ASTM A333 Low-Temperature Pipe: Grades, Impact Testing and Selection

Quick Answer

ASTM A333 covers seamless and welded carbon and alloy steel pipe for low-temperature service and other duties that need notch toughness. The pipe is treated to control its microstructure and Charpy impact tested at a temperature fixed by its grade, and that tested toughness is what the buyer pays for. Grade 6, a carbon-manganese steel with the same minimum strength as A106 Grade B (415 MPa tensile, 240 MPa yield), is tested at −45 °C (−50 °F) and is the workhorse for cold climates and refrigerated propane. Grade 1 is a weaker carbon steel tested at the same temperature, and Grade 10 a stronger one tested at −60 °C (−75 °F). The nickel grades go colder: Grades 7 and 9 to −75 °C (−100 °F), Grade 3 (3.5% Ni) and Grade 4 to −100 °C (−150 °F), and Grade 8 (9% Ni) to −195 °C (−320 °F). For most grades, full-size specimens must average at least 18 J (13 ft·lbf), with only one of the three allowed below 18 J, and none below 14 J. Choose a grade whose test temperature is at or below the minimum design metal temperature (MDMT), then match the system: A350 LF2 Class 1 flanges and A420 WPL6 fittings go with Grade 6. LF2 Class 2 is tested only at −18 °C (0 °F).

A typical enquiry reads: "LTCS pipe, 6 inch, Sch 80, A333, 2,000 metres." It names the standard but not the grade, and A333 grades are impact tested at anything from −45 °C to −195 °C. It gives no design temperature, so nobody can check that the grade suits the duty, and it says nothing about the flanges and fittings that must match.

Low-temperature pipe is bought for one property: toughness at the coldest temperature the line will see. This guide covers what A333 guarantees, how the grades differ, and how to specify pipe, flanges and fittings that stay tough together.

What ASTM A333 covers

ASTM A333 covers nominal (average) wall seamless and welded carbon and alloy steel pipe for use at low temperatures and in other applications that require notch toughness. The current edition is A333/A333M-24; ASME adopts the specification as SA-333. The main requirements:

• Manufacture: seamless, or welded with no filler metal added; Grade 4 is seamless only, and Grade 11 may also be welded with filler metal
• Heat treatment: every grade except Grade 11 is heat treated to control its microstructure; Grade 11 is annealed only by agreement
• Tests: tension and Charpy impact tests, plus a hydrostatic or nondestructive electric test on each pipe
• Sizes: ordered by NPS and schedule or by OD and wall
• Units: inch-pound, unless the order calls up A333M

The scope carries a warning worth reading twice: some sizes may not be available, because heavier walls have an adverse effect on impact properties. Check heavy-wall availability before the design is frozen.

For the other end of the temperature range, see our guides to A335 chrome-moly pipe and A213 alloy tubes.

Why ferritic steel needs a low-temperature specification

Carbon and low-alloy steels are ferritic. Above a transition temperature they fail in a ductile way and absorb a lot of energy; below it they can fracture in a brittle way, at low stress and with little warning. Ordinary carbon steel pipe such as A106 Grade B is not impact tested under its base specification, so its transition temperature is not known.

A333 controls what sets that transition temperature, and then proves the result:

• Grain size: refined by normalising or by controlled hot working
• Chemistry: phosphorus and sulfur held to 0.025% maximum in most grades, a minimum silicon in Grade 6, and nickel in the alloy grades, which lowers the transition temperature
• Proof: Charpy V-notch tests at the grade's own temperature

Austenitic stainless steel pipe has no ductile-to-brittle transition, which is why it is the usual alternative for the coldest services.

The grades at a glance

Values are from A333/A333M-24; the SI figures are the standard's own.

Why Grade 6 is the workhorse

Grade 6 is, in effect, A106 Grade B made tough. It has the same carbon (0.30% maximum), manganese (0.29–1.06%) and minimum silicon (0.10%) limits, and the same minimum strength: 415 MPa (60 ksi) tensile and 240 MPa (35 ksi) yield. The differences are what buy the toughness: phosphorus and sulfur are held to 0.025% (A106 Grade B allows 0.035%), the microstructure is controlled by normalising, controlled hot working or (seamless only) quenching and tempering, and the pipe is Charpy tested at −45 °C (−50 °F).

That combination explains its popularity:

• No nickel, so it costs far less than the nickel grades
• Familiar welding, provided procedures are qualified with impact tests where the code requires them
• Enough reach for most cold-climate outdoor piping and for refrigerated propane, which boils at about −42 °C at atmospheric pressure
• An established system of matching A350 LF2 Class 1 flanges and A420 WPL6 fittings

Grade 1 is tested at the same temperature but has lower minimum strength (380 MPa tensile, 205 MPa yield), so Grade 6 is normally preferred.

Dual certification. Some seamless stock is offered certified to both A106 Grade B and A333 Grade 6. Such pipe has to meet every requirement of both specifications, including A333's heat treatment and impact tests, and both should appear on the stencil and the certificate. If the certificate shows only A106, the pipe is not A333.

The nickel grades and Grade 10

Nickel lowers the transition temperature of ferritic steel, so the test temperature steps down as nickel rises:

• Grades 7 and 9, −75 °C (−100 °F): about 2–2.5% nickel. Grade 9 adds about 1% copper and has the higher minimum yield, 315 MPa
• Grade 3, −100 °C (−150 °F): 3.5% nickel. Ethylene boils at about −104 °C, a few degrees colder, so piping that can reach that temperature needs an impact test at or below the design temperature written into the order, or a different material
• Grade 4, −100 °C (−150 °F): less nickel, with chromium, copper and aluminium
• Grade 8, −195 °C (−320 °F): 9% nickel, quenched and tempered or double normalised and tempered, with a 515 MPa minimum yield. Its test temperature sits well below LNG's boiling point of about −162 °C. Welding 9% nickel steel normally calls for high-nickel filler metals and impact-tested procedures

Grade 10 is the odd one out: a microalloyed carbon-manganese steel tested at −60 °C (−75 °F), with a minimum yield of 450 MPa, close to twice Grade 6's. Where pressure would push Grade 6 into a heavy wall, Grade 10 keeps it thinner, provided the design code gives it an allowable stress, which also eases the heavy-wall toughness problem.

Charpy impact requirements

Each test uses a set of three V-notched specimens, broken at the grade's test temperature. A333 sets the minimum absorbed energy by specimen size:

These limits apply to Grades 1, 3, 4, 6, 7, 9, 10 and 11, with straight-line interpolation between sizes. Grade 8 has its own acceptance criterion.

• Thin walls mean subsize bars, with the lower energy limits above
• The test temperature belongs to the grade: A333 lowers it only where a subsize bar is narrower than 80% of the wall, by the amount in its temperature-reduction table; any other colder test is a special requirement, agreed and written into the order
• Project minimums may be higher: state any higher energy, the specimen size and the test temperature in the enquiry

Heat treatment and why it matters

A333 requires most grades to be normalised at not less than 815 °C (1500 °F), with optional tempering. Seamless pipe may instead be made by controlled hot working, finishing at 845–945 °C (1550–1750 °F), then cooled from not less than 845 °C. Seamless Grades 1, 6 and 10 may also be quenched and tempered. Grade 8 is quenched and tempered or double normalised and tempered, and Grade 11 is annealed only by agreement between purchaser and manufacturer.

The purpose is a fine, uniform grain, which lowers the transition temperature. Welding, hot bending and forging reheat the steel and change that structure locally, so welds need impact-tested procedures, and hot bends should be checked against the code for re-heat treatment and fresh impact tests.

Choosing a grade: MDMT and the codes

The minimum design metal temperature is the lowest temperature the pipe wall may reach under any condition: cold ambient, refrigerated process, start-up, and the autorefrigeration chill when a liquefied gas depressurises. Propane let down to atmospheric pressure falls to about −42 °C.

The A333 test temperature is a property of the material. Whether it is enough is decided under the construction code:

• ASME B31.3 (process piping): paragraph 323.2.2 sets when impact testing is required. Table A-1 gives each listed material's minimum temperature, and carbon steels assigned a letter curve (A to D) take their exemption temperature from Figure 323.2.2A according to thickness. Check A333's entry in the edition you design to
• ASME Section VIII Division 1 (vessels): UCS-66 uses exemption curves by material group and governing thickness, with a further reduction in MDMT allowed where the required thickness is less than the thickness provided
• Welds: weld metal and heat-affected zone need procedures qualified with impact tests where the code requires them
• Every component: flanges, fittings, valves, bolting and gaskets must suit the same MDMT

The matching systems:

The trap is A350 LF2 Class 2, which is impact tested at only −18 °C (0 °F). An order for "LF2" without a class does not guarantee a −45 °C flange.

Worked example: grade and wall check for a −45 °C gas line

A typical case: an NPS 8 (DN 200) gas line in a cold climate. The lowest ambient temperature is −40 °C, and the blowdown study gives a lowest metal temperature of −45 °C, so the MDMT is −45 °C. Design pressure is 50 bar(g) (5.0 MPa, 725 psi), and the line is Schedule 40: 219.1 mm OD × 8.18 mm wall.

Step 1: grade. Grade 6 is tested at −50 °F. Converted exactly, that is (−50 − 32) × 5/9 = −45.6 °C, which A333 states as −45 °C and A350 as −46 °C. An MDMT of −45 °C is covered. If the project quotes −46 °C, avoid an argument about rounding and write "Charpy test at −46 °C or colder" on the order.

If the blowdown case reached −50 °C, Grade 6 would no longer be covered by its standard test. The next steps are Grade 10 (−75 °F = −59.4 °C), Grades 7 and 9 (−100 °F = −73.3 °C) and Grade 3 (−150 °F = −101.1 °C), or Grade 6 with a colder test agreed with the mill and accepted under the code.

Step 2: stress level. Hoop stress at design pressure is P × D ÷ (2t) = 5.0 × 219.1 ÷ (2 × 8.18) = 67.0 MPa, or 27.9% of Grade 6's 240 MPa minimum yield. This is not the code check, which uses the B31.3 wall equation, allowable stress, mill tolerance and corrosion allowance, but it shows the logic: pressure and corrosion set the wall, temperature sets the grade, and a stronger grade does not answer a toughness question.

Step 3: weight. At 7.85 kg/dm³, mass per metre = 0.02466 × (D − t) × t = 0.02466 × (219.1 − 8.18) × 8.18 = 42.55 kg/m (28.6 lb/ft). A 1,800 m line is 76.6 t, or 150 lengths of 12 m.

Step 4: the rest of the system. A350 LF2 Class 1 flanges, A420 WPL6 fittings, and welding procedures qualified with impact tests at −45 °C or colder where the code requires them.

Common Mistakes

1. Ordering "LTCS" or "A333" without a grade. A333 grades are tested anywhere from −45 °C to −195 °C, and an open enquiry invites the cheapest.

   Prevention: State the grade and the MDMT on every enquiry.

2. Setting MDMT from the weather alone. Depressurising a liquefied gas can chill a line well below ambient; propane falls to about −42 °C at atmospheric pressure.

   Prevention: Take the MDMT from process, start-up and blowdown cases as well as ambient.

3. Ordering "A350 LF2" without the class. Class 2 is tested at −18 °C (0 °F), not −46 °C.

   Prevention: Specify LF2 Class 1 for a Grade 6 system.

4. Mixing ordinary carbon steel parts into the line. A234 WPB fittings, for example, carry no mandatory impact test.

   Prevention: Use A420 WPL fittings and A350 flanges of the matching grade, and check valves and bolting for the same MDMT.

5. Forgetting the welds. Tough pipe joined by brittle weld metal or heat-affected zones is a brittle line.

   Prevention: Qualify procedures with impact tests at or below the MDMT where the code requires them.

6. Designing around a heavy wall that cannot be made tough. The standard warns that some heavy-wall sizes may not be available.

   Prevention: Confirm heavy-wall availability early, or consider Grade 10's higher strength to reduce the wall, after confirming that the design code lists an allowable stress for it.

7. Using Grade 3 at ethylene's boiling point. Ethylene boils at about −104 °C, colder than Grade 3's −100 °C test.

   Prevention: Order a colder impact test, or choose Grade 8 or austenitic stainless steel.

8. Assuming low-temperature pipe is sour-service pipe. A333 does not address hydrogen sulfide cracking.

   Prevention: Where H2S is present, add NACE MR0175/ISO 15156 requirements to the order.

Ordering checklist

• Specification and grade: ASTM A333 or A333M, Grade X; ASME SA-333 where the code calls for it
• Type: seamless or welded
• Size: NPS and schedule, or OD and wall; confirm heavy-wall availability
• Lengths and ends: random or fixed lengths; plain or bevelled ends
• Temperature: MDMT, the required Charpy test temperature, and any energy above the A333 minimums
• Quantity: metres, tonnes or lengths
• Matching parts: A350 flange grade and class, A420 fitting grade
• Service: sour service requirements and any supplementary requirements
• Documents: test reports, EN 10204 3.1 or 3.2 certificates (see our guide to material test certificates), third-party inspection
• Delivery: destination, Incoterm, marking and packing

Supply from Kasko Makine

Kasko Makine supplies ASTM A333 low-temperature pipe:

• Grade 1, Grade 3, Grade 4, Grade 6, Grade 7, Grade 9 and Grade 10 — seamless or welded (no filler metal; under A333, Grade 4 is seamless only), 19.05–114.3 mm OD, 2.0–14 mm wall, lengths up to 16,000 mm
• ASTM A333 Grade 6 seamless pipe — 1/4 in to 24 in, Schedule 10 to XXH, Charpy V-notch tested at −45 °C (−50 °F), for low-temperature piping, oil and gas processing, petrochemical plants and cold-climate installations
• ASTM A333 Grade 8 (9% nickel) pipe — for LNG storage and transport, cryogenic piping and ultra-low-temperature service
• Inspection and testing listed: chemical analysis; tensile, flattening, flaring, bend and hardness tests; impact tests; nondestructive testing; hydrostatic testing; surface and dimensional inspection
• Surface finish options: oil dip, varnish, phosphating, shot blasting
• Packing: bare, in bundles or in crates, with wooden end protection; seaworthy export packing on request; crates marked with order number, heat number, dimensions, weight and bundle details

Engineering support: Send the line list or each item's size, schedule and quantity, with the MDMT and the design code. We will check the grade against your MDMT and the flange and fitting grades, and quote the pipe.

Send the grade, size, schedule, minimum design metal temperature and quantity for a quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading

• Carbon Steel Pipe: Grades, Sizes and Standards
• Seamless vs Welded Steel Pipe: Differences and How to Choose
• Pipe Fittings: Types, Materials and Standards
• Pipe Flanges: Types, Faces and Pressure Classes
• NACE MR0175 / ISO 15156: Sour Service Materials
• Industrial Equipment Supply in Kazakhstan

Frequently Asked Questions

Q: What is ASTM A333 pipe?
A: ASTM A333 covers seamless and welded carbon and alloy steel pipe for low-temperature service and other notch-toughness applications. Every grade except Grade 11, which is annealed only by agreement, is heat treated, and every grade is Charpy impact tested at a temperature fixed by its grade, from −45 °C for Grades 1 and 6 to −195 °C for Grades 8 and 11. Tension tests are also required, and each pipe needs a hydrostatic or nondestructive electric test.

Q: What is the difference between A333 Grade 6 and A106 Grade B?
A: They share the same carbon, manganese and silicon limits and the same minimum strength, 415 MPa tensile and 240 MPa yield. A333 Grade 6 holds phosphorus and sulfur to 0.025 percent instead of 0.035, requires normalising, controlled hot working or, for seamless pipe, quenching and tempering, and is Charpy impact tested at −45 °C (−50 °F). A106 Grade B has no mandatory impact test, so its low-temperature toughness is not guaranteed.

Q: What is the minimum temperature for A333 Grade 6 pipe?
A: A333 Grade 6 is Charpy impact tested at −50 °F, which the standard states as −45 °C, and full-size specimens must average at least 18 J. Whether it may be used at a given design temperature is decided by the construction code, such as ASME B31.3 or Section VIII. For metal temperatures below its test temperature, use Grade 10 (tested at −60 °C) or a nickel grade, or agree a colder impact test with the mill.

Q: Which A333 grade is used for LNG and cryogenic service?
A: Grade 8, a 9 percent nickel steel, is impact tested at −195 °C (−320 °F), well below the boiling point of LNG at about −162 °C. It is quenched and tempered or double normalised and tempered, with a minimum yield strength of 515 MPa. Austenitic stainless steel is the usual alternative for cryogenic piping because it has no ductile-to-brittle transition. Grade 3, with 3.5 percent nickel, is tested at −100 °C.

Q: What are the Charpy impact requirements of ASTM A333?
A: For Grades 1, 3, 4, 6, 7, 9, 10 and 11, a set of three full-size 10 × 10 mm Charpy V-notch specimens must average at least 18 J (13 ft·lbf), with no single specimen below 14 J (10 ft·lbf). Smaller specimens taken from thin walls have lower limits, down to a 5 J average for 10 × 2.5 mm bars. Grade 8 has its own criterion, and project specifications may require more.

Q: Which flanges and fittings match A333 Grade 6 pipe?
A: Use ASTM A350 LF2 Class 1 flanges and forgings, impact tested at −46 °C (−50 °F), and ASTM A420 WPL6 butt-weld fittings, impact tested at −50 °F. Do not accept LF2 Class 2 for a −45 °C system, because it is tested only at −18 °C (0 °F). For 3.5 percent nickel Grade 3 pipe, the matching parts are A350 LF3 flanges and A420 WPL3 fittings.

Q: How do I choose the right A333 grade?
A: Start from the minimum design metal temperature, including cold ambient, process, start-up and depressurisation cases. Choose a grade whose impact test temperature is at or below it: Grade 6 to −45 °C, Grade 10 to −60 °C, Grades 7 and 9 to −75 °C, Grades 3 and 4 to −100 °C, and Grade 8 to −195 °C. Then check the code's impact rules and match flanges, fittings and welds.