Influence Mechanism Of Surface Roughness And Curvature Radius On Magnetic Flux Leakage Testing For Bearing Steel Pipe

Magnetic flux leakage(MFL)detecting technology has been widely used in the non-destructive testing of ferromagnetic materials due to its reliability and high efficiency.Bearing steel pipe which used in the manufacture of precision steel parts requires the most stringent quality assurance,and the testing accuracy requirements of the MFL detection equipment are getting higher and higher.Therefore,it is necessary to seek the factors that restrict the detection capability of conventional MFL testing equipment,find out the difficulties and shortcomings,and put forward the corresponding measures to achieve more accurate MFL detection.First,the disturbance caused by surface roughness to the magnetic field distribution on the workpiece surface is studied,it is shown that:the magnetic disturbances formed by the roughness contour peak and contour valley are opposite to each other,and it is obtained that the typical surface MFL signal feature caused by surface roughness is"approximate sine" shape.The vector superposition formulas of the leakage magnetic field near the crack are deduced according to the different relative positions of the micro crack and its nearest roughness profile.The mechanism of the influence caused by the surface roughness on the formation of the leakage magnetic field near the micro crack is revealed.Through the finite element simulations and experiments,it is found that the surface MFL signal increases with the increase of Ra or RSm,and the influence rule of the surface roughness geometric feature on the micro crack MFL signal is obtained.The effects of the surface roughness and crack geometric features on the signal-to-noise ratio(SNR)are studied.With the increase of the surface roughness and the decrease of the crack depth,the signal SNR decreases,and with the increase of the crack width,the signal SNR increases first and then tends to be stable.Obviously,it is more difficult to identify the micro crack with smaller depth and width on rough surface.Based on the finite element simulation results,numerical fitting method is used to obtain the reference theoretical formula which describes the detection limit of the crack on rough surface.And a MFL method of detecting the micro crack on rough surface by adding magnetic material into the air lift-off gap is proposed.Then,the influences of different curvature radius on the circumferential magnetization of the steel pipe are studied and obtained as follows:on one hand,the magnetic flux density of the inner wall is always greater than that of the outer wall for the same steel pipe;on the other hand,with the increase of the curvature radius,the circumferential magnetic intensity inside the steel pipe increases first and then decreases slowly for different pipes.Combining the analysis that the magnetic compression of the background magnetic field in the air has the influence on the leakage magnetic field of the crack,the mechanism of the influence on the MFL detection of the axial crack caused by the pipe curvature radius is revealed.Aiming at the difficulty of detecting the inner or outer surface axial crack on small diameter pipe with high precision,several specific methods are proposed to optimize the magnetization state of steel pipe and the sensor arrangement.Finally,a precision MFL testing method for steel pipe with double-sided arrangement of magnetic probes is proposed.There are two magnetic probe strategies according to the surface condition:the zero lift-off magnetic head is utilized to obtain the highest sensitivity when the surface is smooth;the magnetic head with magnetic material added into the air lift-off gap is utilized to obtain the better SNR for rough surface.The technology is applied to the automatic MFL testing of bearing steel pipe,and the micro crack with the size of 6.25mm(length)X 0.2mm(width)X 0.1mm(depth)can be detected effectively whether the crack is on the inner or the outer wall,which breaks through the detection accuracy limit of the conventional MFL testing equipments with single-sided arrangement of magnetic probe.