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ASTM A790 and ASTM A928
1. Standard specification
Both ASTM A790 and ASTM A928 are standard specifications for ferritic-austenitic stainless steel tubing used in corrosive environments. ASTM A790 specifies the elements and requirements for the manufacture of seamless and welded pipes, while ASTMA928 covers only electric fusion welded pipes.
ASTM A790 requires that duplex and super duplex steel pipes be welded without the addition of any filler metal. However, ASTM A928 requires that the weld be welded with the addition of filler metal.
2. Manufacturing process
The manufacturing processes listed in the ASTM A790 and ASTM A928 standards also provide manufacturers with some freedom of choice. For example, under certain conditions where the raw material (coil or sheet) has already been solution annealed, both standards allow the finished tube to not be annealed.
However, the ASTM A790 standard has restrictions on this situation. When using UNS S31803, S32205, S32750, S32760, and S32520 materials to produce steel pipes, corrosion tests must be supplemented. Special differences between the two standards must be agreed between the purchaser and the manufacturer.
Welding Classifications and Joint Efficiency
Because ASTM A928 deals with electric fusion welding involving filler metals, it introduces specific production classes that alter the performance and testing of the pipe. These classes determine the Joint Efficiency factor used by piping designers in pressure calculations.
- Class 1: Pipes are double-welded using filler metal in all passes and are completely radiographed (100% X-ray inspected). This delivers a Joint Efficiency of 1.0.
- Class 2: Pipes are double-welded using filler metal, but they are not radiographed.
- Class 3: Pipes are single-welded using filler metal, with the weld bead penetrating the root pass completely.
- Class 4: Identical to Class 3, but the internal root pass can be made without filler metal under specific code provisions.
- Class 5: Pipes are double-welded using filler metal and undergo spot radiography.
3. ASTM A790 and ASTM A928 Size
ASTM A790 covers steel pipe diameters from 1/8" (10.29mm) to 30" (762.0mm), with wall thicknesses ranging from 1.24mm to 12.7mm. Based on ANSI B36.10 for steel pipes, larger diameter sizes are available in accordance with A790.
For steel pipes with diameters over 48" (1219.2mm), since they are not covered in the relevant standard ASTM A999, such steel pipes must meet diameter tolerances and out-of-roundness tolerances separately. The wall thickness tolerance ≤ 12.5% required in ASTM A999 is also applicable to steel pipes with a wall thickness exceeding 12.7mm.
ASTM A928 references ASTM A999, which covers diameters from 1/8" (10.29mm) to 48" (1219.2mm). Based on ANSI B36.10 for steel pipes, larger diameter sizes are available in accordance with A928. For ASTM A928 UNS S31803 with a diameter exceeding 48" (1219.2mm), since it is not covered in the related standard ASTM A999, such steel pipes must meet the diameter tolerance and out-of-roundness tolerance respectively. The wall thickness of steel pipes in ASTM A999 ranges from 1.7mm to 59.5mm.
Size Ranges and Wall Thickness Tolerances
Both standards reference underlying dimensional frameworks but cover slightly different maximum sizing parameters for industrial applications.
ASTM A790 typically covers standard outer diameters from 1/8 inch (10.29 mm) up to 30 inches (762 mm), with wall thicknesses keeping close to standard schedules up to 12.7 mm.
For massive project lines exceeding 48 inches (1219.2 mm) outer diameter, specific out-of-roundness and localized diameter tolerances must be negotiated based on standard ASTM A999 parameters.
ASTM A928 expands standard sizing deeper into large-scale industrial distribution networks, covering nominal sizes from 1/8 inch up to 48 inches (1219.2 mm) outer diameter. Because it serves heavy-wall vessel piping and large transport headers, its underlying references account for steel pipe wall thicknesses scaling up much higher, matching heavy industrial requirements from 1.7 mm up to 59.5 mm.
Heat Treatment and Corrosion Testing Supplements
Heat treatment (solution annealing) is vital to restore the 50/50 austenitic-ferritic microstructure after the thermal shock of forming or welding duplex steels.
Under both ASTM A790 and ASTM A928, if the raw base material (such as steel coil or plate) was already fully solution annealed before forming, the manufacturer may bypass annealing the finished pipe under very specific conditions.
However, ASTM A790 imposes stricter quality controls for this exception. When utilizing high-performance alloys such as UNS S31803, S32205, S32750, or S32760 without post-weld annealing, supplementary corrosion tests must be executed to ensure the heat-affected zone has not degraded in pitting resistance.
