Stainless Pipe: A312, A790 Duplex

Quick Reference

ASTM A312 covers austenitic stainless steel pipes (304/304L, 316/316L, 321, 347) for corrosive/high-temp service. ASTM A790 covers duplex (UNS S31803/S32205) and super duplex (UNS S32750/S32760) pipes. Sizes per ASME B36.19. Matching fittings: ASTM A403 (SS) and A815 (duplex).

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Need pipe dimension charts? Download the ASME B36.19 Pipe Schedule Chart for stainless/nickel alloy pipe dimensions and weights.

Stainless Steel Pipe ASTM A312

General Info ASTM A312 Ss Pipes

ASTM A312 is a specification for seamless, welded, and heavily cold-worked austenitic stainless steel pipes intended for high-temperature and general corrosive service.

ASTM A312 stainless steel pipes are used across oil and gas, chemical processing, food processing, pharmaceuticals, and construction.

The table below summarizes the main features of ASTM A312 stainless steel pipes:

Feature	Description
Types	ASTM A312 covers both seamless and welded austenitic stainless steel pipes. Seamless pipes are produced by extruding a billet through a mandrel or piercing a solid bar to form a hollow tube, while welded pipes are manufactured by welding together stainless steel plates or coils.
Grades	Available in several grades, including TP304/304L (general corrosive service), TP316/316L (higher corrosion resistance), and others such as TP321, TP347, TP310, and TP904L for specific applications requiring enhanced properties.
Chemical Composition	Varies depending on grade. Common alloying elements include chromium, nickel, and molybdenum, which impart corrosion resistance, strength, and toughness. The composition may also include carbon, manganese, silicon, and nitrogen in controlled amounts.
Mechanical Properties	Pipes undergo various mechanical tests including tensile testing, hardness testing, and NDT such as ultrasonic testing. Tensile strength, yield strength, elongation, and hardness vary depending on the grade and heat treatment condition.
Dimensions and Sizes	Available from 1/8 inch to 24 inches in nominal diameter. Wall thickness varies depending on the nominal pipe size and grade. Standard pipe schedules such as SCH 5, SCH 10, SCH 40, and SCH 80 are commonly available.
Applications	Oil and gas (pipelines, refineries, petrochemical plants), chemical processing (reactors, storage tanks, process piping), food processing (dairy equipment, beverage production), pharmaceuticals (sterile piping systems), and construction (architectural structures, handrails).
Welding	Readily weldable using conventional welding methods such as TIG, MIG, and manual arc welding. Preheating and post-weld heat treatment may be necessary to prevent sensitization and ensure corrosion resistance in welded joints.

Pro Tip

When ordering 304 or 316 stainless, always request dual-certified “L” grade (304/304L or 316/316L). The cost difference is minimal, and you get better weldability. The “L” (low carbon) version prevents sensitization during welding without sacrificing strength.

ASTM A312 stainless steel pipes are preferred for their excellent corrosion resistance, high-temperature performance, and versatility across industrial applications. When selecting ASTM A312 pipes, consider the grade, size, wall thickness, surface finish, and end connections to match the service requirements.

History

Development of Stainless Steel

The development of stainless steel spans over a century of metallurgical progress. The following table outlines the major milestones:

Period	Milestone	Details
1912	Discovery of Chromium Effects	Harry Brearley, a British metallurgist, discovered that adding chromium to steel significantly improved its resistance to rust and corrosion. This discovery laid the foundation for stainless steel development.
1913	Introduction of Martensitic Stainless Steel	Brearley produced the first stainless steel, known as martensitic stainless steel, by adding around 12% chromium to steel. This alloy exhibited remarkable corrosion resistance and was initially used in cutlery and surgical instruments.
1920s	Development of Austenitic Stainless Steel	Researchers at Krupp in Germany and in the United States independently developed austenitic stainless steel, which contained higher levels of chromium and nickel. Austenitic stainless steel offered superior corrosion resistance and became the most widely used type.
1939-1945	World War II and Stainless Steel Demand	Demand for stainless steel surged due to military applications such as aircraft, naval vessels, and weaponry. Its corrosion resistance, strength, and durability proved indispensable for wartime infrastructure.
Post-1945	Post-War Industrial Boom	Stainless steel demand increased rapidly as industries expanded worldwide. Its versatility, hygiene properties, and aesthetic appeal led to adoption in automotive, construction, chemical processing, and food processing sectors.
1950s-2000s	Advancements in Manufacturing	Improvements in metallurgy, manufacturing processes, and alloy compositions produced specialized grades including duplex stainless steel, precipitation-hardening stainless steel, and high-temperature alloys.
Present	Modern Applications	Stainless steel is now used in kitchen appliances, architectural structures, medical devices, transportation infrastructure, and renewable energy systems. Research continues on improving properties, sustainability, and recyclability.

History of Stainless Steel Pipes

The development of stainless steel pipes followed the broader evolution of stainless steel alloys. Stainless steel pipes have served multiple industries due to their corrosion resistance, durability, and adaptability.

Period	Milestone	Details
Early 1900s	Early Developments in Stainless Steel	The discovery of stainless steel in the early 20th century enabled the development of stainless steel pipes. In 1912, Harry Brearley discovered that adding chromium to steel significantly improved its corrosion resistance, leading to the first stainless steel alloys.
1920s-1930s	Introduction of Stainless Steel Pipes	As demand for corrosion-resistant materials grew, stainless steel pipes became a practical solution. Austenitic stainless steel, with higher levels of chromium and nickel, became the preferred pipe material due to its superior corrosion resistance and durability.
1939-1945	World War II and Industrial Expansion	Stainless steel pipes were used in military applications such as shipbuilding, aircraft construction, and weapon manufacturing. The war effort increased production and drove technological advances in pipe manufacturing.
Post-1945	Post-War Boom and Diversification	The industrial expansion drove demand for stainless steel pipes across oil and gas, chemical processing, food processing, and construction. They found use in pipelines, refineries, petrochemical plants, and architectural structures.
1950s-2000s	Advancements in Manufacturing Processes	Innovations such as seamless pipe manufacturing techniques, improved welding technologies, and advanced surface finishing methods made stainless steel pipes more reliable and cost-effective.
1970s-Present	Specialized Applications and Grades	Manufacturers began producing pipes in various grades and sizes for different applications. Specialized grades such as duplex stainless steel, high-temperature alloys, and corrosion-resistant alloys addressed the specific demands of various industries.
Present	Modern Innovations and Sustainability	Stainless steel pipes remain at the forefront of piping technology, with ongoing research improving performance, sustainability, and lifecycle characteristics. Advances in alloy design, manufacturing processes, and surface coatings continue to improve durability and cost-effectiveness.

The history of stainless steel pipes mirrors the evolution of stainless steel as a material and its role in modern industrial infrastructure.

Stainless Steel Types

Any steel alloy containing at least 10.5% chromium qualifies as stainless steel. The market offers numerous grades based on combinations of alloying elements such as nickel, chromium, molybdenum, titanium, copper, nitrogen, and others. Each alloy has a distinct structure and specific chemical and mechanical properties.

The defining characteristic of stainless steel is its corrosion resistance, due to an outer layer of chromium oxide. This oxide layer acts as a microscopic protective barrier, reacting with oxygen to inhibit corrosion. Stainless steel alloys also exhibit superior toughness in cryogenic applications compared to carbon steel, along with enhanced strength, hardness, ductility, and low maintenance costs.

Stainless steels are grouped into several families, designated as “series.”

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 which give weldability and formability properties to the alloy. The corrosion resistance can be further improved by augmenting the percentage of chrome, moly, and nitrogen in 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 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 properties 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 of 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 various applications, except for thin surfaces as they have low resistance to welding or applications requiring formability (ferritic steels show low formability and ductility). Ferritic stainless steel cannot be hardened by heat treatment. By adding moly to a ferritic grade, the steel can be used in highly aggressive applications such as desalination plants and seawater.

These steel show also remarkable resistance to stress corrosion cracking. Likewise, martensitic steels and ferritic SS are 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 steel contains a balanced mixture of austenite and ferrite phases, typically with higher chromium (19% to 32%) and molybdenum (up to 5%) content. It offers a combination of high strength, corrosion resistance, and resistance to stress corrosion cracking. Duplex stainless steel is commonly used in applications requiring excellent resistance to pitting and crevice corrosion, such as marine environments and chemical processing. Common grades include 2205 (S32205) and 2507 (S32750).

Precipitation hardening (PH 17-4)

PH steels can feature remarkable strength due to the addition, in the alloy, of elements such as copper, niobium, and aluminum.

These steels can be machined to very specific shapes with high tolerance 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)

Chemical Composition

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
TP304	S3040	0.08	2.0	0.045	0.030	1.0	18.0-20.0	8.0-11.0
TP304L	S30403	0.035	2.0	0.045	0.030	1.0	18.0-20.0	8.0-13.0
TP304H	S30409	0.04-0.10	2.0	0.045	0.030	1.0	18.0-20.0	8.0-11.0
TP304N	S30451	0.08	2.0	0.045	0.030	1.0	18.0-20.0	8.0-18.0				0.10-0.16
TP304LN	S30453	0.035	2.0	0.045	0.030	1.0	18.0-20.0	8.0-12.0				0.10-0.16
TP309S	S30908	0.08	2.0	0.045	0.030	1.0	22.0-24.0	12.0-15.0	0.75
TP309H	S30909	0.04-0.10	2.0	0.045	0.030	1.0	22.0-24.0	12.0-15.0
TP309Cb	S30940	0.08	2.0	0.045	0.030	1.0	22.0-24.0	12.0-16.0	0.75		10xC min1.10 max
TP309HCb	S30941	0.04-0.10	2.0	0.045	0.030	1.0	22.0-24.0	12.0-16.0	0.75		10xC min1.10 max
TP310S	S3108	0.08	2.0	0.045	0.030	1.0	24.0-26.0	19.0-22.0	0.75
TP310H	S3109	0.04-0.10	2.0	0.045	0.030	1.0	24.0-26.0	19.0-22.0
TP310Cb	S31040	0.08	2.0	0.045	0.030	1.0	24.0-26.0	19.0-22.0	0.75		10xC min1.10 max
TP310HCb	S31041	0.04-0.10	2.0	0.045	0.030	1.0	24.0-26.0	19.0-22.0	0.75		10xC min1.10 max
TP316	S3160	0.08	2.0	0.045	0.030	1.0	16.0-18.0	11.0-14.0	2.0-3.0
TP316L	S31603	0.035	2.0	0.045	0.030	1.0	16.0-18.0	10.0-14.0	2.0-3.0
TP316H	S31609	0.04-0.10	2.0	0.045	0.030	1.0	16.0-18.0	11.0-14.0	2.0-3.0
TP316Ti	S31635	0.08	2.0	0.045	0.030	0.75	16.0-18.0	10.0-14.0	2.0-3.0	5x(C-N)-0.70		0.10
TP316N	S31651	0.08	2.0	0.045	0.030	1.0	16.0-18.0	10.0-14.0	2.0-3.0			0.10-0.16
TP316LN	S31653	0.035	2.0	0.045	0.030	1.0	16.0-18.0	11.0-14.0	2.0-3.0			0.10-0.16
TP317	S3170	0.08	2.0	0.045	0.030	1.0	18.0-20.0	10.0-14.0	3.0-4.0
TP317L	S31703	0.035	2.0	0.045	0.030	1.0	18.0-20.0	11.0-15.0	3.0-4.0
TP321	S3210	0.08	2.0	0.045	0.030	1.0	17.0-19.0	9.0-12.0				0.10
TP321H	S32109	0.04-0.10	2.0	0.045	0.030	1.0	17.0-19.0	9.0-12.0				0.10
TP347	S3470	0.08	2.0	0.045	0.030	1.0	17.0-19.0	9.0-13.0
TP347H	S34709	0.04-0.10	2.0	0.045	0.030	1.0	17.0-19.0	9.0-13.0
TP347LN	S34751	0.05-0.02	2.0	0.045	0.030	1.0	17.0-19.0	9.0-13.0			0.20-50.0	0.06-0.10
TP348	S3480	0.08	2.0	0.045	0.030	1.0	17.0-19.0	9.0-13.0
TP348H	S34809	0.04-0.10	2.0	0.045	0.030	1.0	17.0-19.0	9.0-13.0

A312 SS PIPES MECHANICAL PROPERTIES

Grade	Tensile Strength N/mm² (min)	Yield Strength N/mm² (min)	Elongation % (min)
TP304	515	205	35
TP304L	485	170	35
TP316	515	205	35
TP316L	485	170	35
TP321 (OD ≤ 17.1 mm)	515	205	35
TP321 (OD > 17.1 mm)	485	170	35

Cross Reference Table Astm/en Stainless Steel Materials For Pipes

Werkstoff/DIN	EN Grade	ASTM Grade
1.4541	X6CrNiTi18-10	A312 Grade TP321
1.4571	X6CrNiMoTi17-12-2	A312 Grade TP316Ti
1.4301	X5CrNi18-10	A312 Grade TP304
1.4306	X2CrNi19-11	A312 Grade TP304L
1.4307	X2CrNi18-9	A312 Grade TP304L
1.4401	X5CrNiMo17-12-2	A312 Grade TP316
1.4404	X2CrNiMo17-13-2	A312 Grade TP316L
1.4462	X2CrNiMoN22-5-3	UNS S31803 (Duplex)
1.4529	X1NiCrMoCuN25-20-7	UNS N08926
1.4539	X1NiCrMoCu25-20-5	UNS N08904 (904L)
1.4547	X1CrNiMoCuN20-18-7	UNS S31254

ASTM A312 Pipe Dimensions

The standard stainless steel pipe dimensions are set by the ANSI ASME B36.19 specification.

Seamless SS pipes are available in the size range 1/8″ thru 24″, and 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

Schedule 5S and 10S

NPS	OD (mm)	OD (in)	5S WT mm (in)	5S kg/m	10S WT mm (in)	10S kg/m
1/8	10.3	0.405	–	–	1.25 (0.049)	0.28
1/4	13.7	0.540	–	–	1.66 (0.065)	0.49
3/8	17.2	0.675	–	–	1.66 (0.065)	0.63
1/2	21.3	0.840	1.65 (0.065)	0.81	2.11 (0.083)	1.00
3/4	26.7	1.050	1.65 (0.065)	1.02	2.11 (0.083)	1.28
1	33.4	1.315	1.65 (0.065)	1.30	2.77 (0.109)	2.09
1 1/4	42.2	1.660	1.65 (0.065)	1.66	2.77 (0.109)	2.69
1 1/2	48.3	1.900	1.65 (0.065)	1.91	2.77 (0.109)	3.11
2	60.3	2.375	1.65 (0.065)	2.40	2.77 (0.109)	3.93
2 1/2	73.0	2.875	2.11 (0.083)	3.69	3.05 (0.120)	5.26
3	88.9	3.500	2.11 (0.083)	4.52	3.05 (0.120)	6.46
3 1/2	101.6	4.000	2.11 (0.083)	5.18	3.05 (0.120)	7.41
4	114.3	4.500	2.11 (0.083)	5.84	3.05 (0.120)	8.37
5	141.3	5.563	2.77 (0.109)	9.46	3.41 (0.134)	11.6
6	168.3	6.625	2.77 (0.109)	11.3	3.41 (0.134)	13.9
8	219.1	8.625	2.77 (0.109)	14.8	3.76 (0.148)	20.0
10	273.1	10.750	3.41 (0.134)	22.7	4.20 (0.165)	27.8
12	323.9	12.750	3.97 (0.156)	31.3	4.58 (0.180)	36.1

Schedule 40S and 80S

NPS	OD (mm)	OD (in)	40S WT mm (in)	40S kg/m	80S WT mm (in)	80S kg/m
1/8	10.3	0.405	1.73 (0.068)	0.37	2.42 (0.095)	0.47
1/4	13.7	0.540	2.24 (0.088)	0.63	3.03 (0.119)	0.80
3/8	17.2	0.675	2.32 (0.091)	0.85	3.20 (0.126)	1.10
1/2	21.3	0.840	2.77 (0.109)	1.27	3.74 (0.147)	1.62
3/4	26.7	1.050	2.87 (0.113)	1.68	3.92 (0.154)	2.20
1	33.4	1.315	3.38 (0.133)	2.50	4.55 (0.179)	3.24
1 1/4	42.2	1.660	3.56 (0.140)	3.39	4.86 (0.191)	4.47
1 1/2	48.3	1.900	3.69 (0.145)	4.06	5.08 (0.200)	5.41
2	60.3	2.375	3.92 (0.154)	5.45	5.54 (0.218)	7.49
2 1/2	73.0	2.875	5.16 (0.203)	8.64	7.01 (0.276)	11.4
3	88.9	3.500	5.49 (0.216)	11.3	7.62 (0.300)	15.3
3 1/2	101.6	4.000	5.74 (0.226)	13.6	8.08 (0.318)	18.6
4	114.3	4.500	6.02 (0.237)	16.1	8.56 (0.337)	22.3
5	141.3	5.563	6.56 (0.258)	21.8	9.53 (0.375)	31.0
6	168.3	6.625	7.12 (0.280)	28.3	10.9 (0.432)	42.6
8	219.1	8.625	8.18 (0.322)	42.5	12.7 (0.500)	64.6
10	273.1	10.750	9.28 (0.365)	60.4	12.7 (0.500)	81.5
12	323.9	12.750	9.53 (0.375)	73.9	12.7 (0.500)	97.4

A312 PIPE DIAMETER TOLERANCE

NPS	Over (in)	Over (mm)	Under (in)	Under (mm)
1/8 to 1 1/2	1/64 (0.015)	0.4	1/32 (0.031)	0.8
> 1 1/2 to 4	1/32 (0.031)	0.8	1/32 (0.031)	0.8
> 4 to 8	1/16 (0.062)	1.6	1/32 (0.031)	0.8
> 8 to 18	3/32 (0.093)	2.4	1/32 (0.031)	0.8
> 18 to 26	1/8 (0.125)	3.2	1/32 (0.031)	0.8
> 26 to 34	5/32 (0.156)	4.0	1/32 (0.031)	0.8
> 34 to 48	3/16 (0.187)	4.8	1/32 (0.031)	0.8

A312 PIPE WALL THICKNESS TOLERANCE

The nominal wall thickness tolerance is +/- 12.5%.

NPS	Over (%)	Under (%)
1/8 to 2 1/2	+20.0	–12.5
3 to 18, t/D ≤ 5%	+22.5	–12.5
3 to 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 pipe 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.

We recommend purchasing the ASTM A312 specification from the ASTM website or the IHS store to get a complete understanding of this topic.

Duplex Pipe

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 in 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, hydrocarbon 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 that 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 showing good high-temperature oxidation resistance, 2205 suffers from embrittlement if kept at temperatures above 300ºC for some time. (a situation that can be rectified only by a full solution annealing treatment).

Common Mistake

Using duplex stainless steel for high-temperature service. The 475°C embrittlement phenomenon starts showing effects at temperatures as low as 250-300°C with prolonged exposure. For temperatures above 250°C, stick with austenitic grades like 316 or go to nickel alloys.

For this reason, Duplex stainless steels are never used for temperatures over 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).

Differences Between Duplex and Stainless Steel Pipe

The main difference between duplex pipes and traditional stainless steel pipes lies in their microstructure and composition, which produce distinct properties and performance characteristics:

Property	Duplex Pipe	Stainless Steel Pipe
Microstructure	Two-phase microstructure consisting of roughly equal proportions of austenite and ferrite phases. This provides a balance of high strength and excellent corrosion resistance.	Typically has a single-phase microstructure predominantly composed of either austenite or ferrite.
Corrosion Resistance	Superior corrosion resistance, particularly in chloride-rich and acidic environments. Excellent resistance to pitting and crevice corrosion.	Provides corrosion resistance, but duplex pipes generally offer better resistance to certain corrosion types, making them suitable for more demanding service.
Strength	Higher strength than austenitic and ferritic stainless steel. Suitable for applications requiring structural integrity and load-bearing capacity.	Strength varies by grade and microstructure. While some grades provide adequate strength for general applications, duplex pipes deliver higher strength levels.
Toughness and Ductility	Maintains good toughness and ductility despite high strength, allowing them to withstand impact and deformation without fracturing.	Also exhibits toughness and ductility, but specific properties vary by grade and microstructure.
Weldability	Generally weldable using standard welding techniques, although precautions may be necessary to avoid harmful phase formation during welding.	Typically weldable using conventional welding methods, with varying susceptibility to welding-related issues such as sensitization and distortion.

Field Note

When welding duplex, control heat input strictly - too high causes sigma phase precipitation, too low causes excessive ferrite in the HAZ. Both reduce corrosion resistance. Always verify austenite/ferrite balance with a ferrite scope after welding critical joints.

Both duplex pipes and traditional stainless steel pipes offer corrosion resistance and durability. However, duplex pipes provide superior strength, corrosion resistance, and a better balance of mechanical properties, making them well suited for demanding applications in oil and gas, chemical processing, and marine engineering.

A790 Duplex Pipe Properties

UNS	Chemical Composition	Min. Tensile (KSI)	Min. Yield(KSI)	Elongation%
UNS S31803	C 0.30 maxCr 21.0-23.0Mn 2.00 maxMo 2.50-3.50N 0.08-0.20Ni 4.50-6.50P 0.030 maxS 0.020 maxSi 1.00 max	90	65	25
UNS S32205	C 0.30 maxCr 22.0-23.0Mn 2.00 maxMo 3.00-3.50N 0.14-0.20Ni 4.50-6.50P 0.030 maxS 0.020 maxSi1.00 max	95	65	25

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 used in 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
• piping
• 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 Pipe Properties

UNS	Chemical Composition	Min. Tensile(KSI)	Min. Yield(KSI)	Elongation%
UNS S32750	C 0.030 max Cr 24.0-26.0Cu 0.5 maxMn 1.20 maxMo 3.0-5.0N 0.24-0.32Ni 6.0-8.0P 0.035 maxS 0.020 maxSi 0.8 max	116	80	15
UNS S32760 (“Sandvik”)	C 0.03 max Cr 24.0-26.0Cu 0.5-1.0Mn 1.0 maxMo 3.0-4.0N 0.2-0.3Ni 6.0-8.0P 0.03 maxS 0.01 maxSi 1.0 maxW 0.5-1.0	109	80	25

Difference Between Duplex & Superduplex Pipes

Duplex and super duplex pipes both have a two-phase microstructure, but they differ in composition and performance. Here are the main differences:

Property	Duplex Pipe	Super Duplex Pipe
Composition	Balanced combination of austenite and ferrite phases, with chromium content 22%-26% and nickel 4.5%-6.5%. Common grades include Duplex 2205 (UNS S32205/S31803) and Duplex 2304 (UNS S32304).	Higher chromium content (typically 24%-27%) and increased levels of molybdenum (up to 6%) and nitrogen (up to 0.3%), resulting in superior corrosion resistance and mechanical properties. Common grades include Super Duplex 2507 (UNS S32750) and Zeron 100 (UNS S32760).
Corrosion Resistance	Excellent corrosion resistance, particularly in chloride-containing environments. Suitable for moderate to high corrosion resistance applications such as chemical processing plants and offshore oil and gas platforms.	Greater corrosion resistance than standard duplex grades, especially in highly corrosive environments such as seawater and acidic solutions. Preferred for marine engineering, desalination plants, and chemical processing.
Strength	High strength and toughness, suitable for structural applications requiring load-bearing capacity and durability.	Higher strength and superior mechanical properties compared to standard duplex grades. Enhanced resistance to stress corrosion cracking and fatigue, suited for demanding applications in harsh environments.
Cost	Generally more cost-effective than super duplex grades, making them a preferred choice where a balance between performance and cost is needed.	More expensive than standard duplex grades due to higher alloy content and superior properties. Typically selected for critical applications where superior corrosion resistance and mechanical performance justify the cost.

Both duplex and super duplex stainless steel pipes offer corrosion resistance and durability, but super duplex pipes provide superior corrosion resistance, higher strength, and better performance in harsh environments. The choice between them depends on the specific application requirements, including corrosion severity, mechanical loading, and budget.

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