Experimental Study On Ferromagnetic Specimen Test Based On Magnetic Conductivity Detection Technology

The magnetic permeability detection technique is a kind of method to evaluate the change of the permeability of specimen,which based on the electromagnetic induction principle,that is proportional to the change rate of the induced voltage and the magnetic flux in the closed magnetic circuit.The detection technique can detect the changes of magnetic permeability in a certain area,like stress concentration,fatigue damage,structural phase transition,aging transformation.And it can detect the change of mechanical toughness and mechanical brittleness,the transformation of martensitic and austenite,the detection of dislocation defect and density changes,and changes of grain boundary.This paper first introduces research background of magnetic permeability detection technology and the research status at home and on abroad.Based on the knowledge of electromagnetism,ferromagnetism,metal physics,electromagnetic detection,heat treatment and so on,the detection mechanism of permeability testing technology is expounded.From the principle of electromagnetic induction and the law of magnetic circuit Ohm,the permeability detection technology detection methods and detection principles are analyzed.According to the principle of permeability detection,a high sensitivity detection sensor is designed for the rod-like component.The influence of the coil diameter,the number of turns of the coil and detection frequency of the excitation voltage and the detection resolution are studied.It is found that the optimal excitation frequency decreases with the increase of the diameter,and the signal resolution increases with the increase of the diameter.The number of turns is independent of excitation frequency,but it is related to the signal resolution.The excitation coil is used as the excitation source.The optimum excitation voltage is 5V and the optimum detection frequency is 250 Hz.And the good excitation coil and the detection coil are 400 turns.The tensile and fatigue tests of Q235 steel and 45 steel were carried out respectively.And the experimental study on heat treatment of 40 Cr steel was carried out.The relationship among the permeability parameters,including tensile stress,residual stress and heat treatment,which were analyzed.In the tensile test,it was found that the change of absolute signal of Q235 steel was 0.33 V and the relative signal change could reach 8%.For 45 steel,the absolute signal change was 0.3V and the relative signal change could reach 7.6%.The response of the detection signal to the residual stress of the tension is very sensitive;The change of the absolute signal of Q235 steel is 0.8V and the relative signal change can reach 20%;For 45 steel,the absolute signal change is 1.0V and the relative signal change can reach 26%.The detection sensitivity of fatigue damage is low;The relative sensitivity of the Q235 steel is 0.13 V,the relative signal change can reach 3.3%;For the 45 steel,the absolute signal change is 0.035 V,and signal changes can reach 1%.In the heat treatment test,the permeability signal decreases with the increase of the quenching temperature.Under the same quenching temperature,permeability signal increases with the increase of tempering temperature.According to the magnetic field detection technology,the specimen of the stress state and the residual stress state can be measured effectively.According to the residual stress left after the stress action,maximum stress experienced by the member can be judged and the stress foreground condition of the component is measured.The conductivity signal value can effectively determine the quality of the heat treatment process.