Duplex stainless steel

Duplex stainless steels are a family of stainless steels. These are called duplex (or austenitic-ferritic) grades because their metallurgical structure consists of two phases, austenite (face-centered cubic lattice) and ferrite (body centered cubic lattice) in roughly equal proportions.
They provide better corrosion resistance, particularly chloride stress corrosion and chloride pitting corrosion, and higher strength than standard austenitic stainless steels such as A2/304 or A4/316. The main differences in composition, when compared with austenitic stainless steel is that duplex steels have a higher chromium content, 20–28%; higher molybdenum, up to 5%; lower nickel, up to 9% and 0.05–0.50% nitrogen. Both the low nickel content and the high strength (enabling thinner sections to be used) give significant cost benefits. Duplex steels also have higher strength. For example, a Type 304 stainless steel has a 0.2% proof strength in the region of 280 MPa (41 ksi), a 22%Cr duplex stainless steel a minimum 0.2% proof strength of some 450 MPa (65 ksi) and a superduplex grade a minimum of 550 MPa (80 ksi).
Duplex steels are used extensively in the offshore oil and gas industry for pipework systems, manifolds, risers, etc. and in the petrochemical industry for pipelines and pressure vessels.
Duplex stainless steels are usually divided into three groups based on their pitting corrosion resistance, characterised by the pitting resistance equivalence number, PREN = %Cr + 3.3 %Mo + 16 %N.
Chemicals composition of grades from EN 10088-1 (2014) Standard are given in the table below:
Mechanical properties from European Standard EN 10088-3 (2014) (for product thickness below 160 mm):
*for thickness ≤ 5 mm (0.20 in)
The minimum yield stress values are about twice as high as those of austenitic stainless steels.
Duplex grades are therefore attractive when mechanical properties at room temperature are important because they allow thinner sections.
Duplex stainless is widely used in the industry because it possesses excellent oxidation resistance but can have limited toughness due to its large ferritic grain size, and they have hardened, and embrittlement tendencies at temperatures ranging from 280 to 500 °C, especially at 475 °C, where spinodal decomposition of the supersaturated solid ferrite solution into Fe-rich nanophase ( a ´ {\displaystyle {\acute {a}}} ) and Cr-rich nanophase ( a ´ ´ {\displaystyle {\acute {a}}{\acute {}}} ), accompanied by G-phase precipitation, occurs, which makes the ferrite phase a preferential initiation site for micro-cracks.
Duplex stainless steel grades must be cooled as quickly as possible to room temperature after hot forming to avoid the precipitation of intermetallic phases (Sigma phase in particular) which drastically reduce the impact resistance at room temperature as well as the corrosion resistance.
Alloying elements Cr, Mo, W, Si increase the stability and the formation of intermetallic phases. Therefore, super duplex grades have a higher hot working temperature range and require faster cooling rates than the lean duplex grades.
Duplex stainless steels are usually selected for their high mechanical properties and good to very high corrosion resistance (particularly to stress corrosion cracking).