Second, depending on the grade it has excellent or good corrosion resistance with near-immunity to stress corrosion cracking. This is a corrosion mechanism that can be the Achilles heel of leaner austenitic grades.
The third benefit is the price stability of duplex grades that results from a lower nickel content than classical austenitic grades. Depending on the market and the application, this makes duplex stainless steel competitive from a price point of view.
Developments in duplex stainless steel
Sweden is the birthplace of duplex stainless steel.The first grades were developed in the 1930s at Avesta, followed by the first modern grades in the 1970s and development of super duplex grades by Sandvik in the 1980s.
Outokumpu then developed lean duplex grades in the early 2000s, starting with Forta LDX 2101. These are the least costly duplex grades with similar corrosion resistance but higher strength than comparable austenitic grades. They have since made a big impact on the market.
Technically, interesting developments have been made during the last 15 years with the introduction of formable duplex stainless steel and the hyper duplex stainless steels. Both these developments show interesting technical properties for niche applications.
High strength, corrosion resistance and attractive price
Over the years, duplex grades have been used in many applications that need a combination of high strength, corrosion resistance and competitive price. For example, oil and gas operators use them for high pressure pipelines and they are also used widely for tubing and piping in the chemical and other process industries.
However, Outokumpu has had to work hard to show customers the advantages of duplex grades. To do that, we’ve developed tools like the tank calculator, which helps engineers quickly produce estimates on how much stainless steel they need to build a tank.
We also work closely with customers. For example, a large chemical company in The Netherlands asked our specialists to join a meeting with a project team that had the task of replacing its fleet of storage tanks around Europe.
The customer’s team included design engineers and researchers who wanted to investigate their options. During a six-hour meeting, we took them through the metallurgy of duplex steel, welding methods, its corrosion resistance and high strength properties, availability, and how it performed during testing. Once we helped them put it all together, the customer team got a full picture of the potential savings. At the end of the session, we had convinced them that duplex was the way to go for their tank replacements at various plants.
The future of duplex
Today, more than 90 years after the development of the first duplex stainless steel, it is still a small percentage of total stainless steel consumption. One reason is that many potential users don’t yet know what duplex grades have to offer. Sometimes we just need to convince one or two decision-makers but other times, we need to convince many people in the decision tree.
It’s a classic chicken-and-egg situation. Customers first need to use duplex stainless steel before they can build it into their standard specifications and include it in their list of approved materials. Still, they won’t use a material that’s not already approved.
As a result, it often takes around ten years to truly understand whether a new grade of stainless steel is a commercial success. However, the growing number of duplex producers sends out the signal that duplex is here to stay and becoming more widely available.
Our long-term prediction is that, as total lifecycle cost and sustainability factors become more decisive, we’ll see duplex grades gradually replacing 304 or 316 stainless steel in a variety of applications.
In 2020 we celebrated 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 the experience and expertise to take the material further.