ASTM A350 LF2 Flanges | Low-Temperature Carbon Steel & NACE MR0175
ASTM A350 LF2 is a low-temperature carbon steel forging grade widely used for pipe flanges, forged fittings, valves and pressure-containing components requiring reliable notch toughness at reduced temperatures.
LTCS ASTM A350 LF2 .combines the strength and weldability of carbon steel with controlled chemistry, fine-grain practice, heat treatment and mandatory impact testing for low-temperature service.
ASTM A350 LF2 flanges are commonly used in oil and gas, petrochemical, natural gas processing, pressure piping and low-temperature process systems.
For H₂S-containing oil and gas environments, LF2 flanges may also be supplied to project requirements based on NACE MR0175 / ISO 15156, subject to the required material condition, hardness, fabrication route and actual sour-service environment.
ASTM A350 LF2 Material Overview
| Item | Specification |
|---|---|
| Material Standard | ASTM A350 / A350M |
| Grade | LF2 |
| Material Type | Low-Temperature Carbon Steel Forging |
| Available Classes | Class 1 / Class 2 |
| Product Forms | Flanges, Forged Fittings, Valves and Forged Components |
| Typical Heat Treatment | Normalized, Normalized & Tempered, or Quenched & Tempered as permitted |
| Maximum Hardness | 197 HBW |
| Main Application | Low-Temperature Pressure Piping |
| Sour Service | Available to specified NACE MR0175 / ISO 15156 requirements |
What Is ASTM A350 LF2?
ASTM A350/A350M covers carbon and low-alloy steel forgings requiring notch-toughness testing for piping components intended primarily for low-temperature service.
LF2 is one of the most widely specified carbon steel grades within this standard.
Typical LF2 products include:
- Weld Neck Flanges
- Blind Flanges
- Slip-On Flanges
- Socket Weld Flanges
- Threaded Flanges
- Long Weld Neck Flanges
- Forged Fittings
- Valve Components
- Custom Forged Components
Unlike ASTM A105, which is commonly used for ambient and higher-temperature carbon steel piping components, ASTM A350 LF2 incorporates low-temperature impact-toughness requirements.
ASTM A350 LF2 Flange Supply Range
Typical supply capability includes:
| Item | Supply Range |
|---|---|
| Grade | ASTM A350 LF2 Class 1 / Class 2 |
| Size | 1/2″ to 24″ to ASME B16.5 |
| Large Diameter | Up to 60″ to ASME B16.47 |
| Pressure Classes | Class 150, 300, 600, 900, 1500, 2500 |
| Facing | RF, RTJ, FF where applicable |
| Standards | ASME B16.5 / ASME B16.47 Series A & B |
| Inspection | EN 10204 3.1 / 3.2 where specified |
| Sour Service | NACE MR0175 / ISO 15156 when specifically required |
Actual size and pressure-class availability depends on flange type, dimensional standard and manufacturing capability.
ASTM A350 LF2 Chemical Composition
Typical LF2 chemistry requirements include:
| Element | Composition |
|---|---|
| Carbon, C | 0.30% max |
| Manganese, Mn | 0.60–1.35% |
| Phosphorus, P | 0.035% max |
| Sulfur, S | 0.040% max |
| Silicon, Si | 0.15–0.30% |
| Nickel, Ni | 0.40% max |
| Chromium, Cr | 0.30% max |
| Molybdenum, Mo | 0.12% max |
| Copper, Cu | 0.40% max |
| Niobium / Columbium, Nb | 0.02% max* |
| Vanadium, V | 0.08% max |
Additional chemistry restrictions apply under ASTM A350/A350M.
The combined amount of Cu, Ni, Cr, V and Mo is controlled, and Cr + Mo is also subject to a combined limit.
Where vacuum carbon deoxidation or specific supplementary requirements apply, additional chemistry controls may be required.
Actual heat chemistry is reported on the Material Test Certificate.
Mechanical Properties
Typical ASTM A350 LF2 Class 1 and Class 2 room-temperature tensile requirements are:
| Property | Requirement |
|---|---|
| Tensile Strength | 485–655 MPa / 70–95 ksi |
| Yield Strength, min. | 250 MPa / 36 ksi |
| Elongation, min. | 22% on applicable round specimen |
| Reduction of Area, min. | 30% |
| Hardness, max. | 197 HBW |
Class 1 and Class 2 have essentially the same basic tensile-property requirements.
Their main distinction is in the impact-test requirements.
LF2 Class 1 vs Class 2
ASTM A350 LF2 is available in Class 1 and Class 2.
| Item | LF2 Class 1 | LF2 Class 2 |
|---|---|---|
| Chemistry | Same basic LF2 chemistry | Same basic LF2 chemistry |
| Tensile Strength | 485–655 MPa | 485–655 MPa |
| Yield Strength | ≥250 MPa | ≥250 MPa |
| Hardness | ≤197 HBW | ≤197 HBW |
| Typical Impact Test Temperature | -46°C / -50°F | -18°C / 0°F |
| Main Difference | Lower impact-test temperature | Higher impact-energy requirement at warmer test temperature |
A common misunderstanding is that Class 2 is automatically “better” because of the higher class number.
That is not correct.
LF2 Class 1 is the more common choice where qualification at -46°C is required. Class 2 uses different impact-test criteria and should be selected only when the project specification calls for it.
Class selection should therefore follow the applicable piping code, minimum design metal temperature and project requirements.
Low-Temperature Impact Toughness
ASTM A350 differs from ordinary carbon steel forging specifications because notch-toughness testing is a fundamental part of the material requirements.
Charpy V-notch testing verifies that the forging retains adequate toughness at the specified test temperature.
For LF2 Class 1, the commonly specified impact test temperature is:
-46°C / -50°F
This makes LF2 widely used for low-temperature process piping and pressure-containing components.
However, this should not be interpreted to mean that LF2 is suitable for every cryogenic application.
Extremely low-temperature applications require material selection according to the actual minimum design metal temperature and applicable code.
ASTM A350 LF2 vs ASTM A105
Both A350 LF2 and A105 are carbon steel forging grades commonly used for flanges.
| Item | ASTM A350 LF2 | ASTM A105 |
|---|---|---|
| Primary Service | Low-temperature service | General pressure piping |
| Impact Testing | Required by A350 | Not universally required |
| Low-Temperature Toughness | Specifically controlled | Not primary grade objective |
| Typical Flanges | WN, BL, SO, SW, THD | WN, BL, SO, SW, THD |
| Common Industry | Oil & Gas / Gas Processing / Petrochemical | General Process Piping |
Where a piping system has a low minimum design metal temperature, LF2 is often selected instead of conventional A105.
ASTM A350 LF2 NACE MR0175 Flanges
ASTM A350 LF2 is frequently specified for flanges used in sour oil and gas service.
The relevant material-selection framework is:
ANSI/NACE MR0175 / ISO 15156
Part 2 addresses cracking-resistant carbon and low-alloy steels used in H₂S-containing oil and gas production environments.
For carbon and low-alloy steels, hardness control is an important factor because excessive hardness increases susceptibility to sulfide stress cracking.
ASTM A350 LF2 itself specifies a maximum hardness of 197 HBW, which is below the commonly recognized 22 HRC hardness threshold associated with sour-service carbon steels.
However:
ASTM A350 LF2 compliance does not automatically mean that every LF2 flange is NACE MR0175 compliant.
Sour-service qualification should also consider:
- Heat treatment
- Final hardness
- Welding and weld repair
- Cold work
- Material chemistry
- H₂S partial pressure
- pH
- Chloride content
- Operating temperature
- Applicable ISO 15156 restrictions
- Project-specific requirements
When NACE service is required, it should be explicitly stated on the purchase order.
Recommended NACE Purchase Description
A typical requirement can be written as:
ASTM A350 LF2 Class 1 Weld Neck Flange, ASME B16.5, NACE MR0175 / ISO 15156, Hardness Controlled, EN 10204 3.1
Where required, the purchase specification may additionally state:
- Maximum hardness
- No weld repair
- HIC testing
- SSC testing
- Restricted chemistry
- Carbon equivalent
- Impact energy requirements
- Third-party inspection
These additional requirements are project-specific and are not automatically part of every ASTM A350 LF2 flange.
Heat Treatment
ASTM A350 forgings are supplied in a heat-treated condition in accordance with the applicable grade requirements.
Permitted treatment routes may include:
- Normalizing
- Normalizing and tempering
- Quenching and tempering
- Other treatment specifically permitted by the standard
Heat treatment is important for developing:
- Required mechanical strength
- Fine-grain structure
- Low-temperature toughness
- Controlled hardness
For sour-service orders, the final heat-treated condition and hardness should be controlled according to the applicable project and NACE/ISO requirements.
Steelmaking Requirements
Steel for ASTM A350 forgings is produced using approved steelmaking routes such as:
- Basic oxygen
- Electric furnace
- Vacuum induction melting
- Other processes permitted by the applicable ASTM specification
Secondary refining and degassing may also be used.
LF2 steel is produced using fully killed, fine-grain practice, helping achieve consistent toughness and mechanical properties.
Adequate discard is required during ingot or primary material processing to reduce harmful segregation and internal defects.
Forging Process
Starting material may include:
- Ingots
- Blooms
- Billets
- Slabs
- Bars
The final component must be mechanically worked into a forging rather than simply machined directly from an unworked casting shape.
Typical manufacturing steps include:
- Steelmaking
- Billet or ingot preparation
- Heating
- Forging or ring rolling
- Heat treatment
- Rough machining
- Mechanical testing
- Impact testing
- Final machining
- Dimensional and NDT inspection
- Marking and certification
ASTM A350 LF2 Weld Neck Flanges
Weld Neck Flanges are one of the most common LF2 flange types for critical low-temperature piping.
They provide:
- Smooth transition from pipe to flange
- Good stress distribution
- Full-penetration butt-weld connection
- Suitability for high-pressure service
- Suitability for cyclic piping systems
Available configurations include:
- Raised Face
- Ring Type Joint
- ASME B16.5
- ASME B16.47
- Standard and custom bore
ASTM A350 LF2 Blind Flanges
Blind Flanges are used to terminate piping systems, pressure vessels and valve openings.
ASTM A350 LF2 Blind Flanges are commonly used in low-temperature systems where impact-tested carbon steel is required.
Available configurations include:
- Raised Face
- Ring Type Joint
- ASME B16.5
- ASME B16.47
- Custom dimensions where specified
Other Available LF2 Flanges
Available product types can include:
- Weld Neck Flange
- Blind Flange
- Slip-On Flange
- Socket Weld Flange
- Threaded Flange
- Long Weld Neck Flange
- Orifice Flange
- Spectacle Blind
- Custom Forged Components
Inspection and Testing
ASTM A350 LF2 flanges can be supplied with:
- EN 10204 Type 3.1 MTC
- EN 10204 Type 3.2 when specified
- Chemical Analysis
- Tensile Testing
- Charpy V-Notch Impact Testing
- Hardness Testing
- Heat-Treatment Records
- PMI where specified
- Dimensional Inspection
- Visual Inspection
- UT
- MT
- Third-Party Inspection
- Full Heat-Number Traceability
For sour-service projects, additional testing can be performed when specifically required by the purchase specification.
Typical Applications
ASTM A350 LF2 flanges are widely used in:
- Oil and gas processing
- Natural gas plants
- Gas transmission systems
- Petrochemical plants
- Refineries
- Low-temperature process piping
- Pressure vessels
- Valve systems
- Refrigeration process equipment
- Offshore facilities
- Sour-service piping systems
Frequently Asked Questions
What is ASTM A350 LF2?
ASTM A350 LF2 is an impact-tested carbon steel forging grade used for flanges, forged fittings, valves and pressure components intended primarily for low-temperature service.
What temperature is LF2 Class 1 impact tested at?
LF2 Class 1 is commonly impact tested at -46°C / -50°F according to the applicable ASTM A350 requirements.
Is LF2 Class 2 better than Class 1?
Not necessarily.
Class 1 and Class 2 have similar chemistry and tensile properties but different impact-test requirements. Class 1 is associated with the lower standard impact-test temperature.
The correct class depends on the project specification.
What is the maximum hardness of ASTM A350 LF2?
The ASTM A350 LF2 maximum hardness is 197 HBW.
Is ASTM A350 LF2 automatically NACE compliant?
No.
LF2 has chemistry, heat-treatment and hardness characteristics that can make it suitable for many sour-service applications, but NACE MR0175 / ISO 15156 compliance must be evaluated against the final product condition and actual H₂S service environment.
What is the difference between A350 LF2 and A105?
A350 LF2 is specifically intended for low-temperature service and includes mandatory notch-toughness requirements.
A105 is a general-purpose carbon steel forging specification for pressure piping components and does not provide the same standard low-temperature qualification.
ASTM A350 LF2 NACE Flange Supplier
We supply ASTM A350 LF2 Class 1 and Class 2 forged flanges for low-temperature oil and gas, petrochemical, pressure-piping and sour-service applications.
Available products include:
- ASTM A350 LF2 Weld Neck Flange
- ASTM A350 LF2 Blind Flange
- ASTM A350 LF2 Slip-On Flange
- ASTM A350 LF2 Socket Weld Flange
- ASTM A350 LF2 Threaded Flange
- ASTM A350 LF2 Long Weld Neck Flange
- ASME B16.5 Flanges
- ASME B16.47 Flanges
- NACE MR0175 / ISO 15156 Project Flanges
For quotation, please provide the flange type, size, pressure class, facing, LF2 class, minimum design temperature, NACE requirement and inspection requirements.
