Martensitic transformation due to plastic deformation and magnetic properties in SUS 304 stainless steel

The relation between magnetic properties and the dislocation structure is studied in SUS 304 stainless steels. The absolute value of the α-phase is obtained by the spontaneous magnetization whose result is consistent with the X-ray analysis. We ascertained the reason that the magnetization is difficult to be saturated; it causes the magnetic shape anisotropy. The needle shaped α-phase is created by plastic deformation, which causes the coercive force, which depends on the maximum value of the anisotropy. The magnetic anisotropy is also measured by torque measurements. The spontaneous magnetization gives the ratio of the α-phase in volume; it attains to about 45% at the 100% strain. The induced Ms and the induced magnetic anisotropy recover by annealing above 700 K. The application to the nondestructive tests is discussed on the basis of the present investigation. The martensitic transformation depends on the internal stress, temperature and the applied stress. The first difficulty is that the magnetic parameters do not correspond simply to the dislocation density. The method due to magnetic measurements is difficult to apply the usage above 700 K, where almost dislocations remain in the substance but the α-phase recovers.