Transition In Structure And Properties Of Ferromagnetic Materials Caused By A Low Frequency Alternating Magnetic Field

Magnetic treatment is a new material processing method with broad potential applications. Since the whole process is taken at ambient temperature, magnetic treatment has its own competitive advantages over other traditional technologies: effective, easy-operated and no additive distortions. However, due to the lack of rules and mechanism of magnetic treatment, at present the applications of this technology in industry are still very limited. Therefore, it is significant to optimize treatment parameters, explore the material properties' change tendency induced by magnetic treatment and further seek the mechanism of this high technique.In this paper, the changes in structures and properties of low alloy steel QStE420 caused by a low frequency alternating magnetic field were detected through expertiments and the mechanism was presented from the viewpoint of domain energy change. The main achievements are listed as follows.Firstly, a set of magnetic treatment device was developed for research purpose based on magnetic circuit design and calculation. Using this device, the excitation voltage, frequency, treatment time and direction can be effectively controlled.Secondly, after magnetic treatment with a certain group of parameters used by previous researchers, the extent of welding residual stress reduction in thin steel plate was investigated by drilling-hole method. The results have indicated that: (1) stress reductions by magnetic treatment are a general rule; (2) for a higher initial stress level the reductions are more remarkable. By orthogonal test, the degrees of four factors affecting residual stress reduction have been found: treatment time>field direction>field frequency>excitation voltage; and also when the above fators taking vules of "40s", "perpendicular to the welding beam", "2Hz" and "225V" in turn, the optimal parameter combination has been obtained.Thirdly, with the optimal parameter combination, the magnetic treatment effects on the mechanical properties (including elastic modulus, poisson ratio, yield strength, tensile strength, elongation and hardness, etc.) of annealed spicemens were measured. It has been found that after treatment the plasticity of specimen slightly increases, the hardness decreases and also the variations in same performance indexes of those spicemens apparently reduces.Further, the images of scanning electric microscope have demonstrated the extence of magnetoplastic deformation. As the frequency parameter changes, the tendencies of magnetostriction and magnetoplastic deformation differ. This has proved that magnetostriction cannot be the main cause of magnetoplastic deformation in steels.At last, on the basis of domain structure image and the analysis of magnetization course, the function between inner stress and the domain wall displacement was put forward and some of experimental phenomon induced by magnetic treatment were interpreted from the viewpoint of changes in domain structures and energy.These research achievements can be used for a further research on magnetic treatment mechanism.