Two Similar Grades, Two Different Duties
ASTM A106 Grade B and API 5L Grade B are both carbon-manganese steel pipe specifications covering seamless and welded pipe, and both set a minimum tensile strength of 415 MPa (60 ksi). Because the strength figures look almost identical, the two grades are often ordered interchangeably, yet they were written for different service. ASTM A106 is a pressure-piping specification for elevated-temperature service, while API 5L is a line-pipe specification for the transportation of oil and gas. A pipe can be dual-certified to both, but only when the mill has qualified each heat against every clause of both documents.
Chemical Composition Compared
| Element | ASTM A106 Grade B | API 5L Grade B (PSL1) |
|---|---|---|
| Carbon, max % | 0.30 | 0.28 |
| Manganese % | 0.29 - 1.06 | 1.20 max |
| Phosphorus, max % | 0.035 | 0.030 |
| Sulphur, max % | 0.035 | 0.030 |
| Silicon % | 0.10 min | not specified |
The practical difference lies in carbon and manganese. A106 Grade B allows up to 0.30 % carbon, while API 5L Grade B caps carbon at 0.28 % and allows manganese up to 1.20 %. API 5L also tightens phosphorus and sulphur to 0.030 % each, whereas A106 permits 0.035 %. Silicon is a controlled minimum of 0.10 % in A106, reflecting the deoxidation practice needed for high-temperature strength. Choosing the API 5L PSL2 route tightens the limits further and restricts the combined vanadium, niobium and titanium content, which is why PSL2 pipe is the usual choice for low-temperature or sour service.
Mechanical Properties
| Property | ASTM A106 Grade B | API 5L Grade B |
|---|---|---|
| Minimum yield strength | 240 MPa | 241 MPa (PSL1); 245 MPa for L245 (PSL2) |
| Minimum tensile strength | 415 MPa | 415 MPa |
| Minimum elongation | 20 % | Calculated from the formula in API 5L, generally higher for common wall thicknesses |
| Impact testing | Only when specified in the order | Ordered option in PSL1; required by PSL2 for certain design temperatures |
A106 Grade B fixes its values directly in the tables of the standard, which makes acceptance testing simple but leaves less room for higher-strength variants. API 5L Grade B expresses elongation through a formula, so the required figure rises with diameter and wall thickness; this is one reason a mill certificate alone does not prove interchangeability.
Applications and Service Scope
ASTM A106 Grade B: boiler tubes and headers, superheaters, economisers, heat exchangers, refinery and petrochemical process piping, and other pressure parts where ASME B31.1 or ASME B31.3 governs the wall thickness calculation.
API 5L Grade B: onshore and offshore transmission pipelines, gathering lines, water injection and utility piping designed to ASME B31.4 or ASME B31.8, where line pipe dimensions, weld quality and field weldability drive the purchase.
Outside diameter and wall thickness series also differ. A106 pipe is normally ordered to ASME B36.10M schedules, while API 5L line pipe is ordered to the dimensional tables of API 5L, which extend to larger diameters and define wall thicknesses for transmission design.
Selection and Ordering Points
Establish the design code first, because ASME B31.1, B31.3, B31.4 and B31.8 point to different pipe specifications for the same design pressure.
State the product specification level for API 5L. PSL2 brings mandatory non-destructive testing, tighter chemistry, a maximum yield-to-tensile ratio of 0.93 and Charpy impact requirements.
For wet sour service, add the material requirements of NACE MR0175 / ISO 15156 and confirm the hardness limits that apply to the finished pipe.
Require an EN 10204 3.1 mill test certificate for either grade and reconcile the heat number with the pipe marking and the hydrostatic test record.
Do not mix the two specifications on one order without dual certification, since traceability may be questioned during third-party inspection.
FAQ
Q: Can ASTM A106 Grade B pipe be used as API 5L Grade B line pipe?
Only when the pipe is dual-certified. The certificate must show compliance with API 5L dimensional tolerances, weld seam inspection and marking, not merely with the A106 mechanical values.
Q: Which grade is better for a high-temperature steam line?
ASTM A106 Grade B is the conventional choice, because its chemistry and silicon minimum are intended for elevated-temperature pressure service under ASME B31.1.
Q: Are the pressure ratings of the two grades the same?
No. Allowable stress comes from the design code tables, and the wall thickness calculation also depends on the specified outside diameter and tolerance, which differ between pressure piping and line pipe.
Q: Does API 5L Grade B pipe need impact testing?
It is optional in PSL1 and ordered as a supplementary requirement, but PSL2 imposes Charpy testing for grades and wall thicknesses covered by the standard when the design temperature or purchaser specification calls for it.
Q: How can a buyer distinguish the two grades in the yard?
Check the stencil or stamp on the pipe, the heat number and the mill certificate: API 5L pipe is marked with the specification and product specification level, while A106 pipe is marked with the grade and the applicable wall designation.
