ASTM A928 UNS S32760 Super Duplex Stainless Steel Welded Pipe

What ASTM A928 Covers and How EFW Pipe Is Made

ASTM A928/A928M is the specification for electric fusion welded (EFW) stainless steel pipe, including duplex and super duplex grades, for corrosive or high-temperature applications. EFW pipe is made by forming plate or sheet into a cylinder and joining the longitudinal seam with an electric arc, usually with filler metal addition, which produces a strong and reliable joint able to withstand high pressure and temperature. Because the pipe starts as flat product, the plate chemistry and the weld consumable can both be specified and verified before the pipe is formed, and the weld can be inspected along its full length by radiographic or ultrasonic means.

UNS S32760 is a super duplex stainless steel with an austenitic-ferritic microstructure and a chromium content of approximately 25% or more. As chromium content increases, corrosion resistance rises, and this grade offers greater resistance to pitting and crevice corrosion than conventional stainless steels and standard duplex alloys. Its high strength suits high-pressure environments, and its low coefficient of thermal expansion relative to austenitic grades lets it operate at elevated temperature without excessive deformation. Additions of molybdenum, nitrogen, copper and tungsten further stabilise the structure and extend the pitting resistance into the super duplex class.

Chemical Composition of Super Duplex UNS S32760

Element, %	Requirement
Carbon, max	0.030
Nitrogen	0.24-0.32
Phosphorus, max	0.035
Silicon, max	0.80
Manganese, max	1.20
Molybdenum	3.0-5.0
Nickel	6.00-8.00
Chromium	24.0-26.0
Sulfur, max	0.020
Copper, min	0.50

The high nitrogen content is essential: it strengthens the austenite phase and helps the weld and heat affected zone recover corrosion resistance after welding. Copper and tungsten are deliberate additions in this grade and contribute to resistance in reducing acids as well as in chloride-bearing environments.

Mechanical Properties and Heat Treatment

Property	Value
Density	7.8 g per cubic centimetre
Melting point	about 1350 degrees C
Yield strength, 0.2% offset	80,000 psi, equal to 550 MPa
Tensile strength	116,000 psi, equal to 800 MPa
Elongation	15%

SA928 UNS S32760 fusion welded pipe is supplied solution annealed at a temperature of 1110 to 1140 degrees C followed by rapid cooling, so that the austenite and ferrite balance of approximately equal parts is restored after welding and no sigma phase is left in the seam or the heat affected zone. The annealing temperature is higher than for standard duplex grades, which reflects the more heavily alloyed composition of super duplex material.

Impact Toughness and Corrosion Testing

Toughness is verified by Charpy impact testing of the weld and heat affected zone at the temperatures specified for the order. Typical reported average results for EFW super duplex pipe are about 150 J per sq cm at plus 20 degrees C and about 90 J per sq cm at minus 50 degrees C, with single values of about 90 J per sq cm and 75 J per sq cm respectively at those temperatures.

Item	Requirement
Steel grade	UNS S32760
Corrosion test	ASTM G48 Method A
Test conditions	As per ISO 17781
Acceptance criteria	As per ISO 17781

The ferric chloride pitting test to ASTM G48 Method A is the standard screening test for this grade, and the temperature and duration of exposure together with the acceptance criteria are taken from ISO 17781, which also governs the metallographic acceptance of the duplex microstructure. Weld procedure qualification and non-destructive examination of the longitudinal seam are agreed between manufacturer and purchaser before production.

Equivalent Grades and Applications

UNS S32760 corresponds to material number 1.4501 in the European designation system, a copper and tungsten bearing super duplex grade. The number 1.4410 is sometimes quoted alongside S32760, but it actually designates the copper free and tungsten free super duplex grade UNS S32750, which is a different alloy and should not be treated as an equivalent. Matching the correct grade, filler metal and heat treatment is essential whenever super duplex pipe is specified for critical duty.

Seawater and desalination systems, including intake, outfall and brine lines.

Offshore oil and gas topsides, flowlines and process piping in chloride-bearing service.

Chemical and petrochemical plants handling chlorides, acids and oxidizing media.

Flue gas desulphurisation, scrubber and pollution control pipework.

Pulp and paper bleach plants and other aggressive process circuits.

Frequently Asked Questions

Q: What is the difference between ASTM A928 and ASTM A790?
ASTM A928/A928M covers electric fusion welded pipe, usually with filler metal addition, while ASTM A790/A790M covers seamless and welded pipe. The choice depends on the pipe making route required for the duty.

Q: Is UNS S32760 the same as UNS S32750?
No. Both are super duplex grades, but S32760 contains copper and tungsten and corresponds to material number 1.4501, while S32750 corresponds to 1.4410 and contains no deliberate copper or tungsten addition.

Q: What heat treatment is required for S32760 welded pipe?
The pipe is solution annealed at 1110 to 1140 degrees C and then rapidly cooled to restore the balanced duplex structure and avoid sigma phase precipitation.

Q: How is corrosion resistance verified?
By the ferric chloride pitting test to ASTM G48 Method A, with test conditions and acceptance criteria taken from ISO 17781, supported by metallographic examination of the microstructure.

Q: Why is super duplex used instead of standard duplex?
Its higher chromium, molybdenum and nitrogen content gives markedly better pitting and crevice corrosion resistance, which is needed in hot seawater and other highly aggressive chloride environments.

Q: Can EFW super duplex pipe replace seamless pipe?
For many corrosive and high-pressure duties it can, provided the welding procedure, filler metal, heat treatment and full length weld examination meet the specification and the project requirements.