As a type of stainless steel, Duplex Steel has two-grain phases - Austenite and Ferrite - in its microstructure. Austenite and Ferrite phases exhibit atomic structures and physical properties that are different. Due to the fact that Duplex Steel contains roughly 50% of each phase, it can exhibit both beneficial properties.
Among these properties are:
- Compared to other stainless steel products, it has a relatively high strength
- Comparatively improved ductility and toughness to ferritic steel; however, these properties remain lower than those of austenitic steel
- The corrosion resistance is excellent.
How Is Duplex Steel Made?
To make Duplex Steel, raw materials including nickel, iron ore, chromium, molybdenum, and other alloying elements are melted down and mixed in a furnace as they are combined. The Duplex Steel stainless steel contains around 25% more chromium, 9% less nickel, and a small amount of nitrogen compared to other stainless steels. Duplex Steel alloy content is defined in ranges and is commonly identified by its chromium content.
Solidifying Duplex Steel as a ferritic solid at room temperature is a process called austenite transformation. Half of the material cools to room temperature as an austenite solid.
What Is Duplex Steel Used For?
Duplex Steel has a high strength and corrosion resistance, as well as relatively low alloy content (particularly nickel), which typically make it a low-cost material for high strength, corrosion-prone applications. As well as pipework systems, manifolds, risers, etc., they are used extensively in the offshore oil and gas industry as well as pipelines and pressure vessels for the petrochemical industry.
Even though duplex stainless steels have high corrosion and oxidation resistance, at relatively low temperatures they form brittle phases in the ferrite, limiting their application at elevated temperatures. The toughness of steel is significantly affected by these phases, and these applications should be avoided.
A brief description of the properties of duplex stainless steel
There are approximately 50% ferrite (a cubic structure centered on the body) and 50% austenite (a cubic structure centered on the face) in the microstructure of duplex stainless steel. In contrast to two-phase alloys that precipitate one phase prior to the other, in a three-phase alloy, they coexist as a stable mixture that grows over time. Stainless steel duplexes are alloyed with either fertilizers (such as chromium, silicon, molybdenum) or austerities (such as carbon, nickel, and nitrogen), which facilitate ferritic and austenitic phase formation.
At high temperatures, the relatively unstable ferrite phase in duplex steels gets converted into the undesirable α’ phase (alpha prime), causing a decrease in their mechanical properties, such as strength and toughness, as well as their corrosion resistance. As a consequence, embrittlement occurs. In duplex metal steels, embrittlement occurs at temperatures as low as 300 degrees Celsius; however, the production of alpha prime starts at 475 degrees Celsius, limiting the maximum service temperature.
In general, duplex stainless steels yield more than martensitic, austenitic, and ferritic grades. It is, however, important to note that they have a limited range of working temperatures because intermetallic phases precipitate at temperatures above 300 C, and they become increasingly brittle as they approach cryogenic temperatures.
Stainless steel grades and standards for duplex stainless steel
During the early days of duplex stainless steel development, only a few grades were available, UNS S31803 being the most popular one. Following that, new grades developed, and they were classified based on the end application of the grade, which could be broken down into two main groups:
- It contains the same properties of stainless steels as hyper duplex and super duplex stainless steels, but they are more prone to corrosion and have been designed to operate in environments with high levels of corrosion
- Stainless steels that are standard and lean have a stronger focus and are commonly used in mildly corrosive environments, such as structural applications.
Based on the composition of duplex stainless steels, the pitting resistance equivalent number (PREN) is used to identify these categories.
PREN=%Cr+3.3×%Mo+16×%N
Duplex Stainless Steel Applications
Unlike ferritic, austenitic, and martensitic stainless steels, duplex steels are limited in formability and machinability, and their production process is extremely complex, so they are typically used in niche applications due to their limitations.
Despite the limitations and advantages of duplex stainless steels, most of their applications require corrosion resistance, including pitting and crevice corrosion resistance, stress corrosion resistance, fatigue corrosion, abrasion corrosion, or resistance to acidic or caustic environments. Here are a few common industrial applications.
- Paper production
Stainless steel with a duplex composition is preferred for the fabrication of vessels used in paper processing. austenitic and ferritic stainless steels cannot withstand the corrosive effects of bleach.
- Desalination
An effective corrosion-resistant material must withstand the corrosion-causing effects of desalinating sea water, a process that involves highly corrosive chlorine under extreme heat. The high strength and corrosion resistance of duplex stainless steel make it a popular material for making evaporators. Due to their thinner cross-sections, they can also be made with thinner cross-sections.
- Oil and Gas
Oil and gas components such as pumps, piping, and manifolds require high resistance to pitting and crevice corrosion, which is why duplex stainless steel with PRE numbers above 40 has been developed.
- Construction
A typical application of duplex stainless steels is in the construction of load-bearing members that must also be corrosion-resistant. Examples include bridges over seawater and structures near the coast.
- Food and Drink Storage
A relatively low cost and excellent corrosion resistance make lean duplex steels an ideal choice for commercial storage as well as the processing of food and drinks.
An Analysis Of The Future Trends Of Duplex Stainless Steel
There is still the development of new grades of duplex stainless steel. Increasing key alloying elements such as chrome, molybdenum, and nitrogen is the primary reason for increasing pitting corrosion resistance further. These alloying elements also come with disadvantages, mainly the destabilization of the ferrite phase, leading to unwanted precipitates, as well as advantages. The challenge is to achieve a balance between desired properties mainly attributed to improved pitting corrosion resistance while limiting the production of undesirable intermetallic phases such as Cr2N by very precise heat treatment. Almost 50 PRE numbers for a duplex steel grade, SAF 2707 HD, have been developed, which is promising for the future of duplex steel in general.
Benefits Of Duplex Stainless Steel
Although duplex stainless steel accounts for only a small percentage of the global stainless steel market, it offers a number of advantages over traditional austenitic and ferritic stainless steel.
- Improved Strength
Duplex grades can be two times stronger than austenitic and ferritic stainless steels.
- High Toughness and Ductility
In addition to their formability under pressure, duplex stainless steel is often more tough than ferritic grades. Though lower in value than austenitic steels, the unique characteristics of duplex stainless steel often outweigh any concerns that may exist.
- High Corrosion Resistance
The most common duplex stainless steel grades offer comparable corrosion resistance (or better) to austenitic grades, especially when alloys with increased nitrogen, molybdenum, and chromium provide high resistance to crevice corrosion.
- Cost Effectiveness
There are tons of benefits associated with duplex stainless steel in addition to having more low molybdenum and nickel requirements. In other words, it is a much less expensive option than many traditional austenitic stainless steel grades. As duplex alloys are often less volatile than other steel grades, it makes it easier to estimate the cost upfront as well as throughout their lifetime. Due to their high strength and corrosion resistance, many parts made with duplex stainless can also be thinner than their austenitic counterparts, which results in lower costs overall.
