Forta LDX 2101 is a lean-alloyed duplex stainless steel developed by Outokumpu to span the clear gap in the market for a grade that offers a combination of high mechanical strength and good corrosion resistance at a reasonable price.

Over 20 years it has proved to be the ideal substitute for coated carbon steel in a wide range of applications including storage tanks, pressure vessels, tubes and pipes and small structural components. Now, bridge designers are turning to Forta LDX 2101 as a flexible solution to help them optimise the Life Cycle Costs of their structures over a service life that can extend from 60 up to 120 years.

Austenitic and ferritic stainless steels

Austenitic-ferritic stainless steels, also referred to as duplex stainless steels, combine many of the beneficial properties of ferritic and austenitic steels. Due to their high content of chromium (Cr) and nitrogen (N), and often also molybdenum (Mo), these steels offer good resistance to localised and uniform corrosion. In fact, the corrosion resistance of even the least resistant Forta duplex stainless steel is better than that for the most commonly used grade of stainless steel, Core 4307/304L.

The duplex microstructure contributes to both high strength and high resistance to stress corrosion cracking (SCC) and also improves abrasion and erosion resistance. Duplex steels also have good weldability.

Outokumpu produces a whole range of duplex products, from the lean-alloyed Forta LDX 2101 up to the super duplex products Forta SDX 2507 and Forta SDX 100.

Taking a lean approach

The lower nickel content of duplex grades compared to austenitic grades with similar corrosion resistance makes them more price stable. This is important, as once a manufacturer has selected a duplex grade for a long-term project they can have greater certainty that their material costs will not increase suddenly.

In developing the lean duplex grade of Forta LDX 2101, Outokumpu has adopted modern steelmaking techniques and innovative alloying technology. The result is a stainless steel that offers good material properties while also being extremely cost-effective. For example, Forta LDX 2101 has approximately twice the strength of austenitic grades with similar corrosion resistance so that thinner gauges can be used in many applications.

The typical chemical composition of Forta LDX 2101 is shown in table 1. Note the relatively low nickel and molybdenum content.

Combining high strength and corrosion resistance

The maintenance budgets for the road, rail and footbridges that play a vital role in the world’s civil infrastructure can often be as significant as their original capital cost. This is especially the case with coated carbon steel structures that require frequent repainting. In contrast, using the right grade of stainless steel can create a structure that resists corrosion and minimises maintenance needs over a typical bridge lifetime. Figure 1 shows the attractive combination of strength and corrosion resistance of Forta LDX 2101.

Great value for mildly corrosive environments

Forta LDX 2101 has good resistance to uniform corrosion due to its high chromium content. It also has good resistance to pitting corrosion, crevice corrosion and chloride-induced stress corrosion cracking (SCC).

These properties mean that Forta LDX 2101 is ideally suited for use on bridge structures in mildly corrosive environments. That typically means structures at least 200 meters from the sea. Structures close to fresh water are not a problem. Further advice on suitability is available from Outokumpu.

Mechanical properties

Forta LDX 2101 is part of the Outokumpu Forta range of stainless steels with high mechanical strength and has a typical minimum yield strength (RpO.2) of 460 MPa. If it is possible to utilise this high strength, then downsizing can be achieved in many applications, leading to cost-efficient solutions.

Suitable for all forming processes

Forta LDX 2101 is suitable for all forming processes applicable to stainless steel. The high yield strength compared to austenitic and ferritic stainless steel can impose some differences in forming behavior depending on the chosen forming technique, such as an increased tendency to springback. This point is particularly relevant to the forming of any high-strength steel. Higher forces may be needed in forming operations, but the possibilities of downsizing can mitigate this.

Forta LDX 2101 also has good weldability and can be welded using an appropriate welding procedure by most of the common methods used for stainless steel.

Small order volumes enable more flexible designs

Carbon steel is essentially a commodity product. The I-beams used in bridge construction are often rolled in one piece on a beam rolling mill and produced in very high volumes. When a design engineer is selecting an I-beam for their bridge design they often choose from a selection of standard sizes. In many cases they will not find the exact size and format they require. Therefore, to comply with design rules they will normally specify the closest next size up.

The stainless steel industry is rather different. Because the volume requirement for stainless steel beams is not so large, it is more common to fabricate beams by automatic welding of rolled plates, giving more flexibility to produce the exact sizes required. Therefore, it is possible to take advantage of the capability to order Forta LDX 2101 in relatively small volumes, down to 2 tons, and to introduce much greater design flexibility.

Flat plate sections can be cut to shape and welded together in the factory to create I-beams that meet a designer’s requirements exactly – a project team might want to specify bespoke flange and web sizes, each specified according to the requirements. Forta LDX 2101 offers that flexibility. If a designer wants different sizes top and bottom or for the beam to taper, all that is possible.

Cost comparison

A typical steel-concrete composite highway bridge design provides a good model to use as a cost comparison between beams in grade S355 carbon steel and Forta LDX 2101. Although the lean duplex will appear more costly per tonne, its high strength means that with optimised design and erection strategies the bridge girders may be downsized. Therefore, a project could be undertaken at similar cost to carbon steel.

Furthermore, maintenance costs will be reduced using Forta LDX 2101, depending on whether the bridge spans a minor or major road, a salt water estuary or a railway cutting. Often, Forta LDX 2101 will have paid for itself when a carbon steel bridge would need its first repair or repainting. These savings accumulate over the life of the bridge. The result is that over a design life of 60-120 years, Forta LDX 2101 is the lowest cost material.

Forta LDX 2101 for Europe’s longest pedestrian and cycling bridge

Forta LDX 2101 was used in the design and construction of the Sölvesborg bridge in Sweden. At 756 meters in length, it is Europe’s longest pedestrian and cycling bridge. The aim was to achieve an attractive design, low maintenance cost and minimum environmental impact on a nearby bird conservation area. 

The arches built from duplex stainless steel deliver a cumulative saving of around 500,000 euro in maintenance and repainting costs. The benefits of avoiding painting and sandblasting every 15 years are also significant from the environmental perspective.

Añorga railway bridge, Spain

Prefabricated assemblies manufactured from 130 tons of Forta LDX 2101 enabled the construction of a bridge structure with 120 year design life requiring little maintenance. Low maintenance was an essential factor for rail operator Eusko Trenbide Sarea as it wanted to avoid closing the railway line for maintenance.

This requirement prompted structural engineers at TECSA in Madrid to select lean duplex stainless steel due to low Life Cycle Cost. The bridge was completed in 2011 and was inspected after four years by a surveyor to evaluate its condition. In spite of potentially corrosive effects of dust from a nearby concrete plant, the bridge was found to be in pristine condition.

Södertälje bridge

Forta LDX 2101 was selected for a pedestrian and cycle bridge over Sweden’s E4 motorway at Södertälje, around 40 km southwest of Stockholm. The bridge design has been patented by fabrication company Stål & Rörmontage, which engineered it to optimise the use of the stainless steel at the same time as minimising the structure’s Life Cycle Cost. The design includes two spans, which were assembled and fabricated in the factory. This meant they could be installed on site during a short highway closure that minimised the impact on traffic.