ASTM A789 UNS S31803 Duplex Stainless Steel Heat Exchanger Tube

Why UNS S31803 Tubing Is Used in Heat Exchangers

ASTM A789/A789M covers nominal wall thickness grades of duplex stainless steel tubing suitable for applications requiring general corrosion resistance, with particular emphasis on stress corrosion cracking. UNS S31803 is the most widely used duplex alloy for heat exchanger tubes: it is a ferritic/austenitic stainless steel with roughly equal proportions of austenite and ferrite, alloyed with chromium and molybdenum, and it combines high strength with good weldability. It shall be made by the seamless or welded process with no filler metal added.

Bright annealed S31803 tube offers excellent resistance to chloride stress corrosion cracking, so it is widely used in marine, food processing, automotive and other industries, and its high strength suits high-pressure applications. One caution applies to every duplex grade: these steels are susceptible to embrittlement when subjected to prolonged use at high temperatures, so the service temperature must be checked against the intended duty.

Chemical Composition of ASTM A789 UNS S31803 Tubing

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

The nitrogen range is important for heat exchangers that are welded during fabrication, because nitrogen stabilises the austenite and helps the weld zone retain its corrosion resistance and toughness.

Mechanical and Physical Properties

Property	Value for UNS S31803 / S32205
Tensile strength, min	620 MPa
Yield strength, 0.2% proof, min	450 MPa
Elongation in 50 mm, min	25%
Hardness commonly reported	about 293 HB, equivalent to 31 HRC maximum

Physical property	Typical value
Density	about 7.8 g per cubic centimetre
Elastic modulus	about 190 GPa
Mean coefficient of thermal expansion, 0 to 100 degrees C	13.7 micro m per m per degree C
Mean coefficient of thermal expansion, 0 to 315 degrees C	14.2 micro m per m per degree C
Thermal conductivity at 100 degrees C	about 19 W per m per K
Specific heat, 0 to 100 degrees C	about 418 J per kg per K
Electrical resistivity	about 850 nOhm m

The low coefficient of thermal expansion compared with austenitic stainless steels reduces thermal stress in exchangers that see repeated start-up and shutdown, and the high yield strength allows thinner tube walls for the same design pressure.

Temperature Range and Service Limits

Tubes of this grade are normally applied up to about 316 degrees C, equivalent to 600 degrees F. Above that range, long-term exposure risks embrittlement, so higher-temperature sections of a plant are usually built from other alloy families. Where the tube side carries cooling water, seawater or brine, the grade's resistance to chloride pitting and crevice corrosion is the main reason for its selection.

Testing, Marking and Packing

Heat exchanger tubes are verified with the mechanical and non-destructive tests called for by the specification and by the purchaser, which commonly include the reverse flattening test, flaring test, flange test, non-destructive examination and moisture testing of the finished bundle. Positive material identification is frequently requested to confirm the alloy before the tubes are installed. Marking follows ASTM A450/A450M and shall indicate whether the tubing is seamless or welded. Mill test certificates are issued to EN 10204 3.1, and all tubes are supplied to the applicable ASTM A789/A789M specification. Tubes are packed in bundles or plywood cases wrapped in plastic and suitably protected for seaworthy delivery.

Frequently Asked Questions

Q: What is ASTM A789 UNS S31803 tube used for?
It is used mainly for heat exchanger and condenser tubes, marine and process piping, and high-pressure tubing where chlorides would attack standard austenitic grades.

Q: What is the maximum service temperature of S31803 tube?
The practical limit is about 316 degrees C. Continuous operation above this temperature risks embrittlement of the duplex structure.

Q: Is bright annealed tubing better than pickled tubing?
Bright annealing gives a cleaner, smoother surface with better corrosion resistance and easier cleaning, which is preferred for food, pharmaceutical and high-purity duty.

Q: Which sizes are produced in A789 S31803?
Seamless tube is typically offered from 1/2 inch to 8 inch, welded tube from 6 inch to 24 inch, with outside diameters from about 6.0 mm to 630 mm and wall thickness from 1 mm to 50 mm.

Q: How are the tubes tested before delivery?
Testing normally includes tensile and hardness verification, flattening, flaring and flange tests, non-destructive examination, moisture testing and positive material identification, with certificates issued to EN 10204 3.1.

Q: What schedules are available?
Common schedules include 10S, 20, 40S, 40, 60, 80S, 80, 100, 120, 140, 160 and XXH, and custom wall thicknesses can be produced for a specific exchanger design.