ASTM A312/A312M is a pipe specification for seamless, straight-seam welded and heavily cold-worked austenitic stainless steel. It is commonly used for process piping ordered by nominal pipe size (NPS) and schedule. This guide explains when A312 is appropriate, how seamless and welded routes differ, and when a tube specification such as ASTM A213 or A249 is the better choice.
ASTM A312 pipe: scope and manufacturing routes
A312 covers both major manufacturing routes under one specification. A purchase order still needs to identify the route because seamless (SML), welded (WLD) and heavily cold-worked welded (HCW) pipe are not interchangeable labels.
- Seamless A312 pipe is produced without a longitudinal weld. Review TeCarve’s stainless steel seamless pipe and tube range.
- Welded A312 pipe is formed and welded along the longitudinal seam, then heat treated and tested as required by the order and applicable edition. Review TeCarve’s welded stainless steel pipe and tube range.
- HCW pipe begins as welded pipe and receives heavy cold work to both wall and weld before final annealing. It should be specified explicitly rather than assumed from the word “welded.”
Pipe versus tube: NPS and schedule are not actual dimensions
An A312 pipe order is normally built around NPS plus schedule, for example NPS 2 Schedule 40S. NPS is a nominal designator: it is not a direct measurement of the outside diameter. The schedule identifies a standardized nominal wall thickness for that NPS. A312 also permits other specified outside-diameter and wall combinations, but those dimensions must be stated clearly in the order.
A213 and A249 are tube specifications. Their orders identify the tube by actual outside diameter and wall thickness rather than an NPS/schedule pair. That distinction matters in heat-transfer equipment, where tube-to-tubesheet fit, wall thickness, surface area and thermal resistance are part of the equipment design.
| Specification | Product and process | Usual dimensional language | Primary service context |
|---|---|---|---|
| ASTM A312/A312M | Seamless, welded and HCW austenitic stainless steel pipe | NPS + schedule; specified OD/wall combinations may also be ordered | General corrosive and process-piping service, including low- and high-temperature duties within the governing design code |
| ASTM A213/A213M | Seamless ferritic and austenitic alloy-steel tube | Actual OD + minimum wall, or average wall when specified | Boilers, superheaters and heat exchangers |
| ASTM A249/A249M | Welded and HCW welded austenitic steel tube | Actual OD + nominal wall thickness | Boilers, superheaters, heat exchangers and condensers |
Minimum mechanical properties for common A312 grades
The following values are room-temperature minimum tensile requirements for ASTM A312 pipe. Strength is shown in N/mm², numerically equal to MPa. Elongation is the minimum for a longitudinal specimen measured over 2 in. or 50 mm (or 4D). The corresponding general transverse minimum for these grades is 25%.
| Grade | UNS | Manufacturing / thickness condition | Tensile strength min. | Yield strength min. | Longitudinal elongation min. |
|---|---|---|---|---|---|
| TP304 | S30400 | All covered routes | 515 N/mm² | 205 N/mm² | 35% |
| TP304L | S30403 | All covered routes | 485 N/mm² | 170 N/mm² | 35% |
| TP316 | S31600 | All covered routes | 515 N/mm² | 205 N/mm² | 35% |
| TP316L | S31603 | All covered routes | 485 N/mm² | 170 N/mm² | 35% |
| TP321 | S32100 | Welded; or seamless with nominal wall ≤3/8 in. (9.5 mm) | 515 N/mm² | 205 N/mm² | 35% |
| TP321 | S32100 | Seamless with nominal wall >3/8 in. (9.5 mm) | 485 N/mm² | 170 N/mm² | 35% |
These are specification minima, not typical mill-test results and not design allowable stresses. Confirm the edition named on the purchase order, the specimen orientation, dimensions, heat treatment, supplementary requirements and governing piping code before acceptance.
A312 versus A213 and A249
One pipe standard versus two tube standards
A312 includes seamless and welded pipe in one standard. Heat-transfer tubing separates the routes: A213 is seamless, while A249 is welded or heavily cold-worked welded. Calling out “A213 welded tube” or “A249 seamless tube” creates a specification contradiction.
Process piping versus heat-transfer equipment
A312 is primarily selected for piping systems that carry process fluids between equipment. A213 and A249 are written for tubes installed inside boilers, superheaters, heat exchangers and condensers. Tube orders often impose tighter control of actual OD and wall because tubesheet fit and wall thickness directly affect fabrication and heat-transfer performance; the exact tolerances must still be stated or verified against the applicable standard and drawing.
Do not translate NPS/schedule directly into a tube drawing
A schedule designation selects a nominal pipe wall for a particular NPS. A heat-exchanger drawing normally controls actual OD and wall thickness. Even when two items appear close in size, their permitted variation, testing, finish and documentation can differ. The correct standard follows the equipment function and drawing—not simply the nearest available diameter.
Where A269 and A270 fit
ASTM A269/A269M covers seamless and welded austenitic stainless tubing for general corrosion-resisting and low- or high-temperature service; it is a general-service tube specification, not a dedicated boiler or heat-transfer specification. ASTM A270/A270M is for sanitary tubing used in dairy, food and related hygienic service, with surface-finish requirements central to the order.
A practical A312 purchase-order checklist
- ASTM A312/A312M designation and required edition
- Grade and UNS designation, such as TP304L / S30403 or TP316L / S31603
- Manufacturing route: seamless, welded or HCW
- NPS and schedule, or explicitly specified OD and wall thickness
- Quantity, cut length and end preparation
- Required hydrostatic or nondestructive electric test and any supplementary tests
- Heat treatment, surface condition, marking and certification requirements
- Applicable design code and service conditions, which are separate from the material specification
Related TeCarve technical references
If the open question is material grade rather than product standard, use the 304, 316 and 316L stainless steel grade and product-form hub. After the grade and specification are defined, review either the seamless pipe range or welded pipe range for the relevant manufacturing route.