Impact-Tested Forgings for Flanges and Fittings Down to -46°C

What is ASTM A350 LF2?

A350 LF2 uses essentially the same carbon-manganese basis as A105 with two deliberate differences: carbon is capped lower at 0.30%, and manganese is allowed up to 1.35%. That combination gives the hardenability needed for good toughness after normalising, while the low carbon keeps the ductile-to-brittle transition below the test temperature.

Impact testing is the defining requirement. Three Charpy specimens are tested at -46°C for Class 1 and the average must reach 20 J, with no single specimen below 16 J. A heat can meet every tensile requirement and still be rejected on Charpy energy, which is why the transition temperature rather than the strength is the real specification of this grade.

LF2 pairs with A333 Grade 6 pipe and A420 WPL6 fittings in a low temperature line, and with A352 LCB cast valves. Mixing an A105 flange into a line built to those specifications is a common and expensive error, because the flange becomes the notch-toughness weak point at exactly the temperature the line was designed for.

Chemical Composition (Weight %)

A350 chemical requirements for LF2. Phosphorus and sulfur sit at the same limits as A105; the carbon cap and the narrow silicon band are what actually control the ductile-to-brittle transition.

The silicon range is the quickest way to distinguish LF2 from A105 on a certificate: A105 allows 0.10-0.35%, LF2 requires 0.15-0.30%. Niobium is capped at 0.02% because even small additions refine the microstructure in a way that moves the transition temperature in the wrong direction for this grade.

Mechanical Properties

A350 tensile and impact requirements for LF2. The impact test is not supplementary; it is part of the specification.

Impact requirement, A350 Table 3: Class 1 is tested at -46°C with an average of 20 J (15 ft·lbf) across three specimens and a single-specimen minimum of 16 J (12 ft·lbf). Class 2 is tested at -18°C with 27 J average. The test temperature is fixed by the class; a supplier offering "impact tested" without temperature and class on the certificate has not supplied LF2.

Physical Properties

Carbon steel does not become brittle in the elastic sense at -46°C; it loses fracture toughness. The elastic modulus actually increases slightly as temperature falls, which is why deflection calculations at low temperature are rarely the governing case.

Heat Treatment, Welding & PWHT

Mandatory heat treatment

A350 requires LF2 forgings to be normalised, normalised and tempered, or quenched and tempered. As-forged supply is not permitted, unlike A105. The certificate must state which cycle was used, because the impact results depend on it.

Normalising versus quenching and tempering

Normalising gives a ferrite-pearlite structure with adequate toughness at -46°C and is the common supply condition. Quenching and tempering raises strength and improves toughness further, and is chosen for heavy sections where normalising cannot cool the core fast enough.

Post-weld heat treatment

PWHT follows B31.3 for P-No. 1, at 593-649°C where the thickness threshold is exceeded. PWHT slightly reduces toughness, so a fitting that only just passes Charpy before heat treatment may fail after it. Impact testing after simulated PWHT is available on request.

Welding

ER70S-6 or E7018, with preheat only for thick sections. Low hydrogen consumables and dry storage matter more than usual here: hydrogen in the weld metal shifts the transition temperature upward and can fail the impact test on a procedure qualification.

Service Limits & Corrosion Behaviour

Temperature envelope

A350 LF2 is rated from -46°C to 343°C in ASME B31.3. The upper limit is lower than A105 because the grade is designed around toughness rather than elevated temperature strength.

Low temperature behaviour

The 20 J average at -46°C gives a margin against brittle fracture in cold climate and refrigerated service. Below -46°C the grade is no longer qualified, and LF3 at -101°C or a stainless steel takes over.

Corrosion

Identical to carbon steel. Low temperature service does not change corrosion behaviour, and the same coatings or allowances apply.

Sour and hydrogen service

197 HBW is below the 22 HRC NACE ceiling, so LF2 is commonly accepted for sour low temperature service when the requirement is stated. Hydrogen-induced cracking resistance still depends on chemistry and hardness control, not on the grade name alone.

ASTM A350 LF2 vs Similar Grades — Selection Guide

A low temperature line is only as tough as its weakest component. Flanges to LF2, pipe to A333 Gr 6 and fittings to A420 WPL6 form a consistent set; substituting any one of them with a carbon steel item defeats the design.

Applicable Standards

HT PIPE Supply — ASTM A350 LF2 Product Forms

Each heat is supplied with Charpy results at -46°C, the heat treatment cycle and hardness. Where the flange will be welded, impact testing after simulated PWHT can be arranged at the time of order.

Project Experience — ASTM A350 LF2

LF2 forgings are supplied for cold climate pipelines, LNG and refrigeration plants, and low temperature process service where -46°C is the design minimum.

We supply the complete low temperature set, pipe to A333 Gr 6 and fittings to A420 WPL6, so that the impact test temperature is consistent across the line.

FAQ — ASTM A350 LF2