Tuesday, January 27, 2009

Low Temperature Metals

The low-temperature properties of metals have created some unusual problems in fabricating cryogenic equipment. Most metals lose their ductility and impact strength at low temperatures, although in many cases yield and tensile strengths increase as the temperature goes down.

Materials selection for low-temperature service is a specialized area. In general, it is necessary to select materials and fabrication methods which will provide adequate toughness at all operating conditions. It is frequently necessary to specify Charpy V-notch (or other appropriate) qualification tests to demonstrate adequate toughness of carbon and low-alloy steels at minimum operating temperatures.

Stainless Steels
Chromium-nickel steels are suitable for service at temperatures as low as -250°C (-425°F). Type 304 is the most popular. The original cost of stainless steel may be higher than that of another metal, but ease of fabrication (no heat treatment) and welding, combined with high strength, offsets the higher initial cost. Sensitization or formation of chromium carbides can occur in several stainless steels during welding, and this will affect impact strength.

However, tests have shown that impact properties of types 304 and 304L are not greatly affected by sensitization but that the properties of 302 are impaired at -185°C (-300°F).

Nickel Steel
Low-carbon 9 percent nickel steel is a ferritic alloy developed for use in cryogenic equipment operating as low as -195°C (-320°F). ASTM specifications A 300 and A 353 cover low-carbon 9 percent nickel steel (A 300 is the basic specification for low temperature ferritic steels). Refinements in welding and (ASME code-approved) elimination of post-weld thermal treatments make 9 percent steel competitive with many low-cost materials used at low
temperatures.

Aluminum
Aluminum alloys have unusual ability to maintain strength and shock resistance at temperatures as low as -250°C (-425°F). Good corrosion resistance and relatively low cost make these alloys very popular for low-temperature equipment. For most welded construction the 5000-series aluminum alloys are widely used.
These are the aluminum-magnesium and aluminum-magnesium manganese materials.

Copper and Alloys
With few exceptions the tensile strength of copper and its alloys increases quite markedly as the temperature goes down. However, copper’s low structural strength becomes a problem when constructing large-scale equipment. Therefore, alloy must be used. One of the most successful for low temperatures is silicon bronze, which can be used to -195°C (-320°F) with safety.

See more on: Belt carbon steel

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