Nondestructive detection of Microstructural Changes due to the Grinding Burn Using Electromagnetic Probes

Grinding burn is one of the most significant problems in grinding process with uncontrolled conditions and degrades the mechanical properties and performance of the part. The aim of the present study is to non-destructively evaluate the grinding burn phenomenon in steel parts. The parameters of the grinding process were changed to produce microstructural changes with different burn grades on the surface of steel parts. The results obtained from microscopic observations and micro-hardness tests revealed the occurrence of two types of burns including thermal softening (surface tempering) and re-hardening (formation of a hard white layer on the surface) burns. In the maximum burn grade, hardness of the surface increased from 600 HV to more than 900 HV, while hardness of 450 HV was obtained in the deeper layers due to thermal softening burn. In this study, the capability of two types of electromagnetic probes, measuring magnetic permeability and magnetic flux leakage, has been studied for detecting the grinding burn. The results show that the outputs of both probes were affected by the microstructural changes. Increase in tempered layer (due to thermal softening burn) enhances the magnetic permeability and reduces the flux leakage sensed by the probes. For the samples with re-hardening burn, effects of two opposing mechanisms (the surface hardening and softening) on the magnetic parameters have been assessed in the deeper layers. The present study indicates that utilizing the designed non-destructive probes, it is possible to detect the grinding burn, especially the thermal softening one, which happens more.