An Introduction to Duplex Stainless Steels

A look at the uses for duplex stainless steel, its history, and the benefits it offers

Duplex stainless steels combine characteristics of the austenitic and ferritic family of alloys to offer benefits from both with fewer downsides -- often at a lower cost than other steel alloys.

However, cost is only one part of a bigger picture.

Choosing the right stainless steel is also about how the steel will perform for your intended use.

Could duplex stainless steel work in your next project?

Let’s take a look at how and when duplex steel might be a good fit for your needs.

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.

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).

Though newer standards use different ranges of chromium, molybdenum and nickel, industries can thank the original grades pioneered in the 1970s for the range of duplex stainless steels in use today.

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.

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

Unified Alloys offers a range of corrosion-resistant stainless steel products to help meet the needs of industries across Canada. For more than 40 years, we’ve offered our expertise to businesses of all sizes looking to utilize stainless steel in everything from building and bridge construction to liquid processing and containment. Our expert advisors are available to help you determine the best alloys and products to help you meet your goals. Call today.