Duplex stainless steels have two phase(Dual phase), in which Austenite components, Apprx. 50%, coexist over Ferrites. It features strong for both intergranular corrosion and stress corrosion cracking.
(1) Mechanical features
Further toughness due to austenite than ferrite stainless steel’s, on the hand, double strength is also a feature resulting from ferrite components. Because of low coefficient of thermal expansion and high thermal conductivity, Duplex steels have lots of needs for heat exchanger tubes. In high temperature condition, its features could change badly, so there are upper-limit temperatures to use it. There are specific available temperatures in ASME and TUV, however, the temperature, 250℃ is commonly highest available one.
(2) Corrosion
Duplex stainless steels show very high superiority in stress corrosion and organic acids resistance. In addition, its great resistance to Chloride increase uses for heat exchanger at VCM projects. Corrosion features are normally reviewed with Pitting Corrosion Resistance Index(PREN), whose high value means big resistance to pitting and crevice corrosion. The critical temperature is another. As pitting and crevice corrosion are dependent for temperature, corrosion-occurrence temperatures are good indicators to review.
CPT : Critical Pitting Temperature
CCT : Critical Crevice Corrosion Temperature
(3) Processability
Low Ni content gives Duplex cost-efficient heat treatment merit. Duplex shows common low temperature features same as general carbon steels. Below – 60℃, impact values dramatically reduce, and over 300℃, ferrite components break down, so the available temperatures range -50 ~ 250℃.
The fraction of austenite and ferrite is most important in duplex, and its imbalance means unwanted feature, brittleness.
(4) Weldability
In general, weldability is good, however, controlling welding temperature is essential. Therefore, adjusting Interpass temperature and Travel speed between passes plays the role of critical factor on multi-layers welding.
On welding, improper temperature causes the fraction to break down, so the standard strictly limits the temperature, 0.5 ~ 1.5KJ/mm, Max. 150℃ in interpass temperature.
APPLICATION
– Heat Exchangers in compress engines, oil refining and chemicals
– Various installations in chemicals and oil industries
– Lots of installations and equipment in Power plants and Ocean plants