DUPLEX AND SUPER DUPLEX PIPES ASTM A790/A928
Duplex and super duplex pipes are widely used in petrochemical and power generation. In this article, we have collected key information about this special type of stainless steel (explaining its features and applications) and show the chemical and mechanical properties of the most common duplex and super duplex pipe grades as UNS S31803, UNS S32205, UNS S32750 and UNS S32760.
WHAT IS DUPLEX STEEL?
Duplex steel UNS 2205 (UNS 31803 / UNS 32205) is a nitrogen-enhanced stainless steel 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 term “duplex” refers to a family of stainless steels that have double structure – i.e. they are neither a fully austenitic SS structure, as 304 stainless, nor a standard ferritic, such as the 430 series. The microstructure of duplex steels consists of austenite pools surrounded by a continuous ferrite phase (approx 40-50% ferrite).
The benefits of duplex steel are that it combines the qualities of a ferritic structure (high resistance to corrosion cracking and high tensile strength) with the qualities of austenitic steel (easy manufacturing and good resistance to corrosion and erosion).The 2205 grade of the duplex family is the most widely used type for temperatures below 600 degrees F.
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 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.
It should be noted 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 ).
DUPLEX PIPES CHEMICAL COMPOSITION AND MECHANICAL PROPERTIES
|Commercial Name||UNS Designation||ASTM Specifications for Duplex Grades||Chemical Composition||Min.Tensile (KSI)||Min.Yield
|UNS S31803||ASME SA-182 (F51), SA-240, SA-798, SA-790, SA-815
ASTM A182 (F51),A240, A276, A479, A789, A790, A815, A928,
C 0.30 max
|UNS S32205||ASME SA-182 (F60), SA-240, SA-798, SA-790, SA-815
ASTM A182 (F60), A240, A276, A479, A480, A798, A790,
A815, A928, A988
|C 0.30 max
P 0.030 max
0.020 max Si1.00 max
Duplex pipes are manufactured according to the ASME B36.19 standard.
SUPER DUPLEX PIPES
WHAT IS SUPER DUPLEX?
Super duplex stainless steel is a variation of the duplex steel alloy which is used in the oil & gas, chemical, power generation, mechanical and desalination industries:
- oil & gas production equipment
- Offshore platforms
- heat exchangers
- process and service water systems
- fire-fighting systems
- injection and ballast water systems
- high-pressure RO plants and seawater piping
- high strength and corrosion-resistant parts in mechanical applications
- FGD systems (power generation)
- Utility and industrial scrubber systems
- Absorber towers
The high chromium and molybdenum content of Super Duplex makes it extremely resistant to uniform corrosion by organic acids like formic and acetic acid. Super Duplex also provides excellent resistance to inorganic acids, especially those containing chlorides. The pitting Resistance equivalent of Super Duplex, calculated by PREN = Cr + 3.3Mo + 16N, will exceed 40 in most material forms.
SUPER DUPLEX PIPES CHEMICAL COMPOSITION AND MECHANICAL PROPERTIES
|Trade Name||UNS||Super Duplex Industry Specifications||Chemical Composition||Min.Tensile
|SAF 2507®||UNS S32750||ASTM A182 F53, A240, A276, A479, A789, A790, A815, A928, A988
|C 0.030 max
Cu 0.5 max
Mn 1.20 max
P 0.035 max
S 0.020 max
Si 0.8 max
|UNS S32760||ASTM A182 F55, A240, A276, A314, A473, A479, A789, A790, A815, A928, A988
|C 0.03 max
Mn 1.0 max
P 0.03 max
S 0.01 max
Si 1.0 max