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The primary requisite for steels for low-temperature service is a high fracture toughness—the ability to resist the rapid crack propagation associated with brittle failure. It is well known that the yield strengths of ferritic steels increase markedly as the temperature decreases and, if the fracture toughness is high enough so that rapidly propagating fractures do not occur, advantage can be taken of this higher yield strength; thus the low temperature becomes an advantage rather than a disadvantage. On the other hand, since fracture toughness decreases as the temperature decreases, if the fracture toughness is low, rapid fracture can propagate at the low temperature from very small defects at nominal stress levels which may be below even the ambient-temperature yield strength. Thus, the usefulness of steels for low-temperature service is primarily dependent upon their having a high enough level of fracture toughness to permit their use at allowable stresses commensurate with their strength at the low temperature without danger of rapidly propagating fractures. Ideally, then, for design purposes or to establish the suitability of steels for reliable structures for low-temperature service, fracture toughness should be evaluated in terms of a limiting stress for brittle fracture in the presence of defects of a limiting size for application at a given temperature. Information of this type would permit the designer to design structures for use at low temperatures which would be safe with respect to brittle fracture in the same manner in which they are accustomed to design structures for elevated-temperature service, based on allowable stresses for such service.
Hodge, John M.
Applied Research Laboratory, Monroeville, Pa