The Influence Of Magnetization State And Environmental Magnetic Field On Magnetic Memory Signal

Metal magnetic memory signal is weak,which is susceptible to many factors,so that metal magnetic memory method can not provide a reliable and accurate detection for engineering practice.In this paper,the influence of magnetization state,environmental magnetic field and heat treatment process on the change law of metal magnetic memory signal were analyzed from experimental and theoretical studies.A static load tensile test was applied to Q235 steel flat specimens with three different magnetization state,and the influence of magnetization state on magnetic memory signal and on magnetomechanical coupling relationship were discussed.The experimental result indicates that in the elastic stage and in the initial stage of plastic deformation,for the magnetized specimens,a applied stress make the initial magnetization state approach nonhysteresis magnetization state constantly,and the variation of magnetization state is determined by the distance between initial magnetization and nonhysteresis magnetization.For the magnetized specimens,the magnetomechanical changing trend of all the surface-sampling points is "gather" on the whole,but for the unmagnetized specimens,because of stress magnetizing specimen,the magnetomechanical changing trend of all the surface-sampling points is "stretch" on the whole.In the steady stage of plastic deformation,for the magnetized and unmagnetized specimens,the variation of magnetization is small,and basically remained stable.A static load tensile test was applied to 45 steel bar specimens under six different environmental magnetic field,and the influence of environmental magnetic field on magnetic memory signal and on magnetomechanical coupling relationship were discussed.The experimental result indicates that environmental magnetic field cannot change the shape of magnetic memory signal curve and magnetomechanical relationship curve,but can change the size of its value.In a certain range of magnetic field,the value of magnetic memory signal induced by stress increases with the increase of the environmental magnetic field,but the value of magnetic memory signal decreases when the environmental magnetic field exceeds a certain threshold value.When the environmental magnetic field is zero or complete offset,stress cannot produce magnetization effect,and magnetomechanical relationship curve is a horizontal line.The microstructures of 40 Cr steel metallographic specimens in five different heat treatment processes were observed and the magnetic memory signal values of the surface-sampling points were measured.The result shows that after the specimen was normally quenched,when the tempering temperature increases,the magnetic memory signal value constantly increases,the tensile strength decreases,but the plasticity enhances.A static load tensile test was applied to 40 Cr steel tensile specimens with five different heat treatment processes,and the influence of heat treatment process on magnetic memory signal was discussed.The experimental result reveals that the change law of magnetic memory signal curve and the gradient curve are similar to same for the original annealing,medium temperature tempering and high temperature tempering specimens,so are the standard quenching and medium temperature tempering specimens.Therefore,the impact of environmental magnetic field must be considered,and initial magnetization state and heat treatment process of the detected component can not be ignored,in the metal magnetic memory testing practice,especially the quantitative detection applications.