ASTM A333 Grade 6 pipe steel: properties, impact testing, equivalents

ASTM A333 Grade 6 is the low-temperature carbon steel pipe: 240 MPa yield, 415 MPa tensile, Charpy tested at −45 °C. Composition, impact requirement and the A106 difference.

Last updated 12 October 2026

What is A333 Grade 6?

ASTM A333 Grade 6 is the low-temperature carbon steel pipe, and the way to understand it is to see what it is not offering. Its strength minima — 240 MPa (35 ksi) yield and 415 MPa (60 ksi) tensile — are exactly those of A106 Grade B and A53 Grade B. Its analysis is close to A106's. No one buys A333 Grade 6 for strength.

What the specification adds is a single requirement neither A106 nor A53 contains anywhere: a mandatory Charpy V-notch test at −45 °C (−50 °F), 18 J average of three specimens with 14 J minimum for any one. Carbon steel loses its ductility abruptly somewhere below room temperature, and where that transition happens depends on composition, deoxidation, grain size and heat treatment — none of which an unimpacted specification controls tightly enough to predict. A333 forces it to be measured.

That is why the composition reads the way it does. Silicon has a minimum of 0.10 %, so the steel must be fully killed and fine grained. Manganese has a minimum as well as a maximum, because manganese is the economical way to push the transition temperature down in a plain carbon steel. Phosphorus and sulphur are capped at 0.025 %, half the A53 allowance, because both embrittle. And the pipe is normally normalised, which refines the grain. These are not quality refinements; they are the mechanism by which 18 J at −45 °C becomes achievable.

The specification then ranks its grades by test temperature rather than by strength, which is unusual and is the key to reading it. Grade 1 and Grade 6 are plain carbon steels tested at −45 °C. Grade 7 (2.5 % Ni) goes to −75 °C, Grade 9 (2 % Ni + Cu) to −100 °C, Grade 3 (3.5 % Ni) to −101 °C, and Grade 8 (9 % Ni) to −195 °C for LNG. Nickel is what buys the extra cold, and the price rises steeply with it — which is why Grade 6 is specified wherever −45 °C is enough.

A333 Grade 6 chemical composition

ASTM A333/A333M, Grade 6 (UNS K03006). Note what the limits tell you. **Silicon has a minimum of 0.10 %**, so the steel is fully killed — fine grained and deoxidised, which is what makes toughness at −45 °C achievable at all. Phosphorus and sulphur are capped at 0.025 %, tighter than the 0.05/0.045 of A53, because both embrittle at low temperature. Manganese carries a **minimum** as well as a maximum, since manganese is what lowers the transition temperature in a plain carbon steel.

A333 Grade 6 mechanical properties

A333 states the minima in ksi: Grade 6 is **35 ksi yield / 60 ksi tensile**, identical to A106 Grade B and A53 Grade B. The room-temperature strength is therefore not what you are buying. What you are buying is the row that neither of those specifications contains — the **mandatory Charpy test**, which is listed separately below because it is the entire purpose of the specification.

Impact toughness

A333 Grade 6 requires a minimum Charpy V-notch energy of 18 J average of three specimens, 14 J minimum for any one J at −45 °C (−50 °F) °C (Transverse Charpy V-notch, full-size specimens where the wall permits; sub-size specimens are tested with reduced energy requirements in proportion to the specimen thickness, per the standard's table. Specimens are taken from each heat and each heat-treatment lot.). These are the figures for Grade 6 and Grade 1. The nickel grades are tested lower and deeper: Grade 7 at −75 °C, Grade 9 at −100 °C, Grade 3 at −101 °C and Grade 8 (9 % Ni) at −195 °C. The test temperature, not the strength, is what distinguishes the grades of this specification from one another.

A333 Grade 6 equivalent grades

Identical = same composition and mechanical limits within rounding. Near = one limit differs (e.g. carbon max or impact temperature). Functional = interchangeable for most uses after an engineering check. Full cross-reference for A333 Grade 6 →

Old and superseded designations

Product forms and tolerance standards

Product forms: seamless pipe, electric-resistance-welded pipe, double submerged-arc welded pipe in larger sizes, normalised, or as agreed heat treated to meet the impact requirement.

Dimensional tolerances: diameter and wall: ASTM A333/A333M, wall schedules to ASME B36.10M; every length hydrostatically tested, or non-destructively inspected by agreement; impact test specimens taken from each heat and heat-treatment lot — the lot definition is part of the specification.

Key property values

A333 Grade 6 strength

240 MPa (35 ksi) minimum yield, 415 MPa (60 ksi) minimum tensile, 30 % minimum elongation longitudinal. Typical mill results are 290–380 MPa yield and 450–530 MPa tensile. The figures are identical to A106 Grade B, which is the point: the grade exists for its toughness, not its strength, and substituting it for A106 gains nothing in a hot or ambient line.

The −45 °C impact requirement

18 J average of three transverse Charpy V-notch specimens at −45 °C, with 14 J minimum for any single specimen. Where the wall is too thin for full-size specimens, sub-size specimens are used and the required energy is reduced in proportion, per the standard's table — a 7.5 mm specimen is held to three quarters of the energy, and so on. Specimens are drawn from each heat and each heat-treatment lot, so the toughness is verified on the material supplied rather than inferred from the grade.

Why carbon steel needs this and stainless does not

Ferritic and pearlitic steels have a ductile-to-brittle transition: below a certain temperature a crack propagates by cleavage with almost no energy absorbed, so a pipe that is perfectly ductile at +20 °C can shatter at −30 °C. The transition temperature is not a property of the grade name — it moves with grain size, deoxidation, phosphorus, and the normalising cycle. Austenitic stainless steels and aluminium have a face-centred cubic structure and no transition at all, which is why 316L needs no impact test at cryogenic temperature and why it is the alternative below about −100 °C where a nickel steel becomes expensive.

Grade ladder by test temperature

This is how the specification is actually navigated. Grade 1 — carbon steel, 380 MPa tensile, −45 °C. Grade 6 — carbon steel, 415 MPa tensile, −45 °C; the default. Grade 7 — 2.5 % Ni, −75 °C. Grade 9 — 2 % Ni + Cu, −100 °C. Grade 3 — 3.5 % Ni, 450 MPa tensile, −101 °C. Grade 8 — 9 % Ni, 690 MPa tensile, −195 °C, quenched and tempered, for LNG. Grades 4, 10 and 11 are alloy grades for particular duties. Choose by the minimum design metal temperature, then check the strength.

Heat treatment and grain size

Grade 6 is normally supplied normalised — austenitised around 900 °C and air cooled — because the resulting fine, uniform ferrite-pearlite grain is what delivers the impact energy. Welded pipe is heat treated after welding so that the weld and the heat-affected zone meet the same requirement as the body. The standard permits other treatments provided the impact test is met, which puts the burden on the result rather than the route. Post-weld heat treatment on site at 595–650 °C is common on heavy wall and is required by the piping code in some thicknesses.

Where it is used and what it replaces

Any carbon-steel line whose minimum design metal temperature falls below the exemption curve of the piping code: LPG and ethylene handling, ammonia refrigeration, cryogenic plant utility lines, cold-climate gas processing, offshore blowdown and depressurisation systems. The practical rule in ASME B31.3 terms is that where a material's MDMT cannot be exempted by curve or by thickness, an impact-tested material is required — and A333 Grade 6 is the cheapest pipe that supplies that documentation at this strength level.

Weldability

Readily weldable with the same consumables and procedures as A106 Grade B — E7018 or ER70S-2/ER70S-6 — but with one addition that governs everything: the weld and heat-affected zone must also meet the impact requirement, so the welding procedure has to be qualified with Charpy testing at −45 °C and not merely for soundness. Low-hydrogen processes, controlled heat input and controlled interpass temperature matter because excessive heat input coarsens the heat-affected-zone grain and destroys the toughness the pipe was bought for. Preheat 100–150 °C for heavy wall or restrained joints; post-weld heat treatment at 595–650 °C where the code requires it for thickness. For Grade 8 (9 % Ni), nickel-base consumables are used instead and the rules are entirely different.

Machining, forming and heat treatment

Machines and threads like any 0.30 % carbon pipe steel, roughly 70 % relative to 1 % C free-cutting steel. End preparation, bevelling and cutting are as for A106. The cautions are about heat and cold work rather than cutting: avoid heavy local cold working, flattening or hard hammering on the pipe, since cold work raises the transition temperature and can push the material above its rated test temperature; and avoid uncontrolled flame heating for bending or straightening, which coarsens the grain. Hot bending at 850–950 °C followed by re-normalising and re-testing is the correct route where a tight bend is needed.

Typical applications

• LPG, ethylene and propylene piping
• Ammonia and industrial refrigeration systems
• Cryogenic plant utility and process lines above −45 °C
• Cold-climate gas gathering and processing piping
• Offshore blowdown, flare and depressurisation lines
• Low-temperature heat exchanger and vessel nozzles
• Any carbon-steel line whose minimum design metal temperature falls below the code exemption curve

Frequently asked questions

Related grades

Verified against ASTM A333/A333M; ASTM A333/A333M — seamless and welded steel pipe for low-temperature service; ASME BPVC Section II Part A (SA-333); ASTM A106/A106M (comparison); EN 10216-4 (P275NL1/NL2 comparison); JIS G3460 (STPL380 comparison). Last checked: October 2026.