ASTM A350

USED FOR:

Governs carbon and low-alloy steel forgings for piping components: flanges, fittings, valves, and similar forged pressure-containing parts, intended for low-temperature service.

Like A105 and A182, this is a forging standard; A350 is the low-temperature counterpart to A105, defined by mandatory notch-toughness qualification rather than a distinct product form.

RELATED STANDARDS:

• Sibling specs: ASTM A105 covers the same forged component types for ambient/moderate temperature service without guaranteed low-temperature toughness; ASTM A182 covers alloy and stainless forgings for elevated-temperature or corrosion-resistant service; A350 fills the gap for carbon/low-alloy forgings that must demonstrate adequate Charpy impact toughness at a specified sub-zero minimum design metal temperature.
• Design code that invokes it: ASME B16.5 and B16.47 list A350 as an acceptable flange material, and ASME B31.3 (particularly its low-temperature/impact-test-exemption provisions) and B31.1 reference A350 allowable stresses from ASME Section II Part D for pressure-temperature rating.
• Dimensional standard that pairs with it: ASME B16.5/B16.47 for flanges and B16.11 for forged socket-weld/threaded fittings define geometry, A350 governs material properties only, not dimensions.

GRADE / TYPE RANGE:
A350 covers several grades, most commonly LF1, LF2, LF3, and LF6 (with LF787 and other higher-strength variants in some editions).

LF2 is the workhorse grade, a carbon-manganese steel broadly comparable in strength to A105 but supplied in the required heat-treated condition (typically normalized, normalized-and-tempered, or quenched-and-tempered as permitted by the specification) and qualified by mandatory Charpy V-notch impact testing for the specified test temperature. It is the default low-temperature substitute for A105 on most projects.

LF1 is a similar carbon steel with slightly different chemistry/property balance, sometimes used where LF2 isn’t specifically called for.

LF3 is a nickel-alloy grade (approximately 3.5% Ni), extending guaranteed toughness down to roughly -150°F (-101°C), used for deeper cryogenic duty such as NGL or ethylene service.

LF6 covers specific higher-strength, quenched-and-tempered options in some editions. As with A420’s WPL grades, carbon content is controlled and nickel content increases as design temperature drops, trading cost and availability for toughness at lower temperature.

Grade selection tracks directly with the piping system’s MDMT and the impact-test-exemption curve in the governing code, mirroring the logic used for A420 fittings on the same line.

DESCRIPTION:
A350 covers carbon and low-alloy steel forgings for flanges, fittings, valves, and similar components, produced by forging and supplied in the heat-treated condition required for the applicable grade (such as normalized, normalized-and-tempered, or quenched-and-tempered as permitted by the specification) to achieve the required mechanical properties and low-temperature toughness.

The standard specifies chemical composition limits with tighter control on elements affecting toughness, mechanical property requirements (tensile, yield, elongation), and, as the defining feature relative to A105, mandatory Charpy V-notch impact testing at a specified test temperature with minimum absorbed energy criteria, tested per heat and heat-treatment lot.

It also addresses hardness testing in some cases, and marking requirements for grade and heat traceability, with impact test certification being a document that’s scrutinized closely during low-temperature project material receipt. In practice, A350 is ordered wherever A105 would otherwise be used, but the piping class’s MDMT falls below the applicable impact-test exemption temperature.

TYPICAL INDUSTRY USE:
Specified for LNG, gas processing, NGL/LPG, refrigerated storage, and cold-climate oil & gas piping systems for forged flanges and fittings, essentially the forging counterpart to A420’s role on buttwelding fittings, and commonly ordered together on the same low-temperature piping class.

IMPORTANT NOTES:

• Sometimes we saw A350 LF2 confused with, or substituted incorrectly for, A105 on line lists that mix ambient and low-temperature systems; since the two grades look similar in bulk strength, a spec-check slip here is easy to miss without a dedicated MDMT review of the piping class.

• NACE MR0175/ISO 15156 compliance is not automatic under base A350; where sour service coincides with low temperature, we mention the need for explicit hardness testing (commonly capped around 22 HRC) and sour-service qualification as an add-on requirement, since the two qualification paths (toughness vs. sulfide stress cracking resistance) are independent.

• We always check that the impact test temperature reported on the forging’s MTR actually covers the line’s MDMT. Similar to the mistake we mentioned on A420, some forgings are stocked pre-qualified to a standard reference temperature that may not be cold enough for a particular design case.

• LF3 forgings, being less commonly stocked than LF2, carry meaningful lead-time risk on cryogenic projects