High-T/sub c/ SQUID microscope study of the effects of microstructure and deformation on the remanent magnetization of steel

We have studied the effects of heat treatment and mechanical stress on the remanent magnetization of ferromagnetic steels using a high-transition-temperature (HTC) Superconducting QUantum Interference Device (SQUID) microscope. Samples were prepared by different heat treatments, which produced varied microstructures, and different rolling treatments, which produced varied levels of deformation. The samples were subsequently magnetized in fields of 50 mT, and the remanent magnetization was measured by rastering the sample over the SQUID using a two-dimensional (2D) translation stage with a scanning range of 50 mm/spl times/50 mm. With a separation between the SQUID and sample of approximately 0.5 mm, this produced a 2D magnetic field image due to the local remanent magnetization of the sample. In addition, microstructural information was determined using optical imaging, allowing us to correlate the effects of heat treatment and mechanical stress on local remanent magnetisation with detailed microstructural information. Since the strength and integrity of steels can be well predicted from microstructural information, correlation of this information with the 2D remanent magnetization images could lead to an effective method for the non-destructive evaluation of ferromagnetic steels through a simple measurement of remanent magnetization.