Effects of High Magnetic Fields on the Martensitic Transformation and on the Mechanical Behavior of Cryogenic Fe-9Ni Steel

摘要

The advancement of high field superconducting magnet technology requires structural steel that can bear large stresses at cryogenic temperatures in the presence of high magnetic fields. One class of candidate materials is high strength ferritic steels, such as Fe-9Ni steel, which are commonly toughened for cryogenic service by a two-phase tempering treatment that forms small austenite ( gamma ) precipitates. A fine dispersion of sub-micron sized gamma is retained upon cooling to cryogenic temperatures and its presence is believed to play an important role in the suppression of the ductile-to-brittle transition of this materials. The retained gamma , however, is metastable with respect to the martensitic transformation, and since the magnetic properties of gamma (paramagnetic) differ from those of the martensite ( alpha ') matrix (ferromagnetic), it is argued that a high magnetic field will influence the stability of gamma . This investigation was devoted to the study of high magnetic field effects on the martensitic transformation of retained gamma and the corresponding effects on the mechanical properties of cryogenic Fe-9Ni steel. Thermomagnetic exposure to 16 Tesla pulsed fields and thermo-magneto-mechanical exposure to 18.1 Tesla steady fields confirmed the presence of magnetically induced gamma implies alpha ' transformation in the liquid helium to room temperature range. The amount of transformation exceeded that expected from equilibrium thermodynamics and resulted in a detrimental effect on the mechanical behavior of this material. The detrimental effect of the gamma implies alpha ' transformation on the mechanical properties correlated to an increase in quasi-cleavage fracture surface area. Microstructural characterization of the gamma implies alpha ' transformation identified the crystallographic degradation of the martensitic matrix as the source of inferior mechanical properties. (ERA citation 10:013571)