ASTM A350 LF2: Low-Temperature Carbon Steel Forgings Explained

ASTM A350 Grade LF2 is the default material specification for carbon and low-alloy steel forgings serving pipelines, valves, and flanges below ambient temperature — typically down to -46°C (-50°F) with impact testing. If you buy ANSI flanges, forged fittings, or valve bodies for LNG-adjacent plants, gas processing in cold climates, or any line class with a minimum design temperature below -29°C, your line class almost certainly calls out LF2. This guide explains how LF2 differs from ordinary A105 forgings, what the impact test requirements mean, which heat treatment conditions are allowed, and how to verify certificates during sourcing.

What A350 LF2 Is — and How It Differs from A105

A105 is the standard carbon steel forging spec for ambient-temperature piping components (ASME B16.5 flanges, B16.11 fittings, valve parts). Chemically, A350 LF2 and A105 overlap closely — both are carbon-manganese-silicon steels with carbon around 0.30% max in LF2 Class 1. The decisive difference is toughness at low temperature: A350 requires Charpy V-notch impact testing at the minimum specified service temperature, while A105 has no sub-size impact requirement at all. In practice, LF2 is best understood as A105 with verified low-temperature toughness plus tighter melting and processing rules.

Requirement	A105	A350 LF2 (Class 1)
Carbon, max	0.35%	0.30%
Manganese	0.60–1.05%	0.60–1.35%
Tensile strength, min	485 MPa (70 ksi)	485 MPa (70 ksi)
Yield strength, min	250 MPa (36 ksi)	250 MPa (36 ksi)
Impact test	Not required	Required at min. design temp (e.g. -46°C)
Heat treatment	Normalized optional	Normalized, normalized + tempered, or Q&T
Typical min. temp	-29°C (with curve limits)	-46°C (Class 1), -101°C for LF3

Class 1 vs Class 2: Impact Testing and Heat Treatment

The A350 specification splits grades into two classes with different test obligations, and this is where most sourcing mistakes happen:

• Class 1: each heat is impact tested in the specified heat-treated condition at or below the minimum temperature the purchaser states on the order. Average energy of three Charpy V-notch specimens must meet the code minimum (typically 20 J / 15 ft-lbf transverse, 27 J longitudinal depending on product and spec revision), with no single value below the minimum permitted average minus a defined allowance.
• Class 2: only tensile and hardness requirements; impact testing is not required, so Class 2 forgings are NOT a substitute for Class 1 in cold service line classes.
• Heat treatment is mandatory: normalizing, normalizing plus tempering, or quenching plus tempering. As-forged material cannot be certified to LF2 Class 1 — a mill offering “LF2 equivalent, as-forged” is selling non-conforming material.
• Test temperature is buyer-defined: always write the minimum design temperature on the PO, not just “LF2”. A certificate reading “impact tested at -29°C” does not qualify the forging for a -46°C line class.

Typical Products Made to A350 LF2

Product	Governing dimensional standard	Common sizes
Weld neck / slip-on / blind flanges	ASME B16.5	½”–24″, Class 150–2500
Large-diameter flanges	ASME B16.47 Series A/B	26″–60″
Socket weld & threaded fittings	ASME B16.11	⅛”–4″, 2000–9000 lb
Valve bodies, bonnets, stems	API 600 / 6D designs	Per valve rating
Forged pipe components (lateral, cross)	MSS SP-75 / project spec	Custom forgings

Manufacturing Route and What It Means for Quality

LF2 forgings start from ingot or continuous cast blooms, are hot forged with a minimum specified reduction ratio, then heat treated. The normalizing treatment (held at 870–915°C, air cooled) refines the grain so that Charpy energy becomes consistent across the section — coarse-grained as-forged steel would fail the -46°C impact test. For heavier sections or when purchasers specify enhanced toughness, normalized-and-tempered or quenched-and-tempered routes deliver lower transition temperatures. Key process checkpoints:

• Grain size: ASTM No. 5 or finer is commonly specified; finer grains shift the ductile-brittle transition temperature down.
• Chemistry tuning: silicon 0.15–0.30% and controlled aluminum (acid-soluble Al ≥ 0.015–0.020%) support grain refinement; some mills add 0.02–0.05% niobium or vanadium for strength without hurting toughness.
• Hydrogen control: vacuum degassing on heats destined for NACE sour service reduces HIC risk on heavier sections.
• Non-destructive testing: magnetic particle inspection of machined surfaces per ASME B16.5, and ultrasonic examination per supplement S5/S6 when specified for thick sections.

Reading the Mill Test Certificate Correctly

A compliant EN 10204 3.1 certificate for LF2 Class 1 must show five things. Missing any one of them means the forging does not prove conformance:

• Heat treatment record: actual holding temperature, soak time, and cooling method — “N+T” without temperatures is not acceptable.
• Impact test temperature: equal to or below your stated minimum design temperature, plus specimen orientation and size (full 10×10 mm or sub-size).
• Three individual Charpy values plus the average, not a single number.
• Tensile results in the heat-treated condition, taken from a test bar that went through the same furnace cycle as the forgings.
• Heat number traceability stamped or dot-peened on the physical part matching the certificate.

Common red flags we see in third-party audits: impact tests quoted at room temperature, Class 2 certificates substituted for Class 1 line items, and “LF2” markings on forgings whose certificates show no heat treatment at all. All three are non-conformances that will fail a project’s material review board.

Substitution Rules and Adjacent Grades

Situation	Valid option	Notes
Down to -46°C, forged components	A350 LF2 Cl.1	Industry default
Down to -101°C	A350 LF3	Ni 3.5% alloy, tougher core
-101°C to -196°C	A350 LF5 / stainless 304L/316L forgings (A182)	Austenitic required
Cast valve bodies	A352 LCB / LCC	Cast equivalent of LF2
Pipe (not forging) in same line class	A333 Gr 6	Seamless low-temp pipe
Wrought fittings (not forged)	A420 WPL6	Welded fittings, same temp class

Frequently Asked Questions

Is A350 LF2 Class 2 the same material as Class 1?

Chemically very similar, but Class 2 forgings carry no impact test requirement, so they cannot be used where the line class mandates verified low-temperature toughness. The class must match the purchase order exactly.

Can A105 flanges be used at -40°C?

Only within the restrictions of the applicable code curve (ASME B31.3 Figure 323.2.2A), which limits A105 to warmer ranges for most wall thicknesses. Below -29°C the standard practice is to switch to LF2 Class 1.

What is the difference between LF2 and A420 WPL6?

Both serve the same -46°C temperature class. LF2 covers forgings (flanges, valve parts); WPL6 covers wrought fittings (elbows, tees, reducers) made from A333-type pipe. They appear side by side on the same line class.

Does LF2 need PWHT after welding?

It depends on wall thickness and the construction code. ASME B31.3 typically exempts welded LF2 components from PWHT up to 19 mm for matching carbon steel fillers; thicker sections or project specs often require stress relief at 595–620°C.