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duplex steels are characterised by high chromium (19–28%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels. the most used duplex stainless steels are the 2205 (22% chromium, 5% nickel) and the 2507 (25% chromium, 7% nickel); 2507 is known as “super duplex” due to its higher resistance to corrosion.
essentially, duplex is a fe-ni-cr alloy that has a two-phase ferritic-austenitic stainless-steel microstructure when it is at room temperature.
the advantage of combining ferritic and austenitic grades is that the resultant metal has a metallurgical structure that consists of two phases and therefore benefits from the properties of both microstructures.
these properties make duplex steel highly sought after in heavy industries, like oil and gas nuclear and chemical processing.
advantages of duplex
duplex stainless steel has an array of various benefits such as:
strength:duplex stainless steels have approximately double the strength of regular austenitic or ferritic stainless steels.
toughness and ductility:duplex stainless steels exceed the toughness and ductility of ferritic grades although they are not as touch as austenitic grades.
corrosion resistance:as with all stainless steels, corrosion resistance depends mostly on the composition of the stainless steel, with chromium, molybdenum and nitrogen content being the most important. duplex stainless steels are extremely corrosion resistant and even in chloride and sulphide environments, duplex stainless steels exhibit very high resistance to stress corrosion cracking (scc). scc is a type of corrosion that takes place when a particular set of factors are present: tensile stress, corrosive environment and a sufficiently high temperature.
heat resistance: duplex stainless steel has higher heat conductivity and lower thermal expansion than austenitic steels. duplex grades can easily be used down to temperatures of at least -50°c because at low temperatures they have better ductility that ferritic grades of steel.
cost:duplex stainless steels have lower nickel and molybdenum contents than their austenitic counterparts. this lower alloying content means that duplex stainless steels can be lower in cost. further to this, it is also possible that the thickness of duplex stainless steel can be reduced as it has an increased yield strength. thinner products mean that significant weight savings can be made.
weldability: duplex stainless steels tend to have good weldability and all standard welding processes can be used although they are not quite as easily welded as the austenitic grades.
duplex stainless steel grades and standards
in the early stages of the development of duplex stainless steels, there were only a few grades, the most popular one among them being duplex stainless steel uns s31803. thereafter, the development of new grades started, and they were determined by their end application, which can be categorised into two main types:
hyper-duplex and super-duplex stainless steels, which were created to operate in very corrosive environments but with less focus on their strength
lean and standard duplex stainless steels, which had more focus on increased strength and were to be used in mildly corrosive environments such as in structural applications.
these categories are now identified by the pitting resistance equivalence number (pren) which is calculated by a formula based on the composition of duplex stainless steels.
pren=%cr+3.3×%mo+16×%n
chemical compositions of duplex stainless steel
lean duplex
astm | en-nr. | c max. | cr | ni | mo | n | mn | cu | w | pre* |
s32101 | 1.4162 | 0,04 | 21,0-22,0 | 1,35-1,7 | 0,1 – 0,8 | 0,20-0,25 | 4,0-6,0 | 0,1-0,8 | – | 26 |
s32202 | 1.4062 | 0,03 | 21,5-24,0 | 1,0-2,8 | 0,45 | 0,18-0,26 | 2,0 | – | 26 | |
s32304 | 1.4362 | 0,03 | 21,5-24,5 | 3,0-5,5 | 0,05-0,6 | 0,05-0,20 | 2,5 | 0,05-0,60 | – | 26 |
standard duplex
astm | en-nr. | c max. | cr | ni | mo | n | mn | cu | w | pre* |
s31803 | 1.4462 | 0,03 | 21,0-23,0 | 4,5-6,5 | 2,5-3,5 | 0,08-0,20 | 2,0 | – | – | 33 |
s32205 | 1.4462 | 0,03 | 22,0-23,0 | 4,5-6,5 | 3,0-3,5 | 0,14-0,20 | 2,0 | – | – | 35 |
duplex with 25% cr
astm | en-nr. | c max. | cr | ni | mo | n | mn | cu | w | pre* |
s32520 |
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