What is the ASTM A268 Standard Specification?
ASTM A268 is the standard specification for ferritic and martensitic stainless steel tubing (including both seamless and welded types) intended for general corrosion-resistant and high-temperature service. This specification covers steel tubing with nominal wall thicknesses used in applications such as heat exchangers, condensers, boiler feedwater heaters, and automotive exhaust systems. It encompasses a wide range of steel grades, primarily belonging to the 400 series of stainless steels.
Referenced Heat Treatment
| Grade Category | Heat Treatment Requirement |
| Ferritic Grades | Anneal at 1200°F [650°C] or higher, then cool as appropriate. |
| Martensitic Grades | Anneal at 1200°F [650°C] or higher, or harden and temper. |
chemical composition
|
Grade |
UNS Designation | C (max) | Mn (max) | P (max) | S (max) | Si (max) | Cr | Ni (max) | Others |
| TP405 | S40500 | 0.08 | 1.00 | 0.040 | 0.030 | 1.00 | 11.5–14.5 | 0.50 | Al 0.10–0.30 |
| TP410 | S41000 | 0.15 | 1.00 | 0.040 | 0.030 | 1.00 | 11.5–13.5 | 0.75 | ... |
| TP430 | S43000 | 0.12 | 1.00 | 0.040 | 0.030 | 1.00 | 16.0–18.0 | 0.75 | ... |
| TP443 | S44300 | 0.20 | 1.00 | 0.040 | 0.030 | 1.00 | 18.0–23.0 | 0.75 | Cu 0.90–1.25 |
| TP446-1 | S44600 | 0.20 | 1.50 | 0.040 | 0.030 | 1.00 | 23.0–27.0 | 0.75 | N 0.25 max |
| 26-1 | S44627 | 0.01 | 0.40 | 0.020 | 0.020 | 0.40 | 25.0–27.0 | 0.50 | Mo 0.75–1.50 |
Mechanical Property Requirements
This table specifies the minimum strength and maximum hardness of steel pipes in the annealed condition.
| Grade | Tensile Strength, min | Yield Strength, min | Elongation in 2 in., min | Hardness, max |
| - | ksi [MPa] | ksi [MPa] | % | Brinell (HBW) / Rockwell (HRB/C) |
| TP405 | 60 [415] | 30 [205] | 20 | 207 HBW / 95 HRB |
| TP410 | 60 [415] | 30 [205] | 20 | 207 HBW / 95 HRB |
| TP430 | 60 [415] | 35 [240] | 20 | 190 HBW / 90 HRB |
| TP443 | 70 [485] | 40 [275] | 20 | 207 HBW / 95 HRB |
| TP446-1 | 70 [485] | 40 [275] | 18 | 207 HBW / 95 HRB |
| 26-1 | 65 [450] | 40 [275] | 20 | 190 HBW / 90 HRB |
For tubing with an outside diameter less than 1/2 inch [12.7 mm], the elongation values specified for strip specimens shall apply. Mechanical property requirements do not apply to tubing with an outside diameter less than 1/8 inch [3.2 mm] or a wall thickness less than 0.015 inch [0.4 mm].
ASTM A268 Stainless Steel Pipe Dimensional Tolerances
| Group | Size, Outside Diameter, in. [mm] | Permissible Variations in Outside Diameter, in. [mm] | Permissible Variations in Wall Thickness,ᴬ % | Permissible Variations in Cut Length, in.ᴮ [mm] | Thin-Walled Tubesᶜ | |
|---|---|---|---|---|---|---|
| Over | Under | |||||
| 1 | Up to ½ [12.7], excl | ±0.005 [0.13] | ±15 | ⅛ [3] | 0 | ... |
| 2 | ½ to 1½ [12.7 to 38.1], exd | ±0.005 [0.13] | ±10 | ⅛ [3] | 0 | less than 0.065 in. [1.6 mm] nominal |
| 3 | 1½ to 3½ [38.1 to 88.9], exd | ±0.010 [0.25] | ±10 | ³⁄₁₆ [5] | 0 | less than 0.095 in. [2.4 mm] nominal |
| 4 | 3½ to 5½ [88.9 to 139.7], exd | ±0.015 [0.38] | ±10 | ³⁄₁₆ [5] | 0 | less than 0.150 in. [3.8 mm] nominal |
| 5 | 5½ to 8 [139.7 to 203.2], incl | ±0.030 [0.76] | ±10 | ³⁄₁₆ [5] | 0 | less than 0.150 in. [3.8 mm] nominal |
Notes (implied by superscripts):
ᴬ: Wall thickness tolerance applies to nominal wall thickness, measured at any point on the tube circumference.
ᴮ: Cut length tolerance applies to specified random / fixed length; negative tolerance is 0 (no underlength allowed).
ᶜ: Thin-walled tube definition is tied to nominal wall thickness for each OD size range, with additional dimensional rules applicable to this category.
excl / exd = excluding (the upper limit of the size range); incl = including (the upper limit of the size range).
Manufacturing and Testing Requirements
To ensure integrity, ASTM A268 mandates rigorous testing protocols distinct from general pipe standards:
Hydrostatic/Nondestructive Testing: Every tube must undergo a hydrostatic test or an approved nondestructive electric test (such as eddy current or ultrasonic) to verify pressure integrity.
Mechanical Tests: Requirements include tension tests, hardness tests, and specific formability tests:
Flaring Test: Required for seamless tubes to ensure ductility during expansion.
Flange Test: Required for welded tubes to verify weld quality.
Reverse Flattening Test: Specifically for welded tubes to inspect the weld seam for defects.
Heat Treatment: Tubes are typically supplied in the annealed condition to relieve stresses and ensure proper microstructure.
Are A268 steel pipes magnetic?
All grades produced under ASTM A268 (such as TP410, TP430, etc.) are strongly magnetic in the annealed condition. This differs fundamentally from common 304/316 austenitic stainless steels, which are non-magnetic or weakly magnetic.
What is the difference between ASTM A268 and ASTM A213?
ASTM A268 focuses on straight-chromium ferritic and martensitic stainless steels (such as TP410) intended for general corrosion resistance and high-temperature service.
ASTM A213 focuses on high-alloy and stainless steels-specifically austenitic grades-designed for demanding thermal conditions and harsh environments requiring the maintenance of structural integrity under high pressure and high temperature, such as in boilers, superheaters, and heat exchangers.
Why are ASTM A268 TP430 or TP446 often selected for environments prone to stress corrosion cracking (SCC)?
Austenitic stainless steels (such as 304L/316L) are highly susceptible to stress corrosion cracking in hot, chloride-containing environments. In contrast, the high-chromium ferritic stainless steels specified in ASTM A268 offer exceptional resistance to this type of failure, making them common choices for heat exchanger and condenser tubing.
What precautions should be taken when welding ASTM A268 steel pipes?
Grain coarsening: Ferritic stainless steels (such as TP430) are prone to grain coarsening at high welding temperatures, which can embrittle the weld zone.
Preheating and post-heating: Martensitic stainless steels (such as TP410) are susceptible to cold cracking during welding; preheating and post-weld heat treatment (PWHT) are typically required.
Filler materials: It is generally recommended to use filler materials that match the base metal, though austenitic filler materials (such as 308L/309L) may be used in certain cases to enhance joint toughness.
Referenced Documents
ASTM Standards:
A 480/A 480M Specification for General Requirements for Flat-Rolled Stainless and Heat-Resisting Steel Plate, Sheet, and Strip
A 763 Practices for Detecting Susceptibility to Intergranular Attack in Ferritic Stainless Steels
A 789/A 789M Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Tubing for General Service
A 790/A 790M Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Pipe
A 941 Terminology Relating to Steel, Stainless Steel, Related Alloys, and Ferroalloys
A 1016/A 1016M Specification for General Requirements for Ferritic Alloy Steel, Austenitic Alloy Steel, and Stainless Steel Tubes