Duplex Stainless Steel
Duplex Stainless Steel
Why chose Duplex and Super Duplex?
First off, both Duplex and Super Duplex have far greater corrosion resistance than the stainless grades 304 and 316.
An important feature of Duplex stainless steel is that it’s pitting and crevice corrosion resistance is greatly superior to that of 304 or 316 grade Stainless Steel. This combined with high mechanical strength and ductility makes it an excellent material for so many applications especially as many fittings are being submerged in salt water.
It would be false economy to use 316 Grade Stainless Steel for marine environments as it is by no means the most robust alloy for high chloride environments.
Duplex stainless steel was developed as a product which could improve upon some of the technical weaknesses of the standard austenitic and ferritic stainless steel grades that are available in the market. Both have low strength, ferritic steels have poor low temperature toughness, and austenitic steels have a very low resistance to stress corrosion cracking. Duplex has a microstructure which is split roughly 50/50 between austenite and ferrite and balancing of these phases provides the following benefits
Higher strength which is about twice of 304 stainless steel.
This leads to reduced plate thicknesses being used in fabrications reducing the weight which is of particular significance in items such as pressure vessels, storage tanks, and structural applications such as bridges.
Good weldability in thick plates.
Good low temperature toughness.
Resistance to stress corrosion cracking of importance in many applications including hot water tanks, process plant, brewing tanks, and desalination plant.
Another key feature of Duplex stainless is its enhanced corrosion resistance. There is no single measure of corrosion resistance but the Pitting Resistance Equivalent Number (PREN) is widely used as a means of comparing the relative corrosion resistance of different steel grades.
To work out the PREN we use a mathematical formula for the chemical composition of steel. PREN = %Cr + (3.3 x %Mo) + (16 x %N)
Here are the approximate PRE pitting corrosion values for 304 and 316 Stainless Steel plus Duplex and Super Duplex grades:
304 = 17-20
316 = 25-30
Duplex = 30-38
Super Duplex = >40
Duplex stainless steel is still being developed and improved by steel manufacturers.