Article
Carbon steel becomes brittle as it gets cold. At room temperature it bends before it breaks; below a certain temperature it can crack suddenly with very little warning. For piping that will see sub-zero temperatures, whether from the process, from depressurisation, or from a cold climate, the steel must be proven tough at the lowest temperature it will meet. That is what low temperature carbon steel, or LTCS, is.
What makes steel suitable for low temperature
The chemistry is close to ordinary carbon steel, but LTCS is made to a fine grain, usually normalised, and impact tested. A Charpy V-notch test strikes a notched specimen at the specified low temperature and measures the energy absorbed. If the steel absorbs enough energy, it is tough enough at that temperature. The test is the whole point: without an impact test at the right temperature, a carbon steel is not LTCS whatever it is called.
The standards
| Product | Standard and grade | Usual test temperature |
|---|---|---|
| Pipe | ASTM A333 Grade 6 | minus 45 °C (minus 50 °F) |
| Flanges and forged fittings | ASTM A350 LF2 Class 1 | minus 46 °C (minus 50 °F) |
| Buttweld fittings | ASTM A420 WPL6 | minus 46 °C (minus 50 °F) |
| Plate | ASTM A516 with supplementary impact testing, or ASTM A537 | as ordered |
A350 LF2 Class 2 is impact tested at a higher temperature than Class 1, so the class must be stated on the order.
Where LTCS is used
- Refrigeration and liquefied gas plants.
- Lines that can cool sharply on depressurisation, such as flare and blowdown systems.
- Plant in cold climates.
- Sour and other critical service where the project calls for impact-tested material throughout.
For temperatures well below about minus 100 °C, carbon steel is not used at all; austenitic stainless steel or nickel steels take over.
What the certificate must show
The Charpy impact results, the test temperature, and the specimen size. Check that the test temperature is at or below your minimum design temperature. A certificate that meets the standard's chemistry and tensile requirements but has no impact results does not certify the material as A333 Grade 6 or A350 LF2.
What to put on the enquiry
The standard and grade, the class for A350, the minimum design temperature, and any supplementary impact requirement beyond the standard. See our note on reading a mill test certificate, and browse pipes and flanges.
Frequently asked questions
What temperature is A333 Grade 6 rated for?
It is impact tested at −45 °C (−50 °F), making it suitable for low-temperature service.
What flange material matches A333 Gr 6 pipe?
ASTM A350 LF2 forged flanges, which are impact tested at −46 °C.
What fittings match A333 Gr 6?
ASTM A420 WPL6 buttweld fittings.
Key takeaways
- Use impact-tested materials for service below about −29 °C.
- A333 Gr 6 pipe, A420 WPL6 fittings and A350 LF2 flanges work together.
- Check impact test results on the MTC.
Need these products?
Sawariya Metal Industries supplies stainless steel pipes, flanges and buttweld pipe fittings from Mumbai to buyers in India and overseas. Send your size, grade, standard and quantity through our enquiry form and we will reply with a clear quotation and the test documents you need. You can also browse our stainless steel pipes, flanges and buttweld fittings.