DETECTION OF DEFECTS OF STRUCTURES FROM FERROMAGNETIC STEEL THROUGH THE LAYER OF ANTICORROSION COVER WITHOUT REMOVAL
DOI:
https://doi.org/10.31471/1993-9981-2021-1(46)-5-14Keywords:
eddy current method, flaw detector, double differential probe, crack, sensitivity, noise, inspection productivityAbstract
The detection of the cracks in ferromagnetic steel components by the eddy current method is formidable because of the additional noise originated from magnetic and structural heterogeneities of the inspected material. In this paper, several techniques proposed for the detection of the defects in ferromagnetic steel components are analyzed. One of the possible approaches is based on the additional magnetization of the inspected zone to minimize magnetic heterogeneity. Another trend is concerned with new selective EC probe development. This inspection problem is exercisable by the double differential EC probes application. These probes were developed some decades ago and are characterized by a specific quasi-absolute signal with maximum amplitude when the probe is situated directly over a crack. The main features of such probes can be characterized with: high sensitivity to elongated (like crack) and to local (like pitting or pore) defects; high sensitivity to surface and subsurface defects through the protective coating or with large clearance between the probe and inspected surface; high penetration for low-frequency probes; high lift-off noise suppression. In this study the possibility to detect the cracks through the anticorrosive coating was investigated. Special detectability diagrams were created to estimate the ultimate thicknesses of dielectric coating for the detection of the defects with different depths. For inspection of the objects with large overall dimensions (like tubes, forgings, etc.) array type EC probe (named EDDYLINE) was developed. High sensitivity width (near 60 mm) was achieved to obtain high inspection productivity. At the same time, the high locality was provided because all EC probes are operated separately. Developed EC inspection techniques based on the EDDYLINE type EC probe were successfully applied for the detection of cracks in ferromagnetic and stainless steel tube forgings.
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References
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