Experimental Research Of Metal Magntic Memory In Weld Of Dissimilar Steel Pipes

The application of the heterogeneous welded steel pipe is becoming more extensive.Because of the non-uniformity of the Chemical composition and mechanical properties,the vicinity of the weld seam has defects easily. Therefore, to use nondestructive testingtechnology to examine the region is essential. With early warning advantage of the metalmagnetic memory, this paper proposed that the detection method is used in the detectionof the heterogeneous welded steel pipe.This problem was studied by using signal processing and experiment. Steel pipes ofQ345,15CrMo and Q245were selected, with stainless steel welding, and three testspecimens were made, which were stretched in the tensile test machine. With metalmagnetism memory tester, signals of magnetic flux leakage field were measured fromexperiment. In order to study the affect which is caused by defects, cracks were producedin the weld seam, and the magnetic leakage field distribution of weld seam with andwithout defects were contrasted. The wavelet denoising can reduce the interference,so thatmetal magnetic memory signal are more obvious. To extract a characteristic signal ofMMM, selective reconstruction of wavelet analysis was presented.Tests show that there is a great limitation of relying on zero-crossig of method ofphase component of magnetic flux leakage field. In weld seam with defect position,zero-crossing appear when load is large enough, but these points are not fixed. However,method of phase component of magnetic flux leakage field reaches maximum in weldseam without defect position, and the maximum is gradually reduced with increasing ofload; gradient usually reaches maximum in the vicinity of the weld, and the maximum isgradually reduced with increasing of load. the gradient in the weld seam witha crack parallel with the detecting direction is smaller than in the weld seam witha crack vertical with the detecting direction, the appearance of zero-crossing in the weldseam with a crack parallel with the detecting direction is earlier than in the weld seamwith a crack vertical with the detecting direction, and they are smaller than gradient in theweld seam without cracks. For the heterogeneous welded steel pipe, experiment results show complete annealingtreatment plays an important role in improving weld quality. The quality of theheterogeneous welded steel pipe is indirectly evaluated by numerical analysis afterheat-treatment having two inspection function of defects in the vicinity of the weld seamand heat treatment quality.On the MMM signal of heterogeneous welded steel pipe processing, traditionalwavelet transform have some shortcomings.According to the shortcomings, selectivereconstruction of wavelet analysis was applied to feature extraction of MMM signals.The results indicate that the method is not only without load restrictions, but also beable to conduct a preliminary estimate the size of the cracks.The research of this paper has a certain significance to guide the application ofMMM in the detection of the heterogeneous welded steel pipe.