Nondestructive determination of case depth in surface hardened steels by combination of electromagnetic test methods

The objective of this study was to improve the accuracy and reliability of nondestructive case depth determination from the current state of the art for application in an industrial environment. In the current state of the art only a single test method is used. In the present study simultaneous measurements were made with four independent electromagnetic test methods. The test methods used were measurement of tangential component of the magnetizing field, magnetic Barkhausen noise analysis, incremental permeability and multi-frequency eddy current measurement. The methods have different penetration depth and sensitivity to microstructure variations, thus complementing each other. The method that measures the tangential component of the magnetizing field is the only method that has a sufficiently deep depth of penetration to be useful for case depth testing. However, measurements with this method can be distorted by material variations other than the case depth. This distortion can be corrected by combining the measurement of the tangential component of the magnetizing field with a method that is mainly sensitive to the distorting effect. Such distorting effects can for example be a thin martensite layer on top of the case or a different quench oil temperature. From the 4 test methods used here 41 parameters were derived that describe the measurement signals. Multiple regression methods were used to select the most suitable parameters and build models from them. This procedure is called calibration. A separate calibration has to be performed for each different material. Models were built and the case depth testing accuracy was evaluated at 2 case hardened specimen groups. For one group the average case depth test error was in a range of ±15 μm. For another group the average test error was in a range of ±100 μm for case depths ranging from 1- 2.5mm. The results of the study show that the case depth testing accuracy and robustness could be improved by combining several independent electromagnetic test methods, thus providing industry with an effective quality control method for fast and reliable quality assurance