Research On The Rules Of Mapping Magnetic Properties To Stresses In Q245R Steel

Q245R is widely used in chemical, oil and other industries as a high-quality carbon structural steel for its simple production process, inexpensive and well process performance. In order to determine the actual stress situation of in-service equipment, and then to prevent the occurrence of fracture failure and major accidents, it becomes extremely important to test the structural stress state of some important structures. The evolution rules of the magnetic properties in different heat treatment state and different loading values for Q245R steel plate are analyzed. The stress state detection method for Q245R steel based on magnetic characteristic parameters is proposed.Pretreatment of samples such as normalizing, annealing, aging heat treatment were conducted. Corrosion and prefabricated defects were also considered. Surface magnetic field intensity was tested using TSC-2M-8 multi-channel magnetic memory tester, also coercivity was tested using MC-04-2 type multi-parameter test system under designed conditions.Specimen surface magnetic field intensity values with different pretreatment states were tested in the applied magnetic and stress field. Samples without applied magnetic field were tested. For specimen without defects, the average absolute value │Hp-1med│of magnetic field intensity decreases with the increasing tensile stress less than the yield strength. It is showed a linear relationship. For the specimen with defects, when the tensile stress below the yield strength, the maximum value of magnetic field strength and the tensile stress is in linear relationship. We can use the relationship of magnetic field strength and stress to test the stress state of the pressure equipment in-service.The surface magnetic intensity will increase with tensile stress for normalized, annealed, and corroded specimen after saturation magnetization. When tensile stress reaches nearly to the yield strength, the surface magnetic intensity growth rate will reduce. After aging, the rules of surface magnetic field strength are significantly different from other heat treated specimen. This is due to the redistribution of surface magnetic domain.Through the correspondence between magnetization characteristic parameters and stress of different materials, we found the rules that characterization of the stress state of material by using the magnetization characteristic parameter. When magnetic magnetization parallel to the tensile direction, the stress promotes domain displacement, and material is easily to be magnetized, and the coercivity value in this direction is less than the value at vertical stretching direction. Before tensile stress approached the yield strength, the coercivity have no obvious change. When stress exceeding the yield strength, the material internal magnetic crystal, magnetic stress anisotropy and micro-defects will change significantly, and the coercive force will also change intensively. The measured coercive force (Hc) values can be used to determine whether the material is in safe state.The mapping function between coercive force and stress was acquired through the test data, which can be used to evaluate the material stress state. The mapping rules between different magnetic parameters and stress are found based on the analysis of other magnetic parameters data. More accurate stress calculation function can be found based on the coercive force values.During the process of a pressure vessel hydrotest, the coercive force value of the weld seam decreased slightly with the increase of testing pressure when the tensile stress is less than 60MPa. The data test along the ring is larger than along the radial direction due to the circumferential stress larger than hoop stress. More comprehensive test should be done to judge the stress state of materials conveniently and quickly for the pressure vessel safety assessment.