WHAT IS DUPLEX STEEL?

the term “duplex” refers to a family of stainless steels that have double structure – i.e. they are neither a fully austenitic ss structure, like 304 stainless, nor a standard ferritic, such as the 430 series.

duplex stainless steel (uns s31803/uns s32205) has been developed to overcome the issues of the 300 series stainless steels as 316l and 317l (chloride stress corrosion cracking, under tensile stress in cases where the metal is exposed to fluids containing chlorides or by steel exposure to high temperatures).

the microstructure of duplex steels consists of austenite pools surrounded by a continuous ferrite phase (approx 40-50% ferrite).

therefore, duplex pipes combine the qualities of ferritic and austenitic stainless steel materials.

uns 2205 duplex steel finds application in marine projects, oil & gas exploration, closed loops water systems, desalinization plants, hydrocarbons storage and transportation, downstream refining, pulp, and paper.

duplex are resistant to stress corrosion cracking. while standard duplex steels have corrosion resistance comparable to standard austenitic steels (but show better strength and resistance to stress-corrosion-cracking), the so-called super duplex steels feature enhanced strength and resistance to any type of corrosion compared to standard austenitic steels.

duplex corrosion resistance

duplex stainless steel shows a superior corrosion resistance to grade 316 for most applications (excellent resistance to localized corrosion including intergranular, pitting and crevice corrosion; the cpt of 2205 is generally at least 35ºc). duplex is also resistant to chloride corrosion cracking (scc) at temperatures up to about 150ºc.

grade 2205 duplex will often perform well in environments which cause premature failure of the standard austenitic grades of series 300 stainless steel. it has also stronger resistance to seawater corrosion, compared to stainless steel 316.

note that, despite duplex stainless steel shows good high-temperature oxidation resistance, 2205 suffers from embrittlement if kept at temperatures above 300ºc for some period of time. (a situation that can be rectified only  by a full solution annealing treatment).

for this reason, duplex stainless steels are never used for temperatures in excess of 300ºc. duplex pipes match the following fittings materials: astm a815 (butt weld fittings) and astm a182 f51 / f53 / f55 (flanges / forged socket weld – threaded fittings).