Effects of Magnetic Fields on Martensite Transformations and Mechanical Properties of Steels at Low Temperatures

Abstract

Most existing or planned high field superconducting magnets use austenitic stainless steels for the magnet structure because these materials are available, paramagnetic, and tough at 4K. Many common austenitic (γ-phase) stainless steels are, however, structurally metastable at 4K. They prefer the bcc (α-phase),and will undergo spontaneous martensite transformations under appropriate conditions. Metastable austenite also appears in other classes of cryogenic structural steel. Some of the promising new high-Mn steels are metastable with respect to the indirect transformation γ→ε→α ,where ε-martensite is a hexagonal phase. Ferritic steels are commonly toughened for low temperature service by the precipitation of metastable austenite within the ferritic matrix. The martensite transformation is promoted both by high magnetic fields [1] and mechanical loads [2] in ways which are imperfectly understood, but which may affect structural performance or reliability in the high stress, high field regions of the large and powerful superconducting magnets now being developed. This paper briefly reviews the combined effects of magnetic fields and mechanical loading on this transformation, reports two initial experiments with a pulsed magnetic field, and describes a new testing facility which has been established to measure cryogenic mechanical properties in steady 8T magnetic fields.