Sub-surface analysis of grinding burns with Barkhausen noise measurements

Barkhausen noise (BN) measurements are commonly used for surface characterization. However, often the need is also to verify the subsurface region because detrimental tensile stresses may be present after different manufacturing steps. Especially in a grinding burn, surface stress may be compressive but changes quickly into tensile stress below the surface. The aim of this study is to find out if regular surface sensitive BN measurement is sensitive also to below surface stresses caused by grinding burns. More specifically the aim is to study the relationship between BN features and subsurface stresses and to identify a model that estimates subsurface stresses. Real samples were collected from actual process. These samples were cylindrical samples manufactured from commercial alloyed steel AISI/SAE L6 that was through hardened prior grinding. Barkhausen noise measurements were carried out for 42 grinding burn locations followed X-ray diffraction based residual stress surface measurements and residual stress depth profiles. Depth information was gained through step-by-step electrolytic removal of thin layers. The stress profiles were preprocessed through interpolation and averaged stress was calculated as a function of depth below surface. Correlation analysis was carried out to evaluate the relationships between BN features and stress at different depths and also between BN features. The main result of the analysis was that BN measurement is dominated by the subsurface tensile stresses instead of the compressive stress at surface. It was also noticed that BN features form two groups corre-sponding to average Barkhausen activity and magnetizing field strength leading to maximum Barkhausen activity. Models for stress at different steps were identified systematically. The performance of models for subsurface stresses were reasonable having R2 values around 0.85 and root mean squared error (RMSE) values around 95 MPa. Base on the results obtained, it can be concluded that BN measurement provides information about subsurface stresses and that stress can be evaluated through modelling allowing detection of grinding burns.