Study On Metal Magnetic Memory Characteristics Of Weld Joints And Defects In Q345R Steel

The purpose of this project is to study the feasibility of magnetic memory method for the detection of welding defects in pressure vessels.The weld and heat affected zone of the container are the most prone to cause container explosion and leakage,so diagnosis of the initiation position of the weld crack is very necessary.In this paper,Q345 R material is selected as the study object.Magnetic memory tests were carried out in the stretching process on specimens with the base materials,welded joints and welding cracks respectively.In the experiment,unloading method is adopted,which first removes the test piece for detection and then continue the loading.Totally 6 pieces are applied,each of which is measured for 20 times.Each piece has two sides,with three testing lines on either.Overall,580 groups of data are obtained.As the original magnetic signal data is chaotic,with occasional factors,it is difficult to clearly and accurately determine whether the specimen yields and where the yield point is.In this paper,a mean local derivative(?)method is put forward in the mathematical characterization of magnetic signal,which can eliminate the interference of accidental factors effectively,and provide the variation pattern of the magnetic signal in different deformation stages.Instructions for application of magnetic memory test in the engineering are provided accordingly.Main conclusions of the paper are as follows.(1)Experiments prove the randomness of the magnetic signal at the unloading stage for the three kinds of pieces.The problem of when to conduct the first magnetic memory test is solved.For pressure containers,the initial magnetic memory test should be carried out after the hydraulic test(after loading).(2)The principle for diagnosis of initial yield is provided,by applying the mean local derivative method.The characteristic of (?) of the yield point is given quantitatively for three kinds of Q345 R pieces.For pieces with base material or weld joints,the characteristic of magnetic memory signal is similar,where the local fluctuation of (?) is 2.In contrast,the fluctuation value at the yield point for the piece with crack is 4.