The completion and subsequent opening of the Holmestrand station in Norway towards the end of 2016 drew international attention to the highly desirable properties of duplex stainless steel. Holmestrand station is set to transform Norway’s rail transport infrastructure. Duplex stainless steel was chosen for critical “hanger” components that support a section of the 12km-long high-speed tunnel, which includes the station. A world first, the tunnel features two parallel tracks enabling two high-speed trains to pass each other at speeds of up to 250km/hr. Duplex stainless steel was chosen for several reasons, not least of which was the stipulation by the NSB AS, the state-owned Norwegian railroad company, that the construction had to be fully functional and maintenance free for at least 100 years. A tough ask, especially considering that an estimated four million movements will occur in that tunnel in that time, and that the railway is the first of its kind to be situated inside a mountain, putting added stresses on the steel. The engineers, however, are completely confident that the properties of duplex stainless steel will more than match the demands required.
What Is Duplex Stainless Steel?Duplex stainless steel (DSS) takes its name from the fact that is has a two-phase microstructure consisting of both ferritic and austenitic stainless steel. Although it’s really only gained in popularity in the past few years, DSS was actually first introduced about 70 years ago. In the early 80s, a new type of Duplex was developed, this time with improved weldability due to its higher nitrogen content.
Key Benefits of Duplex Stainless SteelDSS has a number of attractive benefits, including:
- Toughness and Ductility – while not as tough as austenitic grades, Duplex stainless steel is significantly tougher and more ductile than ferritic grades.
- Strength – DDS is about twice as strong as regular stainless steel, with good fatigue resistance and low thermal expansion.
- Corrosion Resistance – This always depends on the composition of the stainless steel. The chromium, nitrogen and molybdenum content are the most important factors when considering corrosion resistance.
- Price – lower alloying content means duplex stainless steels are often cheaper than other types. In addition, DSS’ yield strength means it’s often possible to reduce its thickness, providing attractive cost and weight savings compared to austenitic grades.