Duplex Stainless Steels

Duplex Stainless Steels

 Duplex stainless steels belong to the stainless steels family and are characterized by high chromium (Cr, 19 % to 32 %) and molybdenum (Mo, up to 6 %) and lower nickel (Ni) contents than austenitic stainless steels and identified by a dual phase micro structure. They have a well balanced two phase structure. The two phases in their microstructure consist of grains of ferritic and austenitic stainless steels. The microstructure contains roughly 50 % austenite and 50 % ferrite. Commercial duplex stainless steels may have 30 % to 70 % austenite and 70 % to 30 % ferrite. Fig 1 shows micro structure of duplex stainless steel.

Microstructure of duplex SS Fig 1 Micro structure of duplex stainless steel

 Duplex grades account for less than 3 % of global stainless steel production, however with a strong growth rate.  They are most commonly used when a combination of high mechanical strength and high corrosion resistance is required.

The idea of duplex stainless steels dates back to the 1920s with the first cast being made at Avesta in Sweden in 1930. These first generation duplex stainless steels provided good performance characteristics but had limitations in the as welded condition. The heat affected zone (HAZ) had low toughness because of excessive ferrite and significantly lower corrosion resistance than that of the base metal. The second generation duplex stainless steels are defined by their nitrogen (N) alloying. This new commercial development began in the late 1970s. With this the duplex steels have begun to ‘take off’ in a significant way. This is mainly due to advances in steelmaking techniques (AOD process) particularly with respect to control of nitrogen (N) content.

The interactions of the major alloying elements, particularly the Cr, Mo, N, and Ni, are quite complex. To achieve a stable duplex structure that responds well to processing and fabrication, care must be taken to obtain the correct level of each of these elements. 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. The solid state transformation during cooling results into phases having different chemical composition with austenite enriched with Ni and N and ferrite enriched with Cr and Mo. Besides the phase balance, there is a second major concern with duplex stainless steels and their chemical composition: the formation of detrimental intermetallic phases at elevated temperatures. Sigma and chi phases form in high chromium, high molybdenum stainless steels and precipitate preferentially at the grain boundaries in the ferrite in the temperature range of 450 deg C to 1000 deg C. The addition of N significantly delays the formation of these phases. Therefore it is necessary that sufficient N is present in solid solution.

Ferrite/austenite phase balance in the microstructure can be predicted by the following equations.

Cr(eq) = %Cr+1.73 %Si + 0.88 %Mo

Ni(eq) = %Ni + 24.55 %C + 21.75 %N + 0.4 %Cu

% Ferrite = -20.93 + 4.01 Cr(eq) – 5.6 Ni(eq) + 0.016 T

where T (in deg C) is the annealing temperature ranging from 1050 deg C to 1150 deg C and the elemental compositions are in weight %.

The high corrosion resistance and the excellent mechanical properties combination of duplex stainless steels can be explained by their chemical composition and balanced microstructure. Firstly, the chemical composition based on high contents of Cr and Mo, improves intergranular and pitting corrosion resistance, respectively. Moreover, additions of nitrogen can promote structural hardening by interstitial solid solution mechanism, which raises the yield strength and ultimate strength values without impairing toughness. Secondly, the two-phase microstructure guarantees higher resistance to pitting and stress corrosion cracking in comparison with conventional stainless steels.

Types of duplex stainless steels

The duplex stainless steels comprise a family of grades with a range in corrosion performance depending on their alloy content. Modern duplex stainless steels are often addressed in the following groups.

  • Lean duplex, such as grades UNS (unified numbering system) S32303, S32004, and S32101, which contain little or no deliberate Mo addition
  • Standard duplex is 22 % Cr Grade S32205, the workhorse grade accounting for more than 80 % of duplex use
  • Duplex with 25 Cr, such as 255 (S32550) and S31260
  • Super duplex, with 25-26 Cr and increased Mo and N, such as S32750, S32760, and S32550. Super duplex by definition is a duplex stainless steel with a pitting resistance equivalent number (PREN) greater than 40. PREN = % Cr + 3.3x (% Mo + 0.5x %W) + 16x %N.
  • Hyper duplex refers to duplex grades with a PREN greater than 48 and examples are S32707 and S33207.

Properties of duplex stainless steels

The duplex structure gives this family of stainless steels the following combination of attractive properties.

  • Strength – Duplex stainless steels are about twice as strong as regular austenitic or ferritic stainless steels. The range of 0.2 % proof stress (PS) is from 400 MPa – 550 MPa. This can lead to reduced section thicknesses and therefore to reduction in weight.
  • Toughness and ductilityDuplex stainless steels have significantly better toughness and ductility than ferritic grades particularly at low temperatures, typically down to minus 50 deg C, stretching to minus 80 deg C. However, these steels do not reach the excellent values of austenitic grades.
  • Weldability – Duplex stainless steels have good weldability in thick sections. However this property is not as straightforward as austenitic stainless steels. But it is much better than ferritic stainless steels.
  • 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 to 6 % molybdenum 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 is a form of corrosion which occurs with a particular combination of factors such as tensile stress, corrosive environment and sufficiently high temperature. SCC can be a problem under certain circumstances for standard austenitic such as grades 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.

The attractive combination of high strength, wide range of corrosion resistance, moderate weldability would seem to offer great potential for duplex stainless steels. However, it is important to understand the limitations of duplex stainless steels and why they are always likely to be ‘niche players’.

The advantage of high strength immediately becomes a disadvantage when considering formability and machinability. The high strength also comes with lower ductility than austenitic grades. Therefore, any application requiring a high degree of formability, for example, a sink, is ruled out for duplex grades. Even when the ductility is adequate, higher forces are required to form the material, for example in tube bending.

Applications of duplex stainless steels

The following are some of the applications for duplex stainless steels.

  • Pressure vessels and storage tanks
  • Structural applications e.g. bridges, building structures, architecture
  • Hot water tanks and hot water boilers
  • Brewing tanks
  • Process plant
  • Swimming pool structures
  • Desalination plants
  • Flue gas desulphurization plants
  • Oil and gas application
  • Bio fuels tanks
  • Food and drink industry
  • Asphalt hauling tanker