Low-temperature carbon steel pipe is a specialised product class where the selection criterion is not just tensile strength but Charpy V-notch toughness at sub-zero service temperatures. Standard carbon steel grades such as ASTM A106 Grade B or API 5L Grade B lose ductility rapidly below about -20°C and can fracture in a brittle manner under impact loading — a failure mode that is catastrophic in pressurised gas systems. ASTM A333 Grade 6 is the standard solution for this problem: a carbon-manganese steel pipe manufactured and impact-tested for service down to -45°C (-50°F), widely specified for natural gas processing, LNG, offshore cold-climate piping, and refrigeration systems.
ZC Steel Pipe supplies seamless ASTM A333 Grade 6 pipe in sizes from 2 inches to 24 inches NPS, to EN 10204 3.1/3.2 mill test certificates with full Charpy V-notch impact records, for EPC projects in the Middle East, West Africa, Southeast Asia, and South America.
What Is ASTM A333?
ASTM A333, titled Standard Specification for Seamless and Welded Steel Pipe for Low-Temperature Service and Other Applications with Required Notch Toughness, covers steel pipe that must retain adequate Charpy V-notch toughness at specified test temperatures. The specification defines multiple grades differentiated by alloy content and minimum service temperature:
| Grade | Key alloy | Min. test temp | Typical application |
|---|---|---|---|
| 1 | Carbon-Mn | -45°C (-50°F) | General LT service (lower strength) |
| 6 | Carbon-Mn | -45°C (-50°F) | Most common grade; gas processing |
| 3 | Ni 3.5% | -100°C (-150°F) | Ethylene, LPG at very low temps |
| 9 | Ni 2.0–2.5% | -73°C (-100°F) | NGL, propane, LPG plants |
| 8 | Ni 9% | -196°C (-320°F) | LNG, liquid nitrogen, LOX |
Grade 6 is the most widely produced grade. It covers both seamless and welded pipe (ERW, DSAW), whereas Grade 8 is seamless only. Grade 6 is interchangeable with ASME SA-333 Grade 6 — SA-333 is the ASME Boiler and Pressure Vessel Code adoption of the ASTM standard, used wherever ASME stamp is required.
Mechanical Properties
ASTM A333 Grade 6 mechanical requirements are modest compared to high-strength grades, reflecting its role as a low-temperature ductile material rather than a high-strength one:
| Property | SI (MPa) | US customary (ksi) |
|---|---|---|
| Tensile strength, min | 415 MPa | 60 ksi |
| Yield strength, min (0.2% offset) | 240 MPa | 35 ksi |
| Elongation in 50 mm (2 in), min | 35% | 35% |
These are minimum values; actual mill test results typically exceed minimums by 10–20% for commercial quality pipe. The elongation of 35% is substantially higher than high-strength grades (which run 15–22%), reflecting the importance of ductile behaviour at low temperatures.
For the complete pipe schedule dimensions and weight tables, see the ASME B36.10M pipe specification tables →
Charpy V-Notch Impact Requirements
The defining requirement for A333 Grade 6 is the Charpy V-notch (CVN) impact test performed at -45°C (-50°F). Tests are conducted on specimens machined from the pipe body — transverse specimens are the controlling requirement for pipe:
| Specimen orientation | Average of 3 specimens (min) | Lowest individual (min) |
|---|---|---|
| Transverse | 20 J (15 ft·lbf) | 13 J (10 ft·lbf) |
| Longitudinal | 27 J (20 ft·lbf) | 20 J (15 ft·lbf) |
Each heat of pipe must be impact-tested. The MTC must report the actual absorbed energy for each specimen in the set. Verify both the average and individual values — a three-specimen set can show a passing average with one borderline low individual. For critical service (e.g., offshore risers, pressure vessels), procurement specifications often add a supplementary requirement for 100% of specimens to exceed the minimum individual value.
Chemical Composition
ASTM A333 Grade 6 is a carbon-manganese steel. The specification intentionally keeps the chemistry simple — no alloy additions — and relies on controlled rolling and fine-grain practice to achieve toughness:
| Element | Requirement |
|---|---|
| Carbon (C) | 0.30% max |
| Manganese (Mn) | 0.29–1.06% |
| Phosphorus (P) | 0.025% max |
| Sulfur (S) | 0.025% max |
| Silicon (Si) | 0.10% min (when required for fine-grain practice) |
The key to low-temperature toughness in Grade 6 is the combination of controlled carbon (lower C improves toughness), adequate Mn (raises the ductile-to-brittle transition temperature DBTT), and fine austenite grain size. Supplementary fine-grain practice using Al or Nb additions is not explicitly mandated by the base specification but is widely used by mills to consistently pass the -45°C Charpy test with margin.
The carbon equivalent (CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15) for Grade 6 typically runs 0.38–0.43, making the material readily weldable with standard procedures.
Standard Sizes
ASTM A333 Grade 6 is available as both seamless and welded pipe. Size availability and typical wall schedules:
| NPS | OD (mm) | SCH 40 WT (mm) | SCH 80 WT (mm) |
|---|---|---|---|
| 2 | 60.3 | 3.91 | 5.54 |
| 4 | 114.3 | 6.02 | 8.56 |
| 6 | 168.3 | 7.11 | 10.97 |
| 8 | 219.1 | 8.18 | 12.70 |
| 12 | 323.9 | 9.53 | 12.70 |
| 16 | 406.4 | 9.53 | 12.70 |
| 20 | 508.0 | 9.53 | 12.70 |
| 24 | 609.6 | 9.53 | 12.70 |
Wall thicknesses follow ASME B36.10M schedules. For LT service piping systems, SCH 40 and SCH 80 cover the majority of process plant requirements. Heavier schedules (SCH 120, 160, XXS) are available in smaller NPS ranges (½" to 10") for high-pressure low-temperature applications.
For a unit conversion tool for OD, wall thickness, and weight: Unit Converter →
Low-Temperature Service — Design Considerations
Ductile-to-Brittle Transition
Carbon steel undergoes a transition from ductile to brittle fracture mode as temperature decreases. The ductile-to-brittle transition temperature (DBTT) varies with chemical composition, grain size, and heat treatment. For A333 Grade 6, the manufacturing controls are designed to ensure the DBTT lies below -45°C with margin. Field failures occur when substitutions are made — e.g., using A106 Grade B (no impact test requirement) in a system rated for LT service. An A106 pipe may show adequate tensile properties at room temperature but shatter at the design low temperature.
Thermal Cycling and Fatigue
LNG and gas processing facilities experience repeated thermal cycles as equipment is commissioned, shut down, and brought back into service. Each cycle imposes thermal stresses in flanges, elbows, and branch connections. For systems that cycle frequently, specify minimum bend radius per ASME B31.3, use forged fittings (ASTM A420 WPL6 — the low-temperature fittings companion to A333 Gr. 6), and verify that all flanges and valves in the system are rated for the minimum design metal temperature (MDMT).
Companion Materials
For complete low-temperature piping systems, all materials must be impact-tested to match the pipe:
- Fittings: ASTM A420 WPL6 (seamless and welded buttweld fittings, LT service)
- Flanges: ASTM A182 Grade F316L, F304L, or ASTM A350 LF2 (carbon steel flanges for LT)
- Valves: API 6D or ASME B16.34 with Charpy-certified body and bonnet
Mixing un-tested carbon steel fittings or flanges into an A333 Grade 6 piping system is a code violation and a safety risk.
ASTM A333 Grade 6 vs ASTM A106 Grade B
Both are seamless carbon steel pipe, but their service temperature ratings are fundamentally different:
| Property | A333 Grade 6 | A106 Grade B |
|---|---|---|
| Tensile strength min | 415 MPa (60 ksi) | 415 MPa (60 ksi) |
| Yield strength min | 240 MPa (35 ksi) | 240 MPa (35 ksi) |
| Elongation min | 35% | 30% |
| Impact tested? | Yes — at -45°C | No |
| Min design temp (ASME B31.3) | -45°C | -29°C |
| Below -29°C without impact testing? | Permitted | Not permitted |
The tensile and yield numbers are identical — substituting A106 for A333 is easy to miss on a dimensional review but creates a latent failure risk at operating temperature. This substitution is one of the most common procurement errors in LT piping systems, particularly when a project uses the same NPS and schedule for both ambient and cold sections.
Purchase Order Guidance
Required PO Line Items
When specifying ASTM A333 Grade 6 pipe, a complete purchase order should include:
- Specification: ASTM A333 / ASME SA-333, Grade 6
- Product form: seamless or welded (state preferred)
- NPS and schedule (or OD × wall thickness)
- Heat treatment: normalised (standard for Grade 6); specify if quench-and-tempered is required
- Test temperature for CVN: -45°C (standard); state if a lower test temperature is required
- NDE: hydrostatic test per the standard; add UT or EMI if required by project specification
- Mill documentation: EN 10204 Type 3.1 MTC
- Third-party inspection: state inspection level (witness, review, or release note)
The Low-Temperature Substitution Trap
The most common procurement error is substituting ASTM A106 Grade B for A333 Grade 6 when a supplier is short of stock. The two grades have the same tensile and yield minimums and the same common schedules. The critical difference — the Charpy impact test at -45°C — does not appear on a cursory dimensional check. Always verify that the MTC lists the Charpy test results and that the test temperature is -45°C or lower. If the MTC shows only tensile and bend test results (the A106 format), the pipe is A106, not A333, regardless of what is stamped on the pipe.
MTR Verification Checklist
Before accepting a consignment of A333 Grade 6 pipe:
- Confirm the heat number on the pipe marking matches the MTC heat number
- Verify the Charpy V-notch test temperature is -45°C (not -29°C or -18°C)
- Confirm all three transverse specimen values: average ≥20 J, minimum individual ≥13 J
- Confirm yield and tensile meet minimums; check elongation ≥35%
- Confirm the manufacturing process (seamless or welded) matches the purchase order
- Verify P and S chemistry limits — A333 limits (P ≤0.025%, S ≤0.025%) are tighter than A106
For complete ASME pipe dimension tables and schedule lookup, see the ASME pipe specification tables →
Use the Unit Converter → to convert between imperial and metric pipe dimensions and wall thicknesses.