Carbon steel shows a particular behavior at low temperatures that doesn't happen with other metals.
The particular behavior is that at low temperatures, always below 4 degrees C, carbon steel becomes fragile. This change from ductile (behavior at higher temperatures) to fragile (behavior at lower temperatures) is called the "ductile / fragile transition".
Unlike its behavior at high temperatures, which is perfectly known (so much that the allowable tensile strength is stated on ASME VIII starting from 600 degrees F or 160 C and up), the behavior of carbon steel at low temperatures is, until now, uncertain. It is not known at which temperature the ductile / fragile transition of a given carbon steel plate, or pipe, will take place. It may happen, for example, at plus 4 ºC or at minus 30 °C. The only thing we know for sure is that it WILL take place at a temperature below 4 ºC.
When carbon steel will be used at low temperatures applications, this uncertainty is a big problem. You can't build a structural steel construction in Alaska, with winter temperatures of minus 40 (C or F are equal at minus 40), using ASTM A 36 carbon steel that will become fragile at minus 10. So, when carbon steel is to be used at low temperatures, it's good practice to require an impact test at the lowest temperature it will have to support. This test may be carried out at the steel mill or at a specialized lab. With this information on hand, the project engineer will have a good idea on whether the material he intends to use is suitable for that application.
A curious thing is that only carbon steel shows this behavior. Other metals don't. That's why aluminum is widely used for cryogenics applications, such as liquid oxygen tanks in steel mills and liquid ammonia tanks in fertilizer plants.
Giovanni S. Crisi
Sao Paulo - Brazil
To Giovanni & dmilesdot,
Thank you very much for your technical explanations, now its enlightened me about impact testing.
To Giovanni, if you dont mind I would like to follow-up my question based on your explanation.
Is carbon steel the only metal required impact testing, just in case? what is the temperature basis that call for metal for impact test, it is the operating temp or the lowest ambient temp. expected in that certain location where project take place?
Best regards,
Ritz
Ritz, if one is working with High Pressure applications from ASME B31.3, ALL materials currently require impact testing.
The Committee has before it a proposal to change to "mostly" carbon steels as stainless steels and nickel alloys, which are common in today's fabrication and construction have exceptional low temperature properties but B31.3 Code, High Pressure Chapter has not kept up to date with these materials as no one from Industry has, until now, requested consideration.
You may find similar circumstances in other codes except that ASME Boiler & Pressure Vessel Codes seem more updated than parallel ASME Piping Codes.
Also, within ASME Boiler & Pressure Vessel Codes, thickness of material is often a consideration.
jon has already answered the first part of your question, i.e., the requirements set forth by the Code of Pressure Piping.
The second part would be to answer which are the requirements of the Pressure Vessel Code (ASME VIII). This I can't do now because I don't have the Code at hand. Please forgive me.
The third part would be to see which are the requirements for rotating equipment (compressors and pumps, for example) that work at subzero temperatures (piping and vessels are already covered by their codes). Does anyone know the answer?
Giovanni S. Crisi
More often than not, standards such as API 620 and in particular appendix Q of the same, several national standards Canadian CSA Z276-01, Mexican NOM-13, and others have specific requirements. As Giovanni has pointed out, some metals behave differently at sub zero temps, especially cryogenic temperatures. With the energy concerns being what they are around the world, research into liqified hydrogen, and other gases, you will very likely see this on a much more frequent basis as the various energy giants around the world attempt to restructure or build new storage and transmission systems for these cryogenic liquids.
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