The ASTM A312 specification covers seamless and welded austenitic stainless steel pipes used for high temperature and/or corrosive service (the most common grades are 304/304L and 316/316L). The A790 specification covers seamless duplex and super duplex pipes UNS S32205/31803 and UNS S32750/32760 (special SS grade with a mixed ferritic/austenitic microstructure). In this article, we illustrate the chemical and mechanical properties of the A312/A790 specifications
STAINLESS STEEL PIPE ASTM A312
STAINLESS STEEL TYPES
As a general rule, any steel alloy that has at least 10.5% of chromium may be considered stainless steel. However, a multitude of grades is available depending on the mix of the alloying elements (Nickel. Chromium, Moly, Titanium, Copper, Nitrogen, etc). Each alloy has a specific structure and chemical and mechanical properties.
The general attribute of stainless steels is that they show resistance to corrosion thanks to an outer chromium oxide layer. Such oxide acts like a microscopic protection layer that reacts with oxygen and blocks corrosion. Further, stainless steel alloys feature better toughness in cryogenic applications than carbon steel, better strength and hardness, improved ductility and low maintenance costs.
Stainless steels may be grouped into a few families, designated as “series”. Let’s take a closer look.
Austenitic Stainless Steel (Series 300)
These are the most common grades of stainless steel. The microstructure of austenitic stainless steels is obtained with the addition of nickel, manganese, and nitrogen that give weldability and formability properties to the alloy. The resistance to corrosion can be further improved by augmenting the percentage of chrome, moly, and nitrogen to the base alloy.
Nevertheless, the basic austenitic grades are vulnerable to stress corrosion cracking (higher percentages of nickel are necessary to enhance the stress corrosion cracking). Austenitic stainless steel cannot be hardened by heat treatment but have can work hardened to high strength levels while retaining a reasonable level of strength and ductility.
Even if austenitic steels are generally non-magnetic, they can show some magnetic property based on the actual alloy composition and the work hardening given during production. Austenitic stainless steels are divided into the series 200 (chromium-manganese-nickel alloys) and 300 (chromium-nickel alloys like 304, 309, 316, 321, 347, etc). Grade 304/304L is the most common austenitic stainless steel that suits most corrosive applications. Any other grade in the 300 series enhances the basic features of SS304.
Martensitic Stainless Steel (Series 400)
Martensitic stainless steels are similar to ferritic steels as they both have remarkable chromium content, however, martensitic steels have higher carbon content up to 1%. The high carbon content allows martensitic steels to be hardened and tempered as standard carbon and chrome alloy steels (but show generally low weldability and ductility).
This type of stainless steel is specified in case of high strength and moderate corrosion resistance requirements. Different from standard austenitic stainless steels, martensitic grades are magnetic. Common martensitic grades are 410, 420 and 440C.
Ferritic Stainless Steel (SS430)
Ferritic stainless steels have significant chrome content but low additions on carbon (generally below 0.1%). The name of this family of stainless steels comes from the fact that their microstructure is quite similar to carbon and low alloy steels.
These steels have a wide range of application, except for thin surfaces as they have low resistance to welding or applications requiring formability (ferritic steels show low formability and ductility). Ferritic stainless steels cannot be hardened by heat treatment. By adding moly to a ferritic grade, the steel can be used in highly aggressive applications as desalination plants and seawater.
These steel show also remarkable resistance to stress corrosion cracking. Likewise martensitic steels, ferritic SS is magnetic. The most common ferritic grades are the 430 (17% chromium), and the 409 (11% chromium), largely used in the automotive sector.
Duplex and Super Duplex Stainless Steel (UNS S32205, S31803, S32750/760)
Duplex stainless steels have a microstructure which is 50% ferritic and 50% austenitic. This composition gives them a better strength than either ferritic or austenitic steels. Further details about this topic are given below (see ASTM A790 pipes).
Precipitation hardening (PH 17-4)
PH steels can feature remarkable strength due to the addition, in the alloy, of elements as copper, niobium, and aluminum.
These steels can be machined to very specific shapes with high tolerances requirements before the final aging treatment. This is different from conventional hardening and tempering of martensitic steels that are subject to distortion during the treatment.
The resistance to corrosion of. precipitation hardening steels are comparable to standard austenitic steels like SS304.
The most common precipitation hardening stainless steel is the 17-4PH, which features 17% chromium and 4% nickel.
STAINLESS STEEL PIPE GRADES CHART (ASTM A312)
The table shows the chemical composition of the most common grades of stainless steel pipes under the ASTM A312-ASME SA312 specification:
|SS Pipes Grade||UNS||C||Mn||P||S||Si||Cr||Ni||Mo||Ti||Nb||N|
A312 SS PIPES MECHANICAL PROPERTIES
|SS Pipes Grade
ASTM A312/ASME SA312
|TP321||OD < 17,1 mm – 515;||OD < 17,1 mm – 205;||35|
|OD > 17,1 mm – 485||OD > 17,1 mm – 170|
CROSS REFERENCE TABLE ASTM/EN STAINLESS STEEL MATERIALS FOR PIPES
|Stainless Steel Pipe Grades|
|Werkstoff/DIN||EN grades||ASTM grades|
|1.4541||X6CrNiTi18-10||A 312 Grade TP321|
|1.4571||X6CrNiMoTi17-12-2||A 312 Grade TP316Ti|
|1.4301||X5CrNi18-10||A 312 Grade TP304|
|1.4306||X2CrNi19-11||A 312 Grade TP304L|
|1.4307||X2CrNi18-9||A 312 Grade TP304L|
|1.4401||X5CrNiMo17-12-2||A 312 Grade TP316|
|1.4404||X2CrNiMo17-13-2||A 312 Grade TP316L|
|1.4462||X2CrNiMoN22-5-3||UNS S 31803 (Duplex)|
|1.4529||X1NiCrMoCuN25-20-7||UNS N 08926|
|1.4539||X1NiCrMoCu25-20-5||UNS N 08904 (904L)|
|1.4547||X1CrNiMoCuN20-18-7||UNS S 31254|
ASTM A312 PIPE DIMENSIONS
The standard stainless steel pipes dimensions are set by the ANSI ASME B36.19 specification.
Seamless SS pipes are available in the size range 1/8″ thru 24″, welded stainless pipes are manufactured in the range 2″ thru 36″ (ASTM A312, ASTM A358, i.e. electric-fusion-welded austenitic chromium-nickel stainless steel pipe, or as rolled).
ASTM A312 PIPE WEIGHT
|Nominal Pipe Size||Outside Diameter||Stainless Steel Pipes Dimensions|
|Schedule 5S||Schedule 10S||Schedule 40S||Schedule 80S|
|(mm)||(inch)||WT / WEIGHT|
|mm (in)||kg/m||mm (in)||kg/m||mm (in)||kg/m||mm (in)||kg/m|
|1/8||10.3||0.405||–||–||1.25 (0.049)||0.28||1.73 (0.068)||0.37||2.42 (0.095)||0.47|
|1/4||13.7||0.540||–||–||1.66 (0.065)||0.49||2.24 (0.088)||0.63||3.03 (0.119)||0.80|
|3/8||17.2||0.675||–||–||1.66 (0.065)||0.63||2.32 (0.091)||0.85||3.20 (0.126)||1.10|
|1/2||21.3||0.840||1.65 (0.065)||0.81||2.11 (0.083)||1.00||2.77 (0.109)||1.27||3.74 (0.147)||1.62|
|3/4||26.7||1.050||1.65 (0.065)||1.02||2.11 (0.083)||1.28||2.87 (0.113)||1.68||3.92 (0.154)||2.20|
|1||33.4||1.315||1.65 (0.065)||1.30||2.77 (0.109)||2.09||3.38 (0.133)||2.50||4.55(0.179)||3.24|
|1 1/4||42.2||1.660||1.65 (0.065)||1.66||2.77 (0.109)||2.69||3.56 (0.140)||3.39||4.86 (0.191)||4.47|
|1 1/2||48.3||1.900||1.65 (0.065)||1.91||2.77 (0.109)||3.11||3.69 (0.145)||4.06||5.08 (0.200)||5.41|
|2||60.3||2.375||1.65 (0.065)||2.40||2.77 (0.109)||3.93||3.92 (0.154)||5.45||5.54 (0.218)||7.49|
|2 1/2||73.0||2.875||2.11 (0.083)||3.69||3.05 (0.120)||5.26||5.16 (0.203)||8.64||7.01 (0.276)||11.4|
|3||88.9||3.500||2.11 (0.083)||4.52||3.05 (0.120)||6.46||5.49 (0.216)||11.3||7.62 (0.300)||15.3|
|3 1/2||101.6||4.000||2.11 (0.083)||5.18||3.05 (0.120)||7.41||5.74 (0.226)||13.6||8.08 (0.318)||18.6|
|4||114.3||4.500||2.11 (0.083)||5.84||3.05 (0.120)||8.37||6.02 (0.237)||16.1||8.56 (0.337)||22.3|
|5||141.3||5.563||2.77 (0.109)||9.46||3.41 (0.134)||11.6||6.56 (0.258)||21.8||9.53 (0.375)||31.0|
|6||168.3||6.625||2.77 (0.109)||11.3||3.41 (0.134)||13.9||7.12 (0.280)||28.3||10.9 (0.432)||42.6|
|8||219.1||8.625||2.77 (0.109)||14.8||3.76 (0.148)||20.0||8.18 (0.322)||42.5||12.7 (0.500)||64.6|
|10||273.1||10.750||3.41 (0.134)||22.7||4.20 (0.165)||27.8||9.28 (0.365)||60.4||12.7 (0.500)||81.5|
|12||323.9||12.750||3.97 (0.156)||31.3||4.58 (0.180)||36.1||9.53 (0.375)||73.9||12.7 (0.500)||97.4|
A312 PIPE DIAMETER TOLERANCE
|NPS||ASTM A312/ASME SA312 OD Tolerance|
|1/8 to 1 1 /2
> 1 1 /2 to 4
> 4 to 8
> 8 to 18
> 18 to 26
> 26 to 34
> 34 to 48
A312 PIPE WALL THICKNESS TOLERANCE
The nominal wall thickness tolerance is +/- 12.5%.
|NPS||ASTM A312/ASME SA312 WT Tolerance, %|
|1/8-2 1 /2||20.0||12.5|
|3~18, t/D≤ 5%||22.5||12.5|
|3~18, t/D> 5%||15.0||12.5|
|≥ 20, welded||17.5||12.5|
|≥ 20, seamless, t/D≤ 5%||22.5||12.5|
|≥ 20, seamless, t/D> 5%||15.0||12.5|
A312 SS PIPE FINISHING AND TESTING
|Polishing||Manufacturers can polish any stainless steel pipe item to a #4 polish, #6 Polish, #7 polish or a #8 mirror finish.|
|Cuttings||Stainless steel pipes can be produced in standard lengths or cut to size (standard lengths are 20′ and 40′ depending on the nominal pipe size).|
|Beveling||Manufacturers can bevel the edges of stainless steel pipes to prepare them for welding.|
|Threading||Stainless steel pipes can be supplied, besides plain and beveled ends, also with threaded ends (generally for NPS below 2 inches)|
|Pipe Honing & Turning||Manufacturers can hone SS pipe and tube, using precision abrasion tools and obtain any desired finish or dimensional tolerance.|
|Heat treating & Annealing||Most stainless steel pipes grades can be heat treated to modify their mechanical properties.|
|Positive Material Identification (PMI)||Manufacturers can perform PMI testing to ascertain the actual content of Nickel, Chrome, and Moly in the pipe.|
|UT Testing||In some cases, UT testing of the stainless steel plates may be required.|
A duplex pipe features, at the same time, the qualities of a ferritic SS pipe (high resistance to corrosion cracking and tensile strength) and of an austenitic SS pipe (easy manufacturing and good resistance to corrosion and erosion). Duplex pipes 2205 (UNS S31803/UNS S32205) are used for applications below 600 degrees (F).
Duplex and super duplex stainless steels are weldable and have moderate formability. These types of stainless steels are magnetic but to a lower extent than ferritic, martensitic and precipitation hardening grades due to the 50% austenitic content.
The common grades under this family are:
- Lean duplex: UNS S32101, S32202 (UR2202), S32304, and S32003.
- Standard duplex: UNS S31803/S32205
- Super duplex: UNS S32760 (Zeron 100), S32750 (2507), and S32550 (Ferralium 255)
- Hyper duplex: UNS S32707 and S33207
The ASTM A790 specification covers seamless duplex and super duplex pipes for demanding applications, with high corrosion, temperature, and pressure. Electro-fusion welded duplex and super duplex welded pipes are covered, instead, by the ASTM A928 specification.
Duplex and super duplex can be considered as ‘higher grade’ stainless steel pipes, and are available on the market in the same dimensional range of conventional A312 stainless steel pipes (size range covered by the ASME B36.19 specification).
There are a few differences between duplex/super duplex pipes and conventional stainless steel pipes, which are explained below.
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).
A790 DUPLEX PIPE PROPERTIES
|UNS||Chemical Composition||Min.Tensile (KSI)||Min.Yield
|UNS S31803||C 0.30 max
Mn 2.00 max
P 0.030 max
S 0.020 max
Si 1.00 max
|UNS S32205||C 0.30 max
Mn 2.00 max
P 0.030 max
S 0.020 max
Duplex pipes are manufactured according to the ASME B36.19 standard.
SUPER DUPLEX PIPE
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 higher chromium and molybdenum content of Super Duplex vs. 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.
A790 SUPER DUPLEX PIPES PROPERTIES
|UNS S32750||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 (“Sandvik”)||C 0.03 max
Mn 1.0 max
P 0.03 max
S 0.01 max
Si 1.0 max