An Introduction to Duplex Stainless Steels - Knowledge

1.What is Duplex Stainless Steel?

Duplex stainless steels get their name from their two-phase microstructure.

While exact ratios vary by grade, most duplex steels have a structure that is roughly 50-percent austenite and 50-percent ferrite.

This gives duplex stainless many of the perks of austenitic and ferritic families of steel while also minimizing the weaknesses.

As with other families of stainless, the exact metallurgic composition of the steel varies by grade, though common components include:

Carbon

Manganese

Silicon

Chromium

Nickel

Phosphorous

Sulphur

Molybdenum, nitrogen, and copper may also be used to further influence characteristics of the final product.

2.A Brief History of Duplex Stainless Steel

The concept of duplex stainless alloys was discussed as early as the 1920s. However, production of the first duplex alloys did not begin until the 1930s.

These earliest duplex alloys were mostly limited to cast production and specific uses as they contained a relatively high amount of carbon.

However, by the end of the 1960s, decarburization allowed metal foundries to create low-carbon steels with high chromium and nickel content while further balancing the ferrite and austenite structure.

This lead to a new generation of duplex alloys with better performance and fewer downsides.

These duplex stainless steels share many similarities with the modern alloys in production today -- in particular, Duplex 2205 (UNS S21803/32205).

Developed in the mid-1970s, this alloy is still popular today providing corrosion resistance superior to common austenitic stainless steel grades, such as 304 (UNS S30400), 316 (UNS S31600), and 317 (UNS S31700).

3.Benefits of Duplex Stainless Steel

While duplex stainless steel only comprises a small percentage of the global stainless steel market, it has a range of benefits when compared to traditional austenitic stainless steel and ferritic stainless steel grades.

Improved Strength

Many duplex grades are as much as two-times stronger than austenitic and ferritic stainless steel grades.

High Toughness and Ductility

Duplex stainless steel is often more formable under pressure than ferritic grades and provides greater toughness. Though they often offer lower values than austenitic steels, the unique structure and characteristics of duplex steel often outweigh any concerns.

High Corrosion Resistance

Depending on the grade in question, duplex stainless steels offer comparable (or better) corrosion resistance as common austenitic grades. For alloys with increased nitrogen, molybdenum, and chromium, steels exhibit high resistance to both crevice corrosion and chloride pitting.

Cost Effectiveness

Duplex stainless steel offers all of the above benefits while requiring lower levels of molybdenum and nickel. This means that it is a lower-cost option than many traditional austenitic grades of stainless steel.The price of duplex alloys is often less volatile than other steel grades making it easier to estimate costs -- both at an upfront and lifetime level.The higher strength and corrosion resistance also means that many parts made using duplex stainless can be thinner than their austenitic counterparts providing lower costs.

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4.Best Uses for Duplex Stainless Steel

Due to their excellent corrosion resistance, increased strength, and affordable pricing, duplex stainless steels are popular in a variety of industries and markets, including:

Offshore and near-shore operations such as oil drilling, desalination, water treatment, and other industrial operations

Chemical and liquid processing

Naval parts and components

Pollution control equipment

Pulp and paper production

Construction

Hot water and brewing tanks