Research On The Test And Evaluation Method For Magnetic Characteristics Of Strain Aged20(G) Steel

20(G) high-pressure pipe is widely used in fertilizer plants and other enterprises. Pipe failures, even explosion accidents, were caused by the strain aging embrittlement in recent years. According to investigation, the manufacturing technology of in-service20steel pipes widely used in fertilizer enterprises were identical to the incident pipes, which means these pipes had the same risk factors. The magnetic characteristics were studied in the process of20(G) strain aging. Based on the test of magnetization characteristics, we proposed NDT methods for evaluating strain aging embrittlement degree. And different degrees of strain aging embrittlement pipeline were identified by building BP neural network.When20(G) strain aging occurred, there were relationships with its heat treatment state, the amount of residual deformation, aging temperature and time, the content of carbon, nitrogen atoms and so on. We put the residual deformation as the primary variable and simulated different degree of strain aging embrittlement sample. TSC-2M-8-type magnetic memory stress concentration measuring instrument and MC-04H-2multi-parameter test system were used for testing metal surface magnetic field and magnetization characteristic parameters in the process of material strain aging treatment. To study the change of material parameters of coercive force mechanism, we test the variation trends of surface residual stress. Hardness of the test material, impact absorption energy at room temperature were tested in order to obtain accurate material strain aging embrittlement degree.Randomly, we selected three kinds of nitrogen contents significantly different pipeline for testing the surface magnetic field characteristics of normalizing, annealing incomplete, hot-rolled state sample. We found that with the increase of residual deformation, the maximum magnetic field strength of the material showed single increasing trend. The mutation occurred in the test data when the residual deformation was5%. Combined with the room temperature impact absorbing energy trends of the sample and we found that if we use this method to evaluate material strain embrittlement degree, it was easy to misjudge strain aging embrittlement of material below5%plastic deformation. Due to the changes of magnetic field strength was small before and after aging treatment, it was easy to misjudge strain aging embrittlement which the material did not occur embrittlement at the residual deformation of more than5%. Different structure state will affect the result of the test.We tested the magnetization characteristics of the material and found that after plastic deformation, the magnetization characteristic parameters coercive force, remanence and hysteresis loss increased and the maximum differential permeability decreased.They all had single increasing and decreasing trend. Plastic deformation of2.5%, the coercive force, the remanence and the hysteresis loss of the material increased two to three times. The maximum differential permeability decreased by about10%. With continued increase in the amount of residual deformation, coercivity force, remanence and hysteresis loss would continue to increase and the maximum differential permeability would continue to decrease. But the magnitude of increase and decrease had slowed. Internal stress of the material would be released when aging treatment occured. The material would be more easily magnetized when stress on magnetic domain was reduced. At the same time, coercivity, remanence and the hysteresis loss was reduced and the maximum differential permeability was increased. However, the magnitude of the characteristic parameters increased and decreased no more than10%of the original data. After high temperature stress relief annealing, each parameter values would be back to the appropriate level of treatment without strain. Combining with the material physical and chemical test results, we found that you could select the coercive force which had biggest change magnitude, strong direction and high measurement accuracy as the evaluation of the degree of strain aging embrittlement characteristic parameters.Mechanism analysis of the coercive force changes of the materials, we found that it was mainly determined by the size and shape of the ferrite. After strain aging treatment, the change of coercive force could be explained by stress theory. By comparing the residual stress test results, we found that the stress theory could explain the process of the change tendency of the coercive force.Based on standard requirement of the impact absorbing energy and criteria impact fracture analysis results, we evaluated the degree of material strain aging embrittlement. Based on the above analysis, we obtained the nondestructive testing scheme for detecing the degree of strain aging embrittlement through20(G) coercive force test. Different structure state will affect the result of the test. Summary of the test parameters, we extracted physical parameters which was in favor of field testing and closely related to the degree of the material strain aging embrittlement. We used a professional software Neurosolutions5.0to establish artificial neural network model. We selected the BP neural network input vector which included the Vickers hardness, the coercive force, residual magnetization, the maximum differential permeability, hysteresis loss of the material. Also, output vector of the material we selected the impact absorbing energy which were sensitive to the degree of the material strain aging embrittlement. By training samples for BP neural network, we got the best convergence of the neural network structure. We tested the network structure and found that the trend of output was the same with real parameters. Finally, we predicted the degree of strain aging embrittlement and the trend of the result was correct. You could distinguish pipes which were serious strain aging embrittlement.