Non-magnetic stainless steels for cryogenic engineering

CIM Bulletin, Vol. 74, No. 831, 1981

H.W. KING, Engineering-Physics, Dalhousie University Halifax, N.S.

Cryogenic engineering is concerned with components and machines which operate for long periods of time at temperatures within a few degrees of absolute zero. The absence of a ductile-brittle transition is thus the prime component of the specification for a suitable low-temperature alloy. The word "cryo" means "frost-forming", and this leads to the second component of the alloy specification, which is resistance to corrosion in humid atmospheres. Further, although most early applications of low-temperature alloys were limited to the storage or transportation ofliquified gases, an increasing proportion of alternative energy storage and conversion technologies are being based on the availability of high-field superconducting solenoids. The third basic component of the specification for a low-temperature alloy is thus the absence of a low-temperature ferromagnetic transition.Although the first two of these components of the specification can be met by using austenitic stainless steels, many of these supposedly "non-magnetic" alloys have been found to undergo martensitic transformations to ferromagnetic phases at very low temperatures. In addition, steels which remain structurally stable may still exhibit other magnetic transitions, e.g. to antiferromagnetic or superparamagnetic states. The successful application of these alloys in cryogenic engineering has thus required an initial basic study of the types of magnetic transition that can occur in such concentrated solid solutions, followed by a more applied study directed at an alloy formulation which will induce beneficial magnetic states at low temperatures.
Keywords: Technology, Metallurgical sciences, Stainless steels, Cryogenics, Phase equilibrium, Ferromagnetic transitions, Alloys, Magnetization, Steel.