Low temperature steels are those which have been chemically formulated to maintain good ductility of parts in temperatures down to -150F. It must be noted that these steels are not considered as cryogenic materials but rather fill the gap between steels used at room temperature and cryogenic temperatures. Normal carbon steels and low alloy steels are prone to embrittlement at temperatures starting below about 32F. Steels for use in lower temperature areas must be formulated to minimize this embrittlement. Generally, lower C contents are found in steels in this group along with varying amounts of Ni up to 4.5%. Cr and Mo are also added in small proportions to special grades in this group to maintain useful strength levels.
These alloys must be heat treated to achieve their full property potential. The heat treatments include a normalize and temper treatment and at times are supplemented by a quench and temper treatment. Similar to carbon steels and low alloy steels, melting of these alloys require attention to the problem that they are gas pickup prone. They will pick up H from anything wet in the charge and they will pick up N from the atmosphere if held liquid in the furnace. Their melting is further complicated by the absorption of O from the atmosphere and requires exacting additions of Al to offset this tendency. The O pickup tendency is also increased in steels when the C content is below .20%. The lower the C content from this point down the more O the steel will dissolve. Control of residual elements is also critical in this group especially for P and S. Castability of these alloys is somewhat impaired because of reduced fluidity due to lower C contents. Because of the intentional formulation for maximum ductility these alloys are very crack insensitive in finishing department operations. Testing of steels in this group normally include a charpy impact test at a reduced temperature for assurance of properties.
Applications for this group of steels include any normal steel part which will be subjected to subzero temperatures. Northern latitude outside service equipment are good examples. It should be kept in mind that even equipment which normally operates at elevated temperatures are subject to low temperature embrittlement when left standing idle in low temperature conditions. It is normal practice to require testing at temperatures of 40F below lowest anticipated expose temperature.