The financial value of high strength
According to Peder, the high strength of duplex grades are an important advantage. He says: “Duplex stainless steel’s high strength makes it possible to reduce the thickness of the material by up to 50 percent. This means you can create lightweight structures which need less stainless steel. In turn, this saves the cost of the material, as well as transport, logistics, handling, fabrication and welding.“That’s why duplex grades were adopted in the pulp and paper industry in the 70s, followed by pressure vessels, storage tanks, containers and seawater pipework systems in the oil and gas industry.
“One popular application in the 80s for the grade Forta DX 2304 was the structural walls of offshore platforms. As a high strength material, it created interior walls that could absorb energy from an explosive blast. However, sacrificial exterior walls were built from lower strength material that would fail in a blast, directing the force of the blast safely away from the platform.
“High strength duplex grades also save lives in escape tunnels for the oil and gas industry. These need to remain intact and provide a safe escape route for workers in the case of an incident.”
Corrosion resistance and long life
Corrosion resistance is another important feature. It gives duplex stainless steel a long life. Peder says: “We produce three families of duplex grades, which provide progressively more corrosion resistance: lean, standard and super duplex.“The lean duplex grade Forta LDX 2101 has corrosion resistance similar to austenitic grades 304 or 316. Then the standard duplex grades such as Forta DX 2205 have dramatically improved corrosion properties. You can upgrade to super duplex and even hyper duplex grades to get progressively higher performance. It’s about matching the material to the environment.
“I’ve worked a lot with storage tanks over the last 10 years. These used to be built from carbon steel, which needs to be shot blasted and then painted with layers of primary paint and epoxy coatings before they were ready to store a chemical product.
“Duplex stainless steel does not need these additional steps. It’s important to carry out some pickling of the welded areas but that’s significantly less intensive than applying a coating and there’s no need to worry about quality issues when applying coatings in humid conditions.”
Bridges and structures
A more recent trend over the last 10 or so years is that architects and structural engineers have discovered duplex stainless steel for bridges. It’s similar to storage tanks as duplex grades do not need regular painting so have the lowest Life Cycle Cost (LCC).According to Peder: “Many European bridges were built after the Second World War. Today, they need extensive maintenance, which is costly and disruptive to traffic.
“Recognizing the hidden cost of maintenance, highways agencies are now using LCC to guide their thinking. It covers initial investment, maintenance, inspection, repair, disruption to the community, and the environmental cost of painting.
In addition, its high strength creates lightweight bridges that are attractive and straightforward to build and that will last for many decades.
It’s also very helpful that the thermal expansion coefficients of duplex stainless steel and concrete are very similar. That avoids problems building up as a bridge adjusts throughout the seasons and it also means that duplex rebar is a good choice for reinforced concrete bridges in seawater.
Peder says: “Over time, chlorides from seawater find their way through the concrete and can corrode the rebar. This causes it to swell to ten times its original volume, cracking the concrete and eventually requiring replacement of the entire structure.
“By specifying duplex stainless steel rebar for sections of bridge that will be in the splash zone, an engineer can extend the life of an entire bridge from 80 to 300 years.
Steel lighter than aluminum
On a personal level, Peder finds one customer’s use of duplex stainless steel particularly appealing. Swedish Steel Yachts, a boat builder, uses duplex to make motor yachts. “They take duplex stainless steel sheets in 2mm thickness, bend them into shape and weld them to internal bulkheads to make a boat that is lighter than an aluminum alternative. The steel’s high strength allows the hull to withstand minor bumps by bouncing back into shape without any warping.“Before launching a boat, Outokumpu Press Plate polishes the plates to the hull to an almost mirror finish. That has the double advantage of minimizing water friction for more energy-efficient travel and eliminating the need for antifouling paint below the waterline. The boats are lightweight, efficient and don’t pollute the sea. It’s a great showcase for Forta SDX 2507, a super duplex grade.”
Future growth markets
Looking ahead, Peder sees adoption of duplex stainless steel being driven by sustainability. He says: “In the early days of renewable energy, wind turbines were built for a life of 15 years so coated carbon was appropriate. But the more you want renewables to be sustainable, the bigger the role for duplex.
Elsewhere, he believes that climate change may drive growth of super duplex and hyper duplex grades: “Gas pipelines feeding coastal regions in Africa may be the most sustainable material due to the high salt concentration and high temperature.
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.