Magnetic Bomb Method--A New Method for Checking Surface Grinding Burns of Parts (2)

The physical manifestations of grinding burns are mainly due to the decrease in hardness caused by the change in the surface metallurgical structure and the residual stress (tensile stress) appearing on the surface. The test instruments shown in Figure 1 are sensitive to them (see Figure 2). The abscissa in Fig. 2a represents the hardness value Rc, and the ordinate represents the amplitude of the output B signal. As the surface hardness value Rc of the workpiece to be inspected changes from high to low, the amplitude of the corresponding B signal output by the detecting instrument will be small to large, that is, the detection signal corresponding to the low hardness is high, and the detection signal corresponding to the high hardness is low. The response of the instrument to the residual stress on the surface is shown in Fig. 2b. It can be seen that when the residual stress changes from small to large, that is, from negative (compressive stress) to positive (tensile stress), the amplitude of the corresponding B signal output from the detector will be low. High change.

Figure 2 can sensitively reflect the hardness and residual stress of grinding burns

2. Evaluate the feature value mp and its calibration

The detection signal generated by the above-mentioned specially designed excitation circuit and sensing device is a quantitative expression of the Barkhansen magnetoelastic effect, which is marked by the characteristic value mn (magnetoelastic parameter). The mp is proportional to the variability of the surface of the workpiece being tested, such as residual stress, and its value can be displayed and output on the screen of the instrument. However, the use of mp to reflect the degree of grinding burns on a workpiece is essentially a comparative measurement method. In order to be able to accurately and accurately describe it, the problem of "calibration" must also be solved. The calibration consists of two items: 1 Determine the boundaries of the nonconforming product. Purposefully produce a batch of samples, including some workpieces with different degrees of burn burn, use the pickling method to make different judgments according to the user's evaluation criteria, and pass several workpieces in the pass/fail critical state. The instrument obtains the corresponding mp value, and then takes the average value as the limit of failure; 2 performs calibration. Calibration is to find the correlation between the eigenvalue mp and the degree of burn burn confirmed by the pickling method. Specifically, it is necessary to determine a correlation coefficient MAGN, and use the dial on the instrument control panel to set, the MGAN value ranges from 0 to 99, and the general mantissa takes 5 or 0. For this reason, two workpieces with different surface states can be found in the previous sample, and a certain position on the workpiece is selected. When the MGAN value interval on the detecting instrument is 5 or 10, the static method is used to read the second. The corresponding mp value of the group, for example, when MGAN is 30, two mp values ​​are measured on two workpieces, and two are obtained when MGAN is 40, until MGAN=90. After the two subtractions are subtracted, a maximum value can be obtained, and the MGAN value at this time is used as the correlation coefficient, which is set on the panel.

Note: When actually performing the “scaling”, you can also use the sample in the first item to find the correlation coefficient MGAN, and then find the limit of the non-conforming product. Otherwise, in the previous operation, there will be some deviation from the limit value due to the arbitrarily set MGAN (generally 50 or 60).

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