ASTM A790 UNS S31803 Duplex Steel Seamless and Welded Pipe for Boiler Duty

ASTM A790 UNS S31803 Pipe for Boiler and Corrosive Service

ASTM A790/A790M is the standard specification for seamless and welded ferritic/austenitic stainless steel pipe intended for general corrosive service and for high-temperature service where the corrosion resistance of the duplex structure is the deciding factor. UNS S31803 is the grade designation of a duplex stainless steel alloy under this specification, also written as Duplex 2205; the S prefix identifies a stainless steel, and 31803 is the Unified Numbering System number for this specific alloy. Pipe made to this specification is supplied as seamless, straight seam welded or heavily cold worked welded product, and is normally ordered together with fittings to ASTM A815 in the same grade so that the whole system is metallurgically consistent.

SA790 UNS S31803 pipe exhibits exceptional corrosion resistance in seawater and other chloride-rich environments. Its resistance to chloride-induced stress corrosion cracking makes it one of the most cost-effective long-term solutions for wet, chlorinated duty. In boiler plants the grade is usually applied in the cooler sections of the water and steam circuit, in condenser and cooling water pipework, in flue gas desulphurisation areas and in feedwater and condensate lines, because prolonged exposure at high temperature embrittles the duplex structure. Duplex S31803 seamless pipe is suitable for high-sensitivity applications requiring precise measurements, although the seamless route gives slightly lower strength than the welded route for the same wall.

Chemical Composition of ASTM A790 UNS S31803 Pipe

Element, %	Minimum	Maximum
Carbon	-	0.030
Manganese	-	2.00
Sulfur	-	0.020
Phosphorus	-	0.030
Silicon	-	1.00
Nickel	4.5	6.5
Chromium	21.0	23.0
Molybdenum	2.5	3.5
Nitrogen	0.08	0.20

Mechanical Properties of Seamless and Welded Pipe

Property	UNS S31803
Tensile strength, min	90 ksi, equal to 620 MPa
Yield strength, 0.2% offset, min	65 ksi, equal to 450 MPa
Elongation, min	25%

The tensile and yield requirements are verified on the finished pipe, and the high specified yield strength means a lighter wall can often meet the same design pressure as an austenitic grade. Hardness is commonly monitored as a supplementary check on the annealing practice, since an over-hard pipe indicates a poorly balanced microstructure.

Wall Thickness Tolerance and Dimensional Range

Outside diameter range	Ratio of wall to diameter	Under tolerance	Over tolerance
1/8 inch to 2 1/2 inch inclusive	All	12.5%	20%
3 inch to 18 inch inclusive	Up to 5% inclusive	12.5%	22.5%
3 inch to 18 inch inclusive	Above 5%	12.5%	15%
Above 18 inch, welded	All	12.5%	17.5%
Above 18 inch, seamless	Up to 5% inclusive	12.5%	22.5%
Above 18 inch, seamless	Above 5%	12.5%	15%

Typical supplied dimensions for this grade are nominal bore from 1/2 inch to 12 inch, with seamless pipe from 1/2 inch to 8 inch nominal bore and welded pipe from 6 inch to 24 inch nominal bore. Wall thickness ranges from 0.5 mm to 45 mm and outside diameter from 6.0 mm to 630 mm, with common schedules including SCH20, SCH30, SCH40, STD and SCH80. Pipe is supplied in single random, double random or cut lengths, in welded, fabricated or seamless form.

Corrosion Performance and Selection Notes

Chloride stress corrosion cracking: the duplex structure is highly resistant, so the grade withstands chlorinated seawater and hot brine where austenitic grades crack.

Pitting and crevice corrosion: chromium, molybdenum and nitrogen together raise pitting resistance well above that of 316L, which extends service life in aerated seawater.

Sulphide stress cracking: duplex grades are favoured in sour service at moderate temperatures, subject to hardness and stress limits agreed for the project.

Temperature limit: duplex is not intended for continuous high-temperature duty, so in boilers it is used in the lower-temperature sections and in water-side pipework.

Welding: electric fusion welded pipe with filler metal addition can be used where slightly lower performance is acceptable, while seamless pipe is preferred for critical high-pressure duty.

Material selection should always match the filler metal, the welding procedure and the post-weld treatment to the grade, since a correct base metal welded with a mismatched consumable will still fail in service.

Frequently Asked Questions

Q: What is the difference between UNS S31803 and Duplex 2205?
They describe the same duplex alloy family. UNS S31803 is the designation used in ASTM A790, and 2205 is the common industry name for the 22% chromium duplex grade.

Q: Can ASTM A790 pipe be used in boilers?
Yes, in the lower-temperature sections such as feedwater, condensate and cooling water circuits. It is not intended for continuous service at high steam temperatures.

Q: How does A790 differ from A928?
ASTM A790 covers seamless and welded pipe without filler metal addition, while ASTM A928 covers electric fusion welded pipe, so the two specifications are selected according to the pipe making route.

Q: What sizes are produced?
Typical supply covers 1/2 inch to 12 inch nominal bore, wall thickness from 0.5 mm to 45 mm, outside diameter from 6.0 mm to 630 mm, in schedules from SCH20 to SCH80.

Q: Is seamless pipe stronger than welded pipe?
For this grade the welded route can develop slightly higher strength, while seamless pipe is chosen for its freedom from a longitudinal weld, which is important in high-pressure and high-integrity duty.

Q: What are the common applications?
Seawater and marine pipework, chemical processing, food plants, heat generators, automotive components, boiler parts and other wet corrosive and moderate temperature duties.