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Austenitic stainless steels are extensively used for service down to as low as liquid helium temperature, (-269 oC). This is largely due to the lack of a clearly defined transition from ductile to brittle fracture in impact toughness testing.
Toughness is measured by impacting a small sample with a swinging hammer. The distance which the hammer swings after impact is a measure of the toughness. The shorter the distance, the tougher is the steel, as the energy of the hammer is absorbed by the sample. Toughness is measured in Joules, (J). Minimum values of toughness are specified for different applications. A value of 40 J is regarded as reasonable for most service conditions.
Steels with ferritic or martensitic structures show a sudden change from ductile, (safe), to brittle, (unsafe), fracture over a small temperature difference. Even the best of these steels show this behaviour at temperatures higher than -100 oC, and in many cases only just below zero.
In contrast austenitic steels only show a gradual fall in the impact toughness value and are still well above 100 J at -196 oC. See Selection of stainless steels for cryogenic applications.
Another factor in affecting the choice of steel at low temperature is the ability to resist transformation from austenite to martensite. This factor is discussed in more detail in Composition effects on the magnetic permeability of austenitic stainless steels.