While duplex stainless steel has been around for decades, it’s currently receiving a lot more attention due to its high mechanical strength, corrosion resistance and price stability. Together,
these properties enable long-lasting, low-maintenance, cost-effective structures such as storage tanks and bridges with thin walls and lightweight beams. In particular, duplex grades are useful for storage tanks at refineries, chemical, petrochemical and processing industries instead of carbon steel or austenitic stainless steel.
One of the early adopters was an ethanol producer in Lima, Ohio, that built a set of storage and fermentation tanks in 2007 from the duplex stainless steel grade of Forta LDX 2101. Since then, countless more tanks have been built around the world.
This article explains the advantages of using duplex stainless steel for storage tanks, how duplex grades are different to austenitic stainless steels and how fabricators have an opportunity to become the go-to experts in working with them as the North American market develops.
Naturally corrosion resistant
The big advantage of stainless steel over carbon steel for storage tanks is that stainless steel naturally resists corrosion without a protective coating. The result is that operators can keep the tank working for longer without having to drain it down for periodic inspection and maintenance.
It also reduces risk of failure. Even expert technicians can overlook tiny imperfections in a carbon steel tank’s coating during inspection. A small scratch can expose the underlying steel to corrosion and ultimately cause sudden structural failure, with risks to process continuity, the environment, employee health and reputation.
By choosing a stainless steel tank, an operator can protect themselves against sudden outages. And they are also buying flexibility as they can use the same tank to store other products without the effort of stripping and replacing the coating.
Corrosion resistance also cuts the order size as tank designers don’t need the 1-mm to 2-mm corrosion allowance required for carbon steel tank walls, which can add up for large tanks.
When it comes to choosing a particular stainless steel grade, it makes sense to use a duplex stainless steel for structural applications like large storage tanks. That’s because, compared with the popular austenitic grades of 304 or 316, duplex stainless steel has high mechanical strength.
The material’s high strength translates into thinner walls for tanks designed according to API 650. Not only does this result in reduced order sizes for the material, lifting and handling requirements during construction are much less time and labor intensive. Another major benefit is a savings in the time and filler material required for welding.
Depending on the size of the tank and the product to be stored, it’s possible to deliver a tank in duplex material at a lower cost than a conventional stainless steel tank or even a carbon steel
tank, especially when looking at a tank’s total lifetime costs.
And, depending on the grade, there may be further commercial advantages. The Forta LDX 2101 used at the Lima ethanol plant is a good example.
Outokumpu developed its Forta LDX 2101 in 2004 for the chemical and process industries, and its high corrosion resistance is due to having a relatively high level of chromium. This enabled the company to reduce the levels of nickel and molybdenum, which provide corrosion resistance in other grades.
This makes Forta LDX 2101 a “lean duplex” grade, making it is less exposed to the market price of nickel, which is well known for its volatility. Nickel prices are currently high, so lean duplex is an even more cost-effective solution.
Until recently, duplex stainless steel was not widely available in the United States. This led to fabricators having less experience with it and adding a price premium to cover the extra time needed for what was perceived as an unusual material.
However, several mills have now started to produce duplex grades and a steady stream of quarto plate and coil is now available from multiple sources. As a result, the market is growing as operators can benefit from the high strength and corrosion resistance. Undoubtedly, this has created an opportunity for fabricators to establish themselves as experts in the growing market.
The key to making the most of duplex stainless steel’s rising popularity is to learn about duplex grades as a family of alloys and adjust working practices to suit. Fabricators can then set competitive prices to win projects against other fabricators working with duplex grades as well as carbon and classic austenitic stainless steels.
On a microscopic level, duplex stainless steel combines the two most common types of stainless steel: austenitic and ferritic stainless steels.
Both of these types have a microstructure based on atoms arranged in a cube. Ferritic steels have one additional atom at the very center of the cube, whereas austenitic steels have an additional atom on every side, like a dice with one dot on every face.
Duplex stainless steels contain a mix of both of these microstructures, which are also known as phases. Ferrite provides better strength and resistance to chloride stress cracking, while austenite is better for general and localized corrosion resistance. The combination of both in duplex grades provides the best of both worlds.
Welding and fabrication
It’s important that fabricators understand this two-phase microstructure when working with duplex grades as they will need to take action in two areas. The first of these is due to the high mechanical strength of duplex grades. Fabricators may need to apply more pressure and expect greater springback when working with sheet metal. This may just be a matter of changing machine parameters when setting up a job.
The second area to be aware of is welding. It is essential to control the heat input and use the correct filler material. This will protect the duplex microstructure and avoid significant shifts between the austenite or ferrite phases that could result in losing either strength or corrosion resistance.
Typically, the heat input should be around 0.5 to 2.5 kJ/mm, although for some duplex grades and for low-temperature applications, heat input should not be higher than 1.5 kJ/mm. Welders should also protect the duplex microstructure by leaving time for cooling between passes. Most duplex grades have a maximum interpass temperature of 150 degrees C, although it may be lower for some steels.
When it comes to filler, recommended metals tend to have higher nickel content to keep the phase balance similar to the base metal. This overcomes the possibility that rapid cooling of welds may result in areas with high ferrite levels in the heat-affected zone.
In addition to the welders, designers and consultants can also benefit by learning more about the potential of duplex grades and tools that are available. For example, Outokumpu recently updated an online tank calculator in its Stainless Steel Finder tool to provide tank sizing data based on the API 650 standard with options of metric and imperial units.
The Outokumpu tool is designed to help as a first step to help tank designers compare duplex, austenitic and carbon steel grades as tank materials. It integrates data on material properties with an algorithm that calculates the minimum thickness required for tank walls. It provides a profile showing the number and thickness of plates required to build a tank, as well as the potential for material savings. The company also offers a more complex offline version that takes account of more parameters.
Another factor that is becoming more important across all industries is sustainability. With careful material selection, it’s possible to reduce the energy and CO2 emissions associated with
stainless steel production.
From an environmental point of view, duplex grades have an advantage as they reduce the amount of material required for a tank and, therefore, cut the amount of material needed for a project.
It’s also possible to choose a supplier with low emissions – while their products might be physically identical, some suppliers use less energy in production. At Outokumpu, great pride is taken in being the lowest carbon stainless steel producer made possible by using energy-efficiency measures at every mill and basing at least 89 percent of production on recycled austenitic scrap. This is significant as every ton of scrap used in production reduces CO2 emissions by 4.3 tons as it takes more energy to process virgin ore than scrap, according to the Fraunhofer Institute in Germany.
And as governments set tighter emissions targets, it’s important to be able to measure and report on the embedded emissions in materials. Outokumpu publishes Environmental Product Declarations that provide certified data on the CO2 emissions and other impacts of the company’s products. In turn, customers can calculate and report on their own projects. This data will also become essential as the chemical, petrochemical and processing industries respond to environmental legislation.
Duplex stainless steel offers exciting prospects to reduce the total cost of ownership of bulk storage tanks. With growing availability in the North American market, duplex is set to become the material of choice.