A350 LF2: Steel Technical Specifications

01Introduction and general characteristics

The ASTM A350/A350M specification (adopted as ASME SA-350 within the Boiler & Pressure Vessel scope) defines, for LF2 forgings, the permitted melting and forging processes, the allowable heat treatments - normalising, normalising and tempering, quenching and tempering - and the chemical analysis, tensile, hardness and impact toughness testing/inspections required to guarantee the properties in sub-zero service.

The distinctive feature of the grade is the mandatory low-temperature notch toughness requirement: for CL1 the standard Charpy test is at -46 °C (-50 °F), with minimum energy levels that ensure a margin against brittle fracture. This characteristic makes LF2 suitable for process equipment in cold climates and for low-temperature piping systems, where it is integrated with ASTM A333 Gr.6 pipe and ASTM A420 WPL6 fittings to preserve mechanical and impact-toughness consistency throughout the entire circuit.

1.1Differences from A105 and general-service steels

Compared with general-service forgings such as ASTM A105, LF2 is designed for cold environments: it requires low-temperature notch impact toughness verification and prescribes heat-treatment conditions aimed at toughness. A105, designed for room- or elevated-temperature service, does not require low-temperature Charpy testing.

In practical terms, this translates into a service window extended toward low temperatures (down to -46 °C for CL1) and into certified toughness, an essential condition where standard carbon steel would exhibit a more critical ductile-to-brittle transition behaviour. For this reason, LF2 belongs to the same functional family of low-temperature materials in the piping circuit (A333 Gr.6, A420 WPL6), favouring a consistent selection across flanges, valves and fittings.

1.2Field of application and service limits

The functional advantage of LF2 is guaranteed low-temperature toughness: the Charpy test prescribed at -46 °C (CL1) provides a safety margin against embrittlement in service. The material is used in cold-latitude oil & gas, refrigerated chemical and process plants and power generation, with a typical service range from -46 °C to approximately +425 °C (800 °F).

It is important to distinguish low-temperature service from deep cryogenics: LF2 is not suitable for LNG temperatures (≈ -162 °C), for which 9% Ni steels or austenitic stainless steels are used. In LNG plants, LF2 can therefore be used on the ambient/low-temperature lines not in cryogenic contact, but not on components in cryogenic service.

The availability of the CL1/CL2 classes and the functional correspondence with A333 and A420 simplify line qualification and reduce the risks of performance mismatch between components.

1.3Reference standards and certifications

The primary reference standard is the ASTM A350/A350M (ASME SA-350) specification, which defines the field of application, chemical and metallurgical requirements, heat treatments and mechanical and impact toughness verifications.

For flanged components, the specification is normally applied with dimensional standards conforming to ASME B16.5/B16.47. In sour service contexts, compatibility with NACE MR0175/ISO 15156 must be attested on a case-by-case basis: it depends on compliance with the hardness limits for forged carbon steels and on the environmental service conditions (H₂S content, pressure, temperature), and must be documented on MTC and PQR. The hardness value often cited for LF2 (≈ 197 HBW) is consistent with such practices, but should not be understood as the sole fixed limit of the base specification.

Conformity documentation remains mandatory (typically MTC according to EN 10204, with the certificate type defined in the order), as does compliance with the prescribed tensile, impact and hardness requirements.

02Chemical composition

LF2 is a carbon/low-alloy steel with elemental limits and summation notes set by ASTM A350/A350M. The composition favours low carbon, controlled impurity content (P, S) and moderate Mn and Si levels, to safeguard low-temperature toughness and weldability.

The specification's footnotes prescribe Σ(Cu+Ni+Cr+Mo+V) ≤ 1.00% and (Cr+Mo) ≤ 0.32% on the heat analysis; lead- or sulphur-enhanced free-cutting steels are not permitted. The carbon equivalent is limited according to the reference IIW formula - CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 - with a guideline value CE ≤ 0.47, a key parameter for setting preheat and any PWHT as a function of thickness.

Table - ASTM A350 LF2 chemical composition (heat analysis, % by mass)

Note: in the case of vacuum carbon deoxidation (supplementary requirement S4), Si is limited to ≤ 0.12%. The CE limit may vary with the maximum thickness of the part. Always check the CE/thickness table in ASTM A350/A350M in the current edition of the standard.

Low C and a moderate CE favour weldability with moderate preheating; Mn and any residual Ni support impact toughness and microstructural stability after normalising or quenching and tempering, factors central to passing the impact test at -46 °C (CL1).

2.1International equivalents (indicative)

LF2 is associated with the UNS K03011 code and with the ASME SA-350 LF2 equivalent for use in boilermaking and pressure equipment. The equivalent for castings, when the process requires casting rather than forging, is ASTM A352 Gr. LCB (with LCC as a higher-Mn-content variant).

The equivalents sometimes cited to European families such as S355J2 (1.0577) or the historical Fe510D / DIN St52-3N (1.0570) are indicative only and not normative: even with partly overlapping Rm ranges, the minimum yield strength differs markedly (LF2 ≈ 250 MPa versus ≈ 355 MPa for S355) and, above all, these are hot-rolled structural families lacking the certified low-temperature impact toughness requirement of LF2. They cannot therefore replace LF2 in low-temperature service.

In design, it is recommended to always report the complete designation (ASTM/ASME, grade, class, any supplementary requirements) and the UNS K03011 on MTC and purchase documents, to avoid ambiguity with adjacent materials not intended for low temperature.

03Mechanical properties

LF2 forgings require tensile and impact toughness verifications that ensure ductility and toughness in sub-zero service, in the permitted delivery conditions (N, NT, QT). At room temperature the acceptance requirements are Rm 485-655 MPa, Rp0.2 minimum 250 MPa, minimum elongation 22% and minimum reduction of area 30%. The distinction between CL1 (Charpy at -46 °C) and CL2 (Charpy at -18 °C) differentiates the impact toughness requirements, not the tensile ones, which remain common to the two classes.

3.1Delivery conditions and metallurgical state

The specification does not provide for the annealed state as a delivery condition: the acceptance requirements (tensile, yield strength, A%, Z%) refer to the normalised, normalised and tempered or quenched and tempered condition, chosen to guarantee toughness and microstructural homogeneity. Any annealing cycles are used as intermediate process treatments, not as a certified final state.

3.2Properties in the quenched and tempered state and delivery condition

In both the quenched and tempered and the normalised/tempered condition, at room temperature LF2 must satisfy Rm 485-655 MPa, Rp0.2 ≥ 250 MPa, A% ≥ 22 and Z% ≥ 30, a ductile basis adequate for pressurised components and bolted joints. The permitted cycles include quenching and tempering with tempering ≥ 593 °C (1100 °F) up to the lower transformation temperature, or normalising followed by tempering ≥ 593 °C. These requirements are consistent with in-line integration with A333 Gr.6 and A420 WPL6.

Table - ASTM A350 LF2 mechanical and impact toughness acceptance requirements

Note: for sub-size specimens (section < 10×10 mm) proportionally equivalent energies apply according to ASTM A370. The two classes share the same tensile requirements: the difference lies in the temperature/impact-energy pairing.

3.3Hardness after heat treatment

The specification requires acceptance hardness testing (clause 7.3), consistent with the metallurgical condition achieved. The hardness value most frequently reported for LF2 forgings is ≈ 197 HBW as a maximum reference, to be verified on the MTC and confirmed when the order invokes NACE MR0175/ISO 15156. The resulting hardness depends on the thermal cycle (tempering temperature, effective thickness, quench severity), which must be set so as to simultaneously meet the hardness limit and the prescribed Charpy impact toughness.

3.4Impact toughness and toughness at low temperature

For CL1 the Charpy V-notch test is required at -46 °C with a minimum average energy of 20 J (15 ft·lbf) over the set of three specimens; for CL2 the test temperature is -18 °C with a minimum average energy of 27 J (20 ft·lbf). The acceptance criterion is assessed on the average and the single minimum of the set, according to ASTM A370. The combination of a microstructure refined by treatment (normalising or quenching and tempering) and controlled toughness is the foundation for use on flanges, fittings and valve bodies for low-temperature plants.

3.5Fatigue and dynamic behaviour

The ASTM A350/A350M specification does not define S-N curves or fatigue limits for LF2: fatigue design in pressure systems relies on the applicable codes (e.g. ASME) and on design qualification, taking into account microstructure, surface condition and stress concentrations. The certified impact toughness at -46/-18 °C reduces the risk of brittle fracture under cyclic loads and thermal transients in cold environments, but fatigue strength remains a function of the constructional detail and must be validated with dedicated tests and checks.

04Physical properties

Physical properties are those typical of carbon steels for low temperatures; they are not prescribed by ASTM A350/A350M and should be understood as indicative reference values. The nominal density is about 7.85-7.86 g/cm³, useful for calculating weights and loads. The elastic modulus at 20 °C is about 200-205 GPa (≈ 29×10⁶ psi). The coefficient of linear thermal expansion in the 20-100 °C range is of the order of 10.4-11.5 μm/m·°C, a figure relevant for joints, bolting and tolerances on flanged elements. Thermal conductivity at room temperature is reported between ≈ 34 and 50 W/(m·K) depending on conventions; the specific heat capacity is ≈ 0.46-0.50 kJ/(kg·K) (≈ 0.11-0.12 Btu/lb·°F).

Table - Typical physical properties of ASTM A350 LF2 (at room temperature)

For thermomechanical analysis, ΔL = α·L₀·ΔT is used to estimate thermal expansion and its effect on flanged assemblies, setting restraints and expansion joints consistent with the minimum and operating temperatures of the service. Heat capacity and conductivity also affect the thermal profiles in welding and preheating and the heat dissipation during machining. For round-bar weights, the steel bar weight calculator is available.

05Heat treatments

LF2 must be supplied in one of the following conditions: normalised (N), normalised and tempered (NT) or quenched and tempered (QT). Normalising involves full austenitising and cooling in still air; in the NT and QT cycles the specification requires tempering at a temperature not below 1100 °F (593 °C), with a minimum holding time of 30 min per 25 mm (1 inch) of maximum thickness - and in any case not less than 30 min - followed by air cooling. The specification does not prescribe an austenitising temperature: steelmakers' datasheets typically place normalising and austenitising in the 870-940 °C range.

Summary of permitted parameters

• Normalising: austenitising (typically 870-940 °C), air cooling.
• Normalising + tempering: temper ≥ 593 °C, holding ≥ 30 min/25 mm, air cooling.
• Hardening + tempering: austenitise, quench in a suitable liquid medium (oil or polymer solution), temper between 593 °C and the lower transformation temperature, holding ≥ 30 min/25 mm, air cooling.

5.1Hardening: temperatures and techniques

In the QT cycle the piece is fully austenitised and then quenched in a liquid medium (oil or polymer solution), with subsequent mandatory tempering to impart low-temperature toughness and microstructural stability. The specification also allows multi-stage procedures (partial re-austenitising followed by quenching) provided the result is equivalent to a full quench and the mechanical and impact toughness requirements are met. Conditions must be calibrated to combine strength and toughness without exceeding hardness levels that would impair weldability and in-service compliance.

5.2Tempering: parameters

Tempering is constrained to ≥ 593 °C (1100 °F), with a minimum holding of 30 min/25 mm (and in any case ≥ 30 min) and air cooling, both in the NT sequence and after QT quenching. The lower limit ensures sufficient toughness in low-temperature service, meeting the Charpy energy levels of CL1/CL2 and microstructural stability. The choice of temperature within the permitted range and of time beyond the minimum is calibrated on effective thickness and mechanical targets. Traceability of the cycles (time-temperature curve, furnace certification, load uniformity) is an integral part of quality control.

5.3Normalising: conditions and applications

Normalising involves austenitising and air cooling, and is permitted both as a final condition and as an intermediate step followed by tempering (NT). The choice between N, NT and QT depends on thickness, geometry and Charpy target: N/NT are preferred for medium-thin sections or where post-treatment dimensional stability and weldability have priority, whereas QT is used for more demanding sections or more stringent mechanical requirements, thanks to deeper through-thickness hardening (through-hardening).

5.4Quality control of heat treatments

Quality control includes tensile checks at room temperature, a Charpy test at the class temperature (-46 °C for CL1, -18 °C for CL2) and hardness tests, consistent with the declared condition. Furnace cycles and compliance of the key parameters (tempering ≥ 593 °C, minimum time per thickness, air cooling) must be recorded and traced, correlating them with the batch results. Where welding is present, any stress relieving (PWHT) applies only if required by the design and must be evaluated with respect to its effects on toughness: stress-relieving cycles on normalised welded LF2 can reduce the Charpy energy, an aspect to be managed with WPS/PQR qualifications and dedicated tests.

5.5Common defects and corrective actions

The most frequent non-conformities in treatment cycles and the related corrective actions:

• Quench cracks or distortions (QT): typical causes are non-uniform austenitising, excessive medium severity or critical geometries. Improve heating uniformity, choose a less aggressive quenching medium, optimise fixturing and, on sensitive sections, prefer NT with compliant tempering.
• Insufficient Charpy toughness: verify that tempering is ≥ 593 °C, with time ≥ 30 min/25 mm and air cooling. If the parameters were not met, repeat the tempering and the tests; if toughness remains insufficient, perform normalising followed by compliant tempering and requalify the batch.
• Over-tempering (drop in Rm/Rp0.2): check the actual part temperature and furnace uniformity, bring tempering back within the prescribed range, reduce the time to the effective minimum and verify with hardness and Charpy.
• Non-uniform hardness on thick sections: increase thermal homogeneity, consider double tempering and, for massive geometries, prefer QT; sample hardness at multiple positions and correlate it with the impact toughness tests.
• Surface decarburization during austenitising: use a suitable atmosphere or protection and remove the decarburized layer with minimal machining before testing, so as not to distort hardness and properties.

Every corrective action must be traced and followed by complete mechanical tests (tensile, hardness, Charpy at the class temperature) to demonstrate the restoration of properties.

06Industrial applications

LF2 is designed for forged components of piping systems that require low-temperature impact testing - flanges, fittings and valve bodies - with certified Charpy toughness at -46 °C (CL1) or -18 °C (CL2). It is an established choice for low-temperature lines in oil & gas, chemical and power, where brittle-fracture prevention is a critical safety requirement. Functional compatibility with A333 pipes and A420 fittings promotes performance uniformity along the line. The grade belongs to the valve steels category.

6.1Sectors where use is not typical

LF2 is not a preferred grade for automotive components or machine-tool parts: it was created for piping and low-temperature process equipment, not for the wear-resistance, dimensional-stability or high-temperature service requirements typical of other steels. In particular cases it may be considered for joints or flanged connections in cold environments where certified toughness is required, but such uses are usually covered by dedicated grades. Any use outside the piping/valves domain requires specific design validation.

6.2Mechanical industry and process plant engineering

In plant engineering, LF2 is used for forged flanges, valve bodies and fittings in low-temperature fluid lines, with Charpy tests required for batch acceptance. Applications include process skids, reduction and control stations and connections to tanks in cold environments. Standards consistency with A333 (pipes) and A420 (fittings) simplifies plant qualification and supports reliable joints.

6.3Specialist sectors and cold climate

In cold-climate oil & gas, chemical and Arctic-area sectors, LF2 is used for forgings intended for -46 °C (CL1) or -18 °C (CL2). In the valve field it is used alongside the cast grades A352 LCB/LCC when the process requires castings rather than forgings. Applications cover ASME B16.5/B16.47 flanges and valve bodies for low-temperature lines. As indicated in §1.2, for extreme cryogenic service (LNG at ≈ -162 °C), materials with a higher Ni content or austenitic materials are instead required.

6.4Comparison with adjacent materials

The primary selection criterion is the minimum design temperature combined with the need for a Charpy test: where guaranteed impact toughness is required, LF2 is preferable to steels without low-temperature requirements; for castings, LCB/LCC with equivalent toughness qualification are used.

07Frequently asked questions

7.1What is ASTM A350 LF2 and where is it used?

It is a carbon/low-alloy steel for forgings with mandatory low-temperature notch impact toughness, intended for flanges, fittings and valve bodies in sub-zero piping systems, in accordance with ASTM A350/A350M (ASME SA-350). The certified toughness of classes CL1/CL2 qualifies it for oil & gas in cold climates, refrigerated chemical and process plants, with a typical service range from -46 °C to +425 °C. In-line, it is matched with A333 Gr.6 pipes and A420 WPL6 fittings.

7.2What is the difference between ASTM A350 LF2 and A105?

A105 is a forging for general service at ambient/high temperature, without any requirement for a low-temperature impact test. LF2 introduces the low-temperature impact toughness requirement and heat-treatment conditions aimed at toughness. Substituting A105 with LF2 is permitted when low-temperature requirements must be met; the reverse is not recommended in the absence of guaranteed toughness.

7.3What distinguishes classes CL1 and CL2, and at what temperatures is the Charpy test performed?

The two classes share the same composition and tensile properties and differ in the impact test: CL1 at -46 °C (minimum average energy 20 J over 3 specimens) and CL2 at -18 °C (minimum average energy 27 J). Acceptance is assessed on the average and single minimum of the set in accordance with ASTM A370. Correct class designation on the order and MTC is essential to satisfy the analysis of climatic loads and thermal transients.

7.4What are the minimum mechanical values at ambient temperature?

For LF2 forgings: Rm 485-655 MPa, Rp0.2 ≥ 250 MPa, A% ≥ 22 and Z% ≥ 30, with hardness typically within ≈ 197 HBW after the final treatment. These values, together with the impact toughness requirements, support the sizing of flanges and valve bodies. The location of the samples (e.g. at T/4) and control of the maximum thickness ensure the representativeness of the tests.

7.5What is the chemical composition and carbon equivalent?

C ≤ 0.30; Si 0.15-0.30; Mn 0.60-1.35; P ≤ 0.035; S ≤ 0.040, with Σ(Cu+Ni+Cr+Mo+V) ≤ 1.00 and (Cr+Mo) ≤ 0.32. The carbon equivalent is kept to CE ≤ 0.47 according to the IIW formula. These limits support post-treatment toughness and reduce the risk of excessive hardening in the heat-affected zone (HAZ) during welding.

7.6Which heat treatments are permitted?

Three conditions: normalised (N), normalised and tempered (NT) or quenched and tempered (QT), with tempering ≥ 593 °C (1100 °F) and minimum times per thickness. The choice of cycle depends on section, geometry and the Charpy target, balancing strength and toughness. Traceability of the cycles is part of acceptance and must be correlated with tensile, hardness and impact toughness.

7.7What are the indicative ranges for normalising/austenitising?

Datasheets place normalising and austenitising typically in the 870-940 °C range, with air cooling for N/NT and liquid-medium quenching for QT, followed by tempering ≥ 593 °C. These ranges promote uniform grain and a microstructure suited to toughness; adjusting the times to the maximum section is decisive for the homogeneity of the forging.

7.8What hardness is permitted at acceptance?

The value most frequently cited for LF2 forgings is ≈ 197 HBW as a maximum reference, to be verified on the MTC and consistent with the declared condition. Hardness testing complements tensile and impact toughness, limiting the risk of insufficient toughness. Where sour-service specifications apply, the limits may be further constrained by the applicable standard.

7.9Is ASTM A350 LF2 weldable?

Yes: weldability is favoured by the moderate carbon and controlled CE, but it requires WPS/PQR qualifications that account for thicknesses, heat input and the toughness required at low temperature. Preheating and HAZ hardness checks guard against excessive hardening and toughness losses, with any PWHT assessed on a case-by-case basis. Joint conformity is verified by mechanical and impact tests consistent with the class of the base material.

7.10How is compliance with NACE MR0175/ISO 15156 (sour service) managed?

Compliance is not automatic: it depends on meeting the hardness limits on base metal, weld metal and HAZ and on the environmental conditions (H₂S, pressure, temperature) of the service. For sour-service applications, process parameters and thermal cycles are selected so as to meet the limits set for forged carbon steels, with evidence on the PQR and MTC. Validation must be formalized in the specification and inspection documentation.

7.11Which dimensional standards apply to flanges?

For flanges, the ASME B16.5/B16.47 dimensional standards usually apply. Conformance to these standards simplifies compatibility with bolting and gaskets. The pressure class and face finish are an integral part of the joint design.

7.12How does it integrate with pipes and line fittings?

LF2 is paired with ASTM A333 Gr.6 pipes and ASTM A420 WPL6 fittings, maintaining toughness consistency along the line at low temperature. This consistency reduces the risks of thermomechanical mismatch and facilitates plant qualification. Selecting the line materials jointly is recommended in the purchasing specifications.

7.13In which conditions and formats is it available?

It is available in forgings and bars (hot-rolled or forged) in the normalised, normalised and tempered or quenched and tempered condition, with diameters and finishes suited to machining requirements. Selection of the condition is related to the required toughness and thicknesses, with certification and traceability as per specification.

08The Siderticino offering for ASTM A350 LF2

Siderticino supplies ASTM A350 LF2 in as-rolled round bars, normalised round bars and as-forged/normalised forgings, covering machining and mechanical-property requirements in accordance with ASTM A350/A350M and classes CL1/CL2. The formats support piping and valve applications for cold environments, where the Charpy test at -46 °C (CL1) or -18 °C (CL2) is an acceptance prerequisite. The offering is geared towards metallurgical and logistical continuity on low-temperature projects, in line with A333 pipes and A420 fittings.

For availability, diameters, delivery conditions, custom cutting and delivery times, see the ASTM A350 LF2 product datasheet and request a quote.