SA 350 LF2 Material – Class 1 & Class 2 Forged Flanges for Cryogenic & Sub-Zero Service
ASTM A350 LF2 flanges are forged carbon steel flanges manufactured to ASTM A350 / ASME SA-350 specification and impact-tested for low-temperature service down to −50°F (−46°C). These A350 flanges are available in all ASME B16.5 and ASME B16.47 types and pressure classes and are widely specified where SA 350 LF2 material is required for reliable notch toughness in sub-zero environments.
What Is ASTM A350 LF2 Flange Material?
ASTM A350 is the standard covering of forged carbon and low alloy steel flange, fittings, valves and parts that are designed to be used in low temperature service. The most commonly specified grade in this standard is LF2 (Low-Temperature Forging Grade 2).
ASTM A350 LF2 flanges are produced using the SA 350 LF2 material which has to possess notch toughness that is at sub-zero temperatures even up to -50 F ( -46 C). In contrast to regular carbon steel flanges that are made to ASTM A105, which is not routinely impact-tested, and is only allowed to serve in ambient and hopefully higher temperature environments, ASTM A350 LF2 flanges are subject to mandatory Charpy V-Notch (CVN) impact testing at the advertised minimum design temperature.
Because of this requirement, A350 flange material is the correct engineering choice for cryogenic pipelines, LNG facilities, cold storage systems, offshore installations in Arctic environments, and process piping systems operating below −20°F.
ASTM A350 LF2 Class 1 vs Class 2 — What's the Difference?
ASTM A350 LF2 Class 1 is the default designation and requires a minimum yield strength of 36 ksi with Charpy impact testing at −50°F.
ASTM A350 LF2 Class 2 allows a higher minimum yield strength of 60 ksi and is selected where elevated mechanical strength is required in addition to low-temperature toughness. Class 2 SA 350 LF2 material typically undergoes more stringent normalization or quench-and-temper heat treatment to achieve higher strength levels.
Common Applications
SA 350 LF2 flanges are specified in LNG terminals and re-gasification units, ethylene and propylene cracking plants, cold-climate upstream oil & gas facilities, cryogenic storage and transfer systems, and any pipeline operating below −20°F where ASTM A105 is disqualified by process temperature.
Manufacturing Process & Heat Treatment of A350 Flange
A350 LF2 flanges are produced as open-die or closed-die forgings, typically from killed carbon steel. Acceptable heat treatment conditions include normalizing, normalizing and tempering, or quench-and-temper. The forging process refines the grain structure and orients the flow lines to maximize toughness — a property that cast equivalents cannot reliably achieve, making forgings the mandatory form for this specification.
Manufacturing Process Flow
Chemical Composition of SA 350 LF2 Material
The table below shows the permissible chemical composition limits per ASTM A350 for ASTM A350 LF2 Class 1 and Class 2 material. Phosphorus and sulfur limits are tighter than ASTM A105 to improve notch toughness and low-temperature performance of A350 flange products.
| Element | LF2 Class 1 | LF2 Class 2 | A105 (ref.) |
|---|---|---|---|
| Carbon (C) | 0.30% max | 0.30% max | 0.35% max |
| Manganese (Mn) | 0.60 – 1.35% | 0.60 – 1.35% | 0.60 – 1.05% |
| Silicon (Si) | 0.15 – 0.30% | 0.15 – 0.30% | 0.10% min |
| Phosphorus (P) | 0.025% max | 0.025% max | 0.035% max |
| Sulfur (S) | 0.025% max | 0.025% max | 0.040% max |
| Nickel (Ni) | 0.40% max | 0.40% max | 0.40% max |
| Chromium (Cr) | 0.30% max | 0.30% max | 0.30% max |
| Molybdenum (Mo) | 0.12% max | 0.12% max | 0.12% max |
| Copper (Cu) | 0.40% max | 0.40% max | 0.40% max |
※ Values per ASTM A350 / ASME SA-350 latest edition. Product analysis tolerances apply. Always verify the heat certificate (MTC) for actual SA 350 LF2 material values supplied.
Mechanical Properties of ASTM A350 LF2 Flanges
Mechanical requirements for ASTM A350 LF2 flanges are defined according to heat treatment condition and class designation. ASTM A350 LF2 Class 2 achieves higher yield strength through quench-and-temper treatment but requires stricter testing control.
| Property | LF2 Class 1 | LF2 Class 2 |
|---|---|---|
| Tensile Strength | 70 – 95 ksi (485 – 655 MPa) | 75 – 100 ksi (515 – 690 MPa) |
| Yield Strength (min) | 36 ksi (250 MPa) | 60 ksi (415 MPa) |
| Elongation (min) | 22% | 22% |
| Reduction of Area (min) | 30% | 30% |
| CVN Impact Energy (avg) | 15 ft·lbf @ −50°F | 15 ft·lbf @ −50°F |
| Hardness (max) | 197 HBW | 235 HBW |
※ Testing is performed per ASTM A370. CVN specimens are full-size (10 × 10 mm) unless section thickness requires sub-size specimens in accordance with ASTM A350 supplementary requirements.
ASTM A350 LF2 vs. Other Flange Grades — When to Specify Which
The comparison below helps engineers select the correct forging material grade based on service temperature, strength requirements, and applicable design code. A350 LF2 is the low-temperature workhorse; other grades serve distinct roles.
| Feature | A350 LF2 | A105 | A694 F65 | A350 LF3 |
|---|---|---|---|---|
| Min. Design Temp. | −50°F | −20°F | −20°F (std) | −150°F |
| CVN Impact Test | ✓ Required | ✗ Not req. | ✗ Suppl. only | ✓ Required |
| Min. Yield (ksi) | 36 / 60 | 36 | 65 | 37.5 |
| Alloy Content | C-Steel | C-Steel | C-Steel | 3.5% Ni |
| Primary Use Case | LNG, cryo, cold climate | General service | Hi-press pipelines | Deep cryo / ethylene |
| Typical Cost Index | Medium | Low | Medium-High | High |
※ Always consult your applicable design code (ASME B31.3, API 6A, etc.) for final material selection. This comparison is for reference only.
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Contact Us TodayApplicable Codes & Standards
ASTM A350 LF2 flanges are governed by a range of interconnected standards covering material, dimensional, and testing requirements:
- ASTM A350 / ASME SA-350 — Material Specification
- ASME B16.5 — Pipe Flanges and Flanged Fittings ½″–24″
- ASME B16.47 — Large Diameter Steel Flanges 26″–60″
- ASTM A370 — Mechanical Testing of Steel Products
- ASTM E23 — Charpy Impact Testing
- ASME B31.3 — Process Piping Code
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