Understanding ASTM A778: Welded, Unannealed Austenitic Stainless Steel Tubular Products
stainless steel welded pipe
The ASTM A778 standard serves as a crucial specification for welded, unannealed austenitic stainless steel tubular products. This standard is particularly relevant for applications involving low to moderate temperatures and general corrosive service, where a post-weld heat treatment for corrosion resistance is not deemed necessary.
Unlike some other prominent stainless steel pipe specifications (e.g., ASTM A312), ASTM A778 specifically focuses on welded and unannealed products. This distinction is key to its application and cost-effectiveness in certain scenarios. The “unannealed” aspect means that the material does not undergo a final heat treatment process after welding, which can simplify manufacturing and reduce costs.
Scope and Manufacturing Process
ASTM A778 covers both straight seam and spiral butt seam welded tubular products. These products are typically manufactured from flat-rolled steel sheet, coil, or plate using a shielded arc-welding process, which can be manual or automatic electric-welding. The standard emphasizes the importance of sound welds, requiring that any injurious weld defects be repaired by removal to sound metal and rewelding. Furthermore, transverse tension and transverse-guided bend tests are mandated for the welded joints to ensure their integrity.
The standard defines a range of outside diameters from 3 inches (75 mm) to 48 inches (1200 mm) and nominal wall thicknesses from 0.062 inches (1.5 mm) to 0.500 inches (12.5 mm). While these are the typical ranges, tubular products with other dimensions can be supplied if they meet all other requirements of the specification.
Key Grades and Their Chemical Composition
ASTM A778 encompasses several grades of austenitic stainless steel, each with a specific chemical composition to achieve desired properties. The table below summarizes the key chemical requirements for common grades:
| Grade | UNS Designation | Carbon, max (%) | Manganese, max (%) | Phosphorus, max (%) | Sulfur, max (%) | Silicon, max (%) | Chromium (%) | Nickel (%) | Titanium | Columbium Plus Tantalum | Nitrogen, max |
| TP 304L | S30403 | 0.030 | 2.00 | 0.045 | 0.030 | 1.00 | 18.0–20.0 | 8.0–13.0 | … | … | 0.10 |
| TP 316L | S31603 | 0.030 | 2.00 | 0.045 | 0.030 | 1.00 | 16.0–18.0 | 10.0–14.0 | … | … | 0.10 |
| TP 317L | S31703 | 0.030 | 2.00 | 0.045 | 0.030 | 1.00 | 18.0–20.0 | 11.0–15.0 | … | … | 0.10 |
| TP 321 | S32100 | 0.08 | 2.00 | 0.045 | 0.030 | 1.00 | 17.0–19.0 | 9.0–12.0 | C | … | … |
| TP 347 | S34700 | 0.08 | 2.00 | 0.045 | 0.030 | 1.00 | 17.0–19.0 | 9.0–12.0 | … | D | … |
Note: ‘C’ for TP 321 indicates titanium content not less than five times the carbon content and not more than 0.70%. ‘D’ for TP 347 indicates columbium plus tantalum content not less than ten times the carbon content and not more than 1.10%.
The “L” grades (TP 304L, TP 316L, TP 317L) denote low carbon content, which improves weldability and resistance to intergranular corrosion. Grades like TP 321 and TP 347 incorporate stabilizing elements (Titanium for TP 321, Columbium plus Tantalum for TP 347) to prevent carbide precipitation during welding, enhancing their corrosion resistance in certain high-temperature applications even without post-weld annealing.
Mechanical Properties: Tensile Requirements
The mechanical properties are crucial for ensuring the structural integrity and performance of the tubular products. For ASTM A778, the primary mechanical requirement is tensile strength. The table below outlines the minimum tensile strength for the specified grades:
| Grade | UNS Designation | Tensile Strength, min, ksi [MPa] |
| TP 304L | S30403 | 70 [485] |
| TP 316L | S31603 | 70 [485] |
| TP 317L | S31703 | 75 [515] |
| TP 321 | S32100 | 75 [515] |
| TP 347 | S34700 | 75 [515] |
These minimum tensile strength values ensure that the welded, unannealed tubes can withstand the anticipated stresses in their intended service environments.
Applications and Advantages
ASTM A778 stainless steel tubular products are commonly employed in a variety of industries where corrosion resistance and cost-effectiveness are key considerations. Typical applications include:
Chemical processing plants: For conveying various corrosive fluids at low to moderate temperatures.
Water treatment facilities: Used in piping systems for water and wastewater treatment.
Food processing plants: Where hygiene and corrosion resistance are paramount.
Architectural and structural applications: For certain building components where the aesthetic and corrosion-resistant properties of stainless steel are desired.
Low-pressure fluid handling systems: Where the high-pressure capabilities of seamless or annealed pipes are not required.
The primary advantages of using ASTM A778 compliant products lie in their cost-efficiency due to the absence of a required post-weld heat treatment, combined with good corrosion resistance, especially with low-carbon or stabilized grades. While not suitable for all applications, particularly those requiring high pressure or high-temperature service with strict heat treatment requirements (for which standards like ASTM A312 might be more appropriate), ASTM A778 provides a reliable and economical solution for a broad range of general corrosive and moderate temperature applications.