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What is a Dual-Phase (Duplex) Stainless?

by Dec 15, 2017News

Balancing costs against the qualities you need most is of vital concern during stainless steel fabrication. Picking the right alloy allows you to get the characteristics that are necessary for the job without breaking the bank. Dual-phase stainless steel, which more commonly known as duplex stainless steel, can satisfy many requirements economically.

What are the features of a dual-phase steel?

Duplex stainless steels have a two-phase microstructure. The microstructure is a mix of Austenitic and Ferritic stainless steel. This family of stainless steels is relatively new, having been first engineered in the 1970s. However, it quickly found wide use once it was discovered.

By keeping a lower Nickel content than Austenitic steels, Duplex achieves the balance between Austenitic and Ferritic structures.

Duplex steel takes good characteristics from both Austenitic and Ferritic which means it has a corrosion resistance comparable to that of Austenitic grades but has higher mechanical properties.  It also has a good weldability and does really well when it comes to toughness at low temperatures compared to ferritic steels. It is specifically very popular when it comes SCC (Stress Chloride Cracking) and Pitting corrosion.

Is dual-phase steel economical?

Two-phase stainless steels typically contain lower levels of nickel and molybdenum than austenitic steels. This makes it relatively economical especially during the times when Nickel price goes up.

S31803, commonly referred to as Duplex, for instance, is high in chromium but contains only around 4.5 to 6.5% Nickel. The higher strength allows reduced section thickness and therefore reduced weight in applications such as boats, pressure vessels, bridges etc. Overall, it reduces the cost of the project.

Corrosion resistance and Duplex stainless steel

PREN (Pitting resistance equivalent number) is a good measure of comparing corrosion resistance of various alloys. PREN formula is, PREN = %Cr + (3.3 x %Mo) + (16 x %N). Duplex has a PREN value of approx.. 35 which about 10 numbers more than Austenitic grade 316. Higher the PREN Number, better the corrosion resistance to pitting.

As indicated earlier, Duplex steels have a very good SCC (Stress Chloride Cracking). SCC occurs when there is a corrosive environment coupled with tensile stress.

At Great Plains Stainless Steel, we take great pride in our knowledge of stainless steel grades and alloys. Not sure what you need for your next project? We can help. Get in touch today to discuss your needs. We’ll identify the right materials and the right construction for you.