Eddy Current False Indications in Austenitic Steel and Titanium Alloys Heat Exchanger Tubes Activated by Stress

Abstract

Eddy current non-destructive examination with the internal bobbin type probes is a mandatory test for heat exchanger tube manufacturing due to its high sensitivity to the different type discontinuities. In this paper, we want to highlight the problems related to the false indications in the nonmagnetic austenitic steel and titanium alloys due to magnetic inclusions. There are known origins for magnetic inclusions in stainless steel like chips or filings as a result of ferrous tool application, alloy depletion concerned with oxidation or corrosion, and magnetite deposits. The little-known origin of magnetic inclusions is concerned with stress induced martensitic transformation is shortly analyzed in this report as a possible mechanism for the creation of the magnetic inclusion in austenitic steel as in titanium alloy tubes. The phase transformation and as the result the presence of magnetic inclusions is generated by the activation of the spin magnetic moments of the free electrons under stress influence. These inclusions can radically influence the eddy current signal due to quite large difference in relative magnetic permeability but can be differentiated from real defects by re-inspection of the suspicious tubes at reduced operational frequency. During the eddy current examinations of stainless steel and titanium tubes by an internal probe, the false indications in tubes have been detected. The presence of these isolated indications is located always in the same length of the tested tube. The indications were detected in each tube and the signal amplitude was estimated in the range of 40-50% of the acceptance reference level. In stainless steels, the false indications generated are higher than in titanium alloys.