Segregation of Varyingly Tempered Samples by Using Non-destructive Eddy Current Testing

The core objective of this research is to demonstrate the suitability of non-destructive Eddy Current Testing (ET) for discriminating differently tempered samples of ASTM A231 (Chromium Vanadium Cr-V) spring steel. The fundamental concept is that anything that influences eddy currents will influence the impedance of instruments’ coil and be discernable by the test circuit. The key factors causing impedance change are the test material conductivity, test frequency, proximity of the magnetizing coil to the test material and material’s permeability. Trends of ET signatures obtained from samples tempered at different temperatures (350, 400, 450, 500^oC) shows increase in both impedance and permeability with increase in tempering temperature. In microstructures analysis, the martensite (magnetically hard material) content is observed to be reduced at higher tempering temperatures as it is being converted to ferrite and cementite thus causing reduction in sample’s hardness; the same results are successfully verified by hardness testing. Thus increase in ET parameters (Permeability and Impedance) with temperature could be due to reduction in percentage of tempered martensite which is magnetically hard in comparison to ferrite and cementite. ET signatures of samples tempered at 350^oC for varying duration of time (10, 15, 20, 50, 60mins) showed increase in permeability and impedance with increment in tempering time. Microstructure analysis exposed more uniform distribution of phases and increase in the percentage of magnetically soft phases (ferrite and cementite) in samples tempered for long duration. Thus increase in ET parameters with time increment could be due to increase in the percentage of magnetically soft phases (ferrite and cementite) resulting in reduced hardness. To check repeatbility of results, the same procedure was successfully repeated at a lower temperature of 350^oC. The obtained results proved the success of this technique in identification of microstructures changes resulting from different heat treatments on the basis of permeability and impedance.  Inverse relationships of ET parameters with respect to hardness are obtained and owing to good linearity, these relationships can be extrapolated for prediction of useful service life of components. The research can be expanded for comparative analysis of differently tempered parts on industrial scale thus reducing the usage of lengthy metallographic evaluation.