Study On The Evaluation Of Weld Quality Based On Metal Magnetic Memory

The formation of welded structure is a complex metallurgical process,and the welded structure will be affected by tension,fatigue,high temperature and corrosion during operation.These factors will lead to stress concentration and welding defects,resulting in failure and fracture of welded structure.The metal magnetic memory detection technology is a new and green non-destructive testing technology.It is based on the force-magnetic effect of ferromagnetic specimen under the action of the geomagnetic field,and it can diagnose the macroscopic flaws and stress concentration of the loaded specimens in an early stage,so as to prevent the occurrence of disaster.Therefore,the combination of magnetic memory testing technology and weld detection will have broad application prospects in the quality evaluation of welds.Based on the magnetic memory testing technology,this paper mainly explored the effects of welding technology,material matching and welding defects on the mechanical properties,magnetic properties and fatigue life of welded structures from two aspects of experimental research and theoretical research,and established corresponding parameters of magnetic memory characteristics in different elasto-plastic deformation stages,make a fundamental research for the magnetic memory testing technology in the stress magnetization law and quantitative evaluation of weld defects.The ray detection,tensile test and magnetic memory signal measurement were performed on Q235 steel welding members under different welding currents and welding members of different materials(Q235,45#,40Cr),the mechanical properties and magnetic memory signals of specimens under different stress were explored.The experimental results indicated that the welding specimens with different welding current have different mechanical properties,and there is a close relationship between the magnetic memory signal variation law and the yield strength and tensile strength of low carbon steel weldments during the tensile process;The good welding components with different materials have different mechanical properties and magnetic memory characteristics.Matching conditions,mechanical properties and magnetic memory signal characteristics are intrinsically linked,and the welding strength and broken position are related to the material matching conditions.The study found that according to the basic principles of metal magnetic memory testing technology,the average magnetic signal information and magnetic signal gradient information under different stress,and the use of two-parameter information fusion technology,can effectively determine the mechanical properties and strength matching of the weld.The research results can provide reference for the application of magnetic memory testing technology in carbon steel weld quality evaluation.The ray detection,fatigue test and orthogonal magnetic memory signal measurement were performed on 40 Cr welding plates containing different invisible damages,and explored the characteristics of magnetic memory signals under the fatigue stress.The experimental results showed that the magnetic characteristics under single measurement method cannot characterize the change characteristics of the weld plate under fatigue cyclic loading,and the results are accidental;The study found that the magnetic field vector gradient integral feature and the magnetic field vector combined gradient feature can be used to evaluate the fatigue damage process of the weld plate,and established a fatigue damage model with the magnetic vector gradient feature as the damage parameter,so that the fatigue life of the welded component with invisible damage can be evaluated accurately.Based on the micromagnetic theory and the principle of energy minimization,the detection mechanism of metal magnetic memory was studied.The study showed that the change of the magnetic field strength on the surface of the component can be reflected by the transformation of internal energy.By calculating the distribution of the magnetization vector in the fixed domain of the magnetic domain in the defect region,the horizontal component of the magnetization intensity had a maximum value,and the vertical component showed a significant nonlinear change,that is,there was a phenomenon of "crossing zero";In order to further verify the theoretical analysis,the magnetic memory signal was measured on the butt weld.The obtained experimental results were consistent with the theory,that is,the characteristics of the leakage magnetic field at the component defects were consistent with the variation characteristics of the internal magnetization vector.Therefore,through the analysis of micro-magnetic principles,it was shown that the use of metal magnetic memory testing technology to evaluate the quality of the weld has a certain feasibility.