Duplex stainless steels combine the best features of both the austenitic and ferritic families of alloys to provide the best of both worlds with fewer drawbacks — and frequently at a cheaper cost than other steel alloys. Cost, on the other hand, is merely one aspect of a larger picture. It’s also important to consider how stainless steel will function in your application. Is duplex stainless steel a viable option for your next project? Let’s look at how and when duplex steel might be a good solution for your requirements.
What Is Duplex Stainless Steel?
The two-phase microstructure of duplex stainless steel gives it its name. The structure of most duplex steels is around 50 percent austenite and 50 percent ferrite, but specific ratios vary by grade. This provides duplex stainless steel with many of the benefits of austenitic and ferritic steels while limiting their flaws.
The exact metallurgic composition of the steel varies with the grade, as it does with other stainless steel families, but common components include:
A Summary of Duplex Stainless Steel’s History
Duplex stainless steel alloys have been discussed since the 1920s. However, the first duplex alloys were not manufactured until the 1930s. Because they included a relatively significant proportion of carbon, these early duplex alloys were primarily confined to cast manufacture and particular usage.
Decarburization, on the other hand, allowed metal foundries to make low-carbon steels with high chromium and nickel content while also balancing the ferrite and austenite structure towards the end of the 1960s. As a result, a new generation of duplex alloys emerged, with improved performance and fewer drawbacks. These duplex stainless steels have a lot of similarities to today’s current alloys, especially Duplex 2205 (UNS S21803/32205).
This alloy, which was developed in the mid-1970s and is still in use today, provides corrosion resistance that is superior to conventional austenitic stainless steel grades like 304 (UNS S30400), 316 (UNS S31600), and 317 (UNS S31700) (UNS S31700). The industry may thank the initial grades pioneered in the 1970s for the wide range of Duplex Steel UNS S31803 Flanges in use today, even though subsequent standards employ different levels of chromium, molybdenum, and nickel.
Benefits of Duplex Stainless Steel
Although duplex stainless steel accounts for a tiny portion of the worldwide stainless steel industry, it offers a number of advantages over typical austenitic and ferritic stainless steel grades.
Many duplex stainless steel grades are up to twice as strong as austenitic and ferritic stainless steel grades.
High Toughness and Ductility
Duplex stainless steel is tougher and more formable under pressure than ferritic stainless steel. Despite having lower values than austenitic steel, duplex steel’s unique structure and features frequently offset any problems.
High Corrosion Resistance
Duplex stainless steels have equivalent (or superior) corrosion resistance to ordinary austenitic grades, depending on the grade. Steels with higher levels of nitrogen, molybdenum, and chromium are more resistant to crevice corrosion and chloride pitting.
Duplex stainless steel offers all of the above benefits while using less molybdenum and nickel. As a result, it is less expensive than many typical austenitic stainless steel grades. Prices for duplex alloys are usually less volatile than for other steel grades, making it easier to budget both upfront and over time. Many duplex stainless components can be smaller than their austenitic counterparts because of their higher strength and corrosion resistance, resulting in lower pricing.
Best Uses for Duplex Stainless Steel
Duplex stainless steels are widely used in a range of sectors and markets due to their superior corrosion resistance, enhanced strength, and low cost.
- Oil drilling, desalination, water treatment, and other industrial operations take place off- and near-shore.
- Processing of chemicals and liquids
- Parts and components for ships
- Pollution-control technology
- Production of pulp and paper
- Tanks for hot water and brewing