When selecting seamless steel pipes for industrial pipelines, especially for extreme temperature applications, ASTM A335 Grade P11 and ASTM A333 Grade 6 are among the most commonly used materials. Though both standards specify seamless pipes, they are designed for opposite temperature ranges and have distinct mechanical, chemical, and dimensional properties.
This article provides a comprehensive technical comparison to help engineers and procurement professionals make informed decisions when choosing between these two grades.
ASTM A335 Grade P11 Steel Pipes
General Overview
| Feature | ASTM A335 Gr. P11 | ASTM A333 Gr. 6 |
| Material Type | Ferritic alloy steel | Fine-grain carbon steel |
| Application | High-temperature service | Low-temperature service |
| Typical Use | Boilers, heaters, power plants | Cryogenic piping, LNG, gas distribution |
| Heat Treatment | Normalized + tempered or full annealing | Normalized, quenched & tempered, or hot-formed + furnace cooling |
| Pipe Type | Seamless only | Seamless and welded |
| Standard Spec | ASTM A335 / ASME SA335 | ASTM A333 / ASME SA333 |
ASTM A333 Grade-6 Steel Pipes
Chemical Composition Comparison (% by weight)
| Element | A335 Gr. P11 | A333 Gr. 6 |
| Carbon (C) | 0.05–0.15 | ≤ 0.30 |
| Manganese (Mn) | 0.30–0.60 | 0.29–1.06 (can increase with reduced C) |
| Phosphorus (P) | ≤ 0.025 | ≤ 0.025 |
| Sulfur (S) | ≤ 0.025 | ≤ 0.025 |
| Silicon (Si) | 0.50–1.00 | ≥ 0.10 |
| Chromium (Cr) | 1.00–1.50 | ≤ 0.30 |
| Molybdenum (Mo) | 0.44–0.65 | ≤ 0.12 |
| Nickel (Ni) | — | ≤ 0.40 |
| Copper (Cu) | — | ≤ 0.40 |
| Vanadium (V) | — | ≤ 0.08 |
| Niobium (Nb) | — | ≤ 0.02 (up to 0.06 by agreement) |
Note:A335 P11 is a chromium-molybdenum (Cr-Mo) alloy, providing high temperature strength and oxidation resistance. A333 Gr.6 is low-alloy carbon steel tailored for low-temperature ductility.
Mechanical Properties Comparison
| Property | A335 Gr. P11 | A333 Gr. 6 |
| Tensile Strength (MPa) | ≥ 415 | ≥ 415 |
| Yield Strength (MPa) | ≥ 205 | ≥ 240 |
| Elongation (%) | 30% (longitudinal standard) | 30% (longitudinal standard) |
| Impact Test Requirement | Not mandatory | Mandatory at –45°C |
| Test Specimens | Round or rectangular samples per ASTM requirements, adjusted for wall thickness |
Elongation Adjustment Rule:
For both grades, if wall thickness is below 7.94 mm (5/16″), elongation requirements are reduced proportionally:
A335 P11: for every 0.79 mm (1/32″) decrease, reduce longitudinal elongation by 1.5%, transverse by 1.0%.
A333 Gr.6: same adjustment applies.
Charpy Impact Requirements for A333 Gr. 6
Standard Test Temperature: –45 °C
Specimen Size: Usually 10 × 10 mm
Minimum Average Energy: 18 J (for 10×10 mm)
Single Value Minimum: 14 J
If specimens are undersized, the required test temperature must be reduced according to ASTM A333 Table 6.
Dimensional Tolerances
Outside Diameter Tolerances (mm)
| OD Range (mm) | A335 Gr. P11 | A333 Gr.6 |
| 10.3–48.3 | +0.4 / –0.4 | +0.4 / –0.4 |
| >48.3–114.3 | +0.79 / –0.79 | +0.8 / –0.8 |
| >114.3–219.1 | +1.59 / –0.79 | +1.6 / –0.8 |
| >219.1–323.9 | +2.38 / –0.79 | +2.4 / –0.8 |
| >323.9 | ±1% D | +4.8 / –0.8 (varies up to 1219mm OD) |
Wall Thickness Tolerances
A335 P11:
For OD ≤73 mm: +20% / –12.5%
For OD >73 mm:
If t/D ≤ 5% → +22.5% / –12.5%
If t/D > 5% → +15% / –12.5%
A333 Gr.6:
No point shall be less than 12.5% below the nominal wall thickness.
If pipe is ordered to minimum wall, no under-thickness allowed.
Application Considerations
| Criteria | Recommended Grade |
| High-temperature strength | ASTM A335 Gr. P11 |
| Oxidation/corrosion resistance | ASTM A335 Gr. P11 |
| Low-temperature toughness | ASTM A333 Gr. 6 |
| LNG / cryogenic pipelines | ASTM A333 Gr. 6 |
| Power plant, boiler piping | ASTM A335 Gr. P11 |
| Projects with impact test need | ASTM A333 Gr. 6 |
Conclusion
ASTM A335 Gr. P11 and ASTM A333 Gr. 6 serve two distinct ends of the temperature spectrum:
A335 P11 is optimized for high-temperature environments, offering strength, stability, and oxidation resistance thanks to its Cr-Mo alloy composition.
A333 Gr.6 is the go-to solution for low-temperature applications, where ductility and impact resistance are crucial, especially below freezing.
When selecting pipe materials, it’s critical to match not only the temperature service range, but also verify heat treatment, tolerance, and impact test requirements. At Haihao Group, we offer both grades in full compliance with ASTM and ASME standards, complete with test certifications, third-party inspection, and custom dimensions.
Contact Haihao Group for expert assistance in selecting the right steel pipe materials for your next project. Email:sales@haihaogroup.com
