Defects caused by machining include cutting streaks and cutting heat. Streaks and scratches on the surface of the part act as a notch surface, and stress concentration occurs at the notch during use. Especially in the case of impact loads and alternating loads, the parts quickly break down. Rough grinding, when the pressure of the grinding wheel is too high, the ground surface is heated, causing the material to anneal, phase change, and even begin to melt. Pay attention to this problem when grinding hardened parts. The stress generated during the grinding process is the main cause of the grinding crack. The grinding crack is perpendicular to the grinding direction or has a mesh shape with a depth of up to 1 mm. When steel having a high carbon content and a carburized layer having a network of cementite are ground, cracks are particularly likely to occur. The spinning wheel crack analyzed below is an example.
The spinning wheel material is GCrl5, and the processing technology is blanking-forging type-destressing annealing-roughing-normalizing-spheroidizing annealing-finishing-quenching-tempering-grinding. After the forming, it was found that the rim of the rim had a crack from the top to the bottom in the longitudinal direction.
Observe the crack under a metallographic microscope and see several bright areas along the rim of the rim as shown in Figure 10-15. The crack originates from a white bright layer, see Figure 10-16. It was observed that the material had more severe banded carbide segregation.
Figure 10-15 White rim of the rim of the rim × 200
Figure 10-16 Crack originating from white bright layerHardness
The microhardness was measured from the surface white bright layer area to the test piece, and the hardness distribution is shown on the right side of Fig. 10-15.
Composition analysis was performed under a scanning electron microscope equipped with an energy spectrometer. The white bright layer contains 2.33% Cr, and only the white bright layer contains 1.48% Cr, and the internal banded carbide segregation contains 2.5% Cr.
2. Analysis Conclusion There is banded carbide segregation in the material. The large amount of carbides dissolved in the austenite at the segregation during quenching heating causes the martensite point to decrease in the amount of retained austenite. The presence of more carbides and retained austenite in the steel will seriously affect the conduction of grinding heat, and the residual tensile stress after grinding will also increase. The grinding surface is instantaneously heated rapidly, and the instantaneous temperature of the surface layer is higher than the critical temperature of the phase transformation and is cooled at a speed of 800-1000 ° C / s. The surface layer is quenched to produce a white bright layer. In addition, the surface layer during the rapid heating and cooling process, especially at the flange of the large plastic deformation, generates a large residual tensile stress.
The white bright layer has a hardness as high as HV1029. The softening layer hardness HV775 appears only in the white bright layer where the heated temperature is lower than the phase transition point. In order to confirm that the white bright layer is a quenched structure, the sample is tempered at 500 ° C, and after tempering, the self-bright layer becomes dark, and a tempered structure appears.
The conclusion is the segregation of the banded carbides of the raw materials. Thermal stress, tissue stress and secondary quenching of the surface during the grinding process result in longitudinal cracks at the flange.
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