duplex outperforms most of austenitic and ferritic alloys when they are used individually.

Origins of Duplex can be traced back to Sweden where this steel type was developed during 1930s. However, the material started gaining popularity at the very end of the century with the advancement of metal processing technologies.

The structure of Duplex has also been significantly optimized over the years with new grades being developed constantly.

Addition of certain chemical components and different production methods can produce four basic Duplex forms:

Lean Duplex usually contains no molybdenum and has lower amounts of nickel. It is also possible to produce lean Duplex with molybdenum added (S32003 grade). Lean Duplex is widely used for construction of storage tanks.

Regular Duplex possesses standard amounts of both nickel and molybdenum – grade 2205 is a good representative of this type. Among other industries, regular Duplex is used widely in the pharmaceutical sector.

Super Duplex has high amounts of molybdenum and chromium (24-26%) – grade 2507 would be an excellent example. The material is often used for production of tubes and pipes for oil, gas, and chemical industries.

Hyper Duplex contains even higher volumes of molybdenum and chromium (27-32%) and is represented by the grade S32707. This Duplex type is used in energy and oil sectors and provides excellent results with subsea applications.

All Duplex grades contain nitrogen which, in combination with molybdenum and chromium, provides excellent protection against corrosion.

Duplex Stainless Steel – Main Benefits

As stated above, Duplex is normally performing better than individual steel types found within its microstructure. Better said, the combination of positive characteristics coming from austenite and ferrite elements provides a better overall solution for a great number of different production situations.

Here is how Duplex compares to austenitic and ferritic alloys when it comes to several important production and performance factors:

Anti-corrosive properties – The effect of molybdenum, chromium, and nitrogen on corrosion resistance of Duplex alloys is immense. Several Duplex alloys can match and exceed anti-corrosive performance of popular austenitic grades including 304 and 316. They are especially effective against crevice and pitting corrosion.

Stress corrosion cracking – SSC comes as a result of several atmospheric factors – temperature and humidity being the most evident ones. Tensile stress just adds to the problem. Normal austenitic grades are highly susceptible to stress corrosion cracking – Duplex stainless steel is not.

Toughness – Duplex is tougher than ferritic steels – even at lower temperatures while it does not actually match the performance of austenitic grades in this aspect.

Strength – Duplex alloys can be up to 2 times stronger than both austenitic and ferritic structures. Higher strength means that metal remains firm even with reduced thickness which is especially important for reducing weight levels.

Ductility – Duplex is fairly more ductile than ferritic steel. However, its ductility cannot rival that of the austenitic type.

Weldability – Austenitic steels are easier to weld than Duplex. However, this material is still more weldable than the ferritic steel. Duplex welding requires careful process planning to control hydrogen, nitrogen, oxygen, and ferrite levels for optimal corrosion resistance and structural stability. Low thermal expansion of Duplex compensates for some of its shortcomings when it comes to welding.

Price – With less nickel in its composition, Duplex is certainly a more affordable solution than austenitic grades that are similar in all other aspects. Additionally, the ability to use Duplex in thinner forms (due to its strength) decreases the need for extra weight and adds to its cost-effectiveness.

For further reading about Duplex Stainless Steel - click on the link to access the full article.

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