Thermodynamic and kinetic effects of static magnetic field on phase transformations in low-alloy steels

Overview & Synopsis

This work is devoted to the quantification of the magnetic field effect on phase transformations in iron based alloys and steels. Pure iron, Fe-xNi substitutional alloys, Fe-xC-Mn steels and Fe-C-Mn-xSi steels are used to explore a wide range of transformations and microstructures. A high temperature dilatometer and a magnetic balance for magnetic susceptibility measurements are used for the in situ characterisation of diffusional phase transformation in magnetic field up to 16T. A third device has been developed for the treatment of materials in magnetic field and includes, for the first time, a quenching step using water, oil or pulsed gas. Non-equilibrium transformation temperatures measured up to 16T in a large set of materials are found to be increased by the application of a magnetic field. In a similar approach as the model used to estimate the effect of hydrostatic pressure on the phase equilibrium, the shift induced by the magnetic field is calculated. Microstructure characterisation of rapidly cooled plain carbon steels has shown large structural modifications induced by the magnetic field. From a mixture of bainite and martensite in the sample treated without any magnetic field, the austenite transforms into allotriomorphic ferrite and Widmanstätten ferrite in a matrix of bainite and martensite in the presence of magnetic field. This work brings solid evidence that magnetic field can be used as a new degree of freedom in physical metallurgy. Combined with existing techniques, it could lead to the successful processing of improved materials.