Magnetic-field-induced microstructural features in a high carbon steel during diffusional phase transformation

In this work, a high purity, high carbon steel was heat treated without and with a 12-T magnetic field. The microstructural features induced by magnetic field during its diffusion-controlled austenite decomposition were investigated by means of optical microscopy and SEM/EBSD. It is found that the magnetic field increases the amount of the abnormal structure, which is composed of proeutectoid cementite along the prior austenite boundaries and ferrite around it, because magnetic field increases the austenite grain size and promotes the transformation of carbon-depleted austenite to ferrite. No specific orientation relationship between abnormal ferrite and cementite has been found in the non-field- or the field-treated specimens. Magnetic field evidently promotes the spheroidization of pearlite, due to its effect of enhancing carbon diffusion through raising the transformation temperature and its effect of increasing the relative ferrite/cementite interface energy. As magnetic field favors the nucleation of the high magnetization phase-pearlitic ferrite, the occurrence of the P-P2 OR that corresponds to the situation that ferrite nucleates prior to cementite during pearlitic transformation is enhanced by the magnetic field.