Duplex stainless steel

Duplex stainless steels are called “duplex” because they have a two-phase microstructure consisting of grains of ferritic and austenitic stainless steel. The picture shows the yellow austenitic phase as “islands” surrounded by the blue ferritic phase. When duplex stainless steel is melted it solidifies from the liquid phase to a completely ferritic structure. As the material cools to room temperature, about half of the ferritic grains transform to austenitic grains (“islands”). The result is a microstructure of roughly 50% austenite and 50% ferrite.

Duplex stainless steel

Duplex stainless steels have a two-phase microstructure of austenite and ferrite grains.

The duplex structure gives this family of stainless steels a combination of attractive properties:

Strength: Duplex stainless steels are about twice as strong as regular austenitic or ferritic stainless steels.

Toughness and ductility: Duplex stainless steels have significantly better toughness and ductility than ferritic grades; however, they do not reach the excellent values of austenitic grades.

Corrosion resistance: As with all stainless steels, corrosion resistance depends mostly on the composition of the stainless steel. For chloride pitting and crevice corrosion resistance, their chromium, molybdenum and nitrogen content are most important. Duplex stainless steel grades have a range of corrosion resistance, similar to the range for austenitic stainless steels, i.e from Type 304 or 316 (e.g. LDX 2101©) to 6% molybdenum (e.g. SAF 2507©) stainless steels.

Stress corrosion cracking resistance: Duplex stainless steels show very good stress corrosion cracking (SCC) resistance, a property they have “inherited” from the ferritic side. SCC can be a problem under certain circumstances (chlorides, humidity, elevated temperature) for standard austenitics such as Types 304 and 316.

Cost: Duplex stainless steels have lower nickel and molybdenum contents than their austenitic counterparts of similar corrosion resistance. Due to the lower alloying content, duplex stainless steels can be lower in cost, especially in times of high alloy surcharges. Additionally, it may often be possible to reduce the section thickness of duplex stainless steel, due to its increased yield strength compared to austenitic stainless steel. The combination can lead to significant cost and weight savings compared to a solution in austenitic stainless steels.

Please download the updated brochure "Practical Guidelines for the Fabrication of Duplex Stainless Steels" for more information.

Stainless steel stops leaks
Repair sleeves are found in the tool box of virtually every water utility. They allow quick repair of leaking water pipes without replacing them. Often fabricated from molybdenum-grade Type 316 stainless steel, they reduce water wastage and are yet another way that molybdenum helps sustain Earth’s resources.
Strong sustainable storage tanks
Duplex stainless steel storage tanks are increasingly being used due to their long, low-maintenance service life. Surprisingly, they can also reduce initial costs for tank owners. The higher strength of duplex grades permits thinner walls requiring less steel. Because costly protective coatings or cathodic protection are not necessary, they can compete on installed cost with carbon steel tanks.
Low sulfur on the high seas
Maritime shipping remains one of the most cost-efficient global transport methods, especially compared with air freight. Its rapid growth since the 1970s and the containerisation of cargo have kept pace with the global economy, however this has also increased its environmental impact. New regulations limiting the sulfur content of engine fuel mean that ships must either use cleaner, more expensive fuel, or install equipment to take sulfur out of exhaust gases, a process in which molybdenum plays a key role.
Guide to high-performance alloys
It can be difficult and time consuming for engineers to find materials properties beyond the basic ones reported in producer data sheets. This is especially true for lesser-used high-performance alloys. The new High-Performance Alloys Database addresses this problem. It contains a large number of materials properties for each of the 100 or so featured alloys, many involving molybdenum.
Stainless steel sparkles in NYC
The iconic New York City skyline is celebrated in movies and photos. But as a global financial, cultural and business center, it is also constantly changing – particularly now. Buildings are rising at a breathtaking pace and many feature sustainable designs. Molybdenum-containing stainless steel is often key to making them not only beautiful, but also resilient and durable, thus reducing their carbon footprint.
Propelling the boating world
The propeller shaft may be the most important component of any motorized vessel. It drives the propeller, hour after hour, day after day. Molybdenum provides improved strength and corrosion resistance in several high-performance stainless steel grades used in demanding shaft applications.
Molybdenum’s nuclear mission
Nuclear power currently supplies some 11% of the world’s energy needs. Without debating its pros and cons, everyone would agree that the spent fuel already in existence from more than 50 years of generation needs safe handling and disposal. Imparting greatly increased corrosion resistance to stainless steel, molybdenum is making a positive contribution to the ongoing safety of spent fuel management throughout the world.
Molybdenum at work in the dentist’s office
Dental instruments need to be hard for the dentist to work efficiently and precisely, and tough enough not to break during procedures. At the same time they have to be hygienic, corrosion resistant and easily sterilized. The molybdenum-containing hardenable surgical stainless steel Type 440A fits these requirements and new, even harder and tougher grades are on the horizon.
Moly rescues a lady in distress
Since its dedication in New York harbor on October 28, 1886, The Statue of Liberty has become one of the world’s best-known sculptures. However, after nearly 100 years in the aggressive marine environment, galvanic corrosion between the iron framework and the copper skin caused major structural deterioration. Molybdenum-containing stainless steel played a crucial role in restoring this iconic landmark.
Duplex rigging for glass sails
Canadian-born California architect Frank Gehry’s project for Fondation Louis Vuitton exhibits unprecedented aesthetic innovation and technological sophistication. The spectacular glass sails of the roof appear so light and airy, in large part, thanks to the delicate high-strength molybdenum-containing duplex stainless steel support structure.
Wireline for downhole tools
Wireline is cable used to lower oil- and gas-well tools and measuring equipment downhole. Wireline must be strong, dependable and resistant to the increasingly corrosive conditions encountered in today’s deeper wells. Molybdenum imparts the required corrosion resistance to the stainless steel and nickel alloys used in this application.
101: Hot forming and heat treatment of duplex stainless steels
102: Bending, cold forming, and springback of duplex stainless steels
103: Machining duplex stainless steels
104: Dissimilar metal welds and weld joint design for duplex stainless steels
105: Welding parameters for duplex stainless steels
Structural stainless steel
Decorative stainless steel panels are widely used in façades, roofs and elevators because they are attractive and long-lasting. Increasingly, engineers now use stainless steel for load-bearing structures in challenging environments. In many cases stainless steel is cost-effective if the whole life cycle of the installation is considered.
Molybdenum scrap saves resources
A recent study found that about one quarter of the molybdenum used each year is recycled material from scrap sources. The rest is newly mined, primary molybdenum. Scrap therefore plays an important role in meeting demand and contributing to sustainability.
Material longevity and its inherent impact on long-term project sustainability are changing the way buildings are designed. Memorials like the new Four Freedoms Park in New York have to last for hundreds of years. Corrosion resistant 2205 duplex stainless steel was necessary so that the sculptural handrails were as durable as the massive blocks of granite in this highly-acclaimed new monument.
Super duplex to keep the Vasa safe
A major historic ship preservation project is currently under way in Stockholm.  The galvanized and epoxy coated mild steel bolts used to hold the ship together after it was raised are failing due to corrosion. Molybdenum is a key alloying element in the high-strength, corrosion-resistant super duplex stainless steel Sandvik SAF 2507TM and Sandvik SAF 2707 HDTM bolts that will hold the great ship Vasa together to achieve a minimum design life of 100 years.
The Use of 2205 Duplex Stainless Steel for Pharmaceutical and Biotechnology Applications