"As a mechanical engineer, I see the benefits of duplex stainless steel as high strength, formability and corrosion resistance. I have a lot of experience in sheet metal forming and fabrication, but I’ve also been responsible for machining, welding, and construction, as well as structural design."
Erik was supporting the team that developed, the formable duplex grades that were launched in the 2010s. “The idea was to create an alloy with the high strength of a duplex grade and the formability of an austenitic grade. To achieve the ductility, we did a great deal of work on the Transformation Induced Plasticity (TRIP) effect, where the material hardens during forming. TRIP had up till then only been achieved on austenitic grades. To successfully realize it in duplex grades was a major achievement.
“This concept is interesting for high pressure heat exchangers. It can typically be considered in sheets of 0.6 mm thickness with complex shapes formed in them. When you’re operating at high pressures, it’s good to have a material with high strength to avoid plate collapse. Previously, it wasn’t possible to form the same complicated pattern in the duplex plates as is possible with an austenitic stainless steel.”
Another grade development that Erik participated in was Forta LDX 2404, a lean duplex grade. It was designed to fill a gap between the existing grades Forta DX 2304 and Forta DX 2205 in terms of corrosion resistance and alloy composition cost. The outcome is a grade with the strength of duplex and a corrosion resistance in between Forta DX 2304 and Forta DX 2205” explains Erik.
How to build with duplex grades
Erik has passed on his knowledge of forming and fabrication through Outokumpu’s sheet metal forming,
welding, and machining handbooks. In combination with the welding handbook, these are valuable resources for customers such as engineers, consultants, workshops and technical managers.
“The high strength of duplex grades is a real asset, but because it’s so strong, you have to be careful to choose the right tooling and machine settings for forming and machining.”
“When it comes to welding a duplex grade, it’s important to know the energy input, filler material, and the right shielding gas to use. You also need a welder who is qualified for the task,” cautions Erik.
Erik sees the market for duplex stainless steel being driven by total cost of ownership:
“My personal opinion is that we sell duplex stainless steel because customers want its high strength and resistance to corrosion, and in applications where the construction can be optimized for the strength of the duplex grades the total cost can be competitive to alternatives.
“The sales of lean duplex grades have soared at times when the alloy cost has been very high. For example, we launched Forta LDX 2101 in 2000 with some interest from the market but when the nickel price jumped from around $8k to $50k per tonne in 2006-7, customers were practically lining up at our gate for orders. Customers still buy lean duplex grades for their lower cost today.
“Today, we’re looking for applications where strength is the most important attribute. A duplex stainless steel pressure vessel, for instance, requires less material and fabricating it is cheaper than in austenitic stainless steel. Alternatively, for a chemical transport tanker, a duplex grade offers a thinner and lighter structure, which means you can carry more cargo or reduce your fuel consumption.”
The future of duplex
For Erik, the future of duplex grades is being shaped by the Eurocode and ASTM technical standards in Europe and the US respectively.
He says: “Duplex grades were first included in the Eurocode in the 1990s and that’s paved the way for their adoption across construction and industry. It created construction and design rules for structural engineers. ASTM is currently working on its own set of design rules. These will be important as they will be the basis for national standards all over the world.
“A lot of this work has been done in cooperation with Steel Construction Institute in London and in many international research projects”.
“This work to improve the rules will probably never be finished. It’s a matter of providing more data and modernizing our guidance.”
We are celebrating the 90 years of duplex stainless steel. Duplex stainless steel was originally developed by Outokumpu in Avesta, Sweden in 1930. As the original inventor of duplex we have both experience and expertise to take the material further.