Comparative study of eddy current and Barkhausen noise nondestructive testing methods in microstructural examination of ferrite–martensite dual-phase steel

• Normalized impedance decreased as the ASTM grain size number increased.
• An increase in martensite percentage resulted in a decrease in normalized impedance.
• As the martensite in the DP steels increased, the MBN signals increased.
• Barkhausen jumps increased with increasing the ASTM grain size number.
• Both ECT and MBN had a high potential to detect microstructural changes of DP steels.

The magnetic properties of ferrite–martensite dual-phase steels were evaluated using eddy current and Barkhausen noise nondestructive testing methods and correlated with their microstructural changes. Several routes were used to produce different microstructures of dual-phase steels. The first route was different heat treatments in γ region to vary the ferrite grain size (from 9.47 to 11.12 in ASTM number), and the second one was variation in intercritical annealing temperatures (from 750 to 890 °C) in order to produce different percentages of martensite in dual-phase microstructure. The results concerning magnetic Barkhausen noise are discussed in terms of height, position and shape of Barkhausen noise profiles, taking into account two main aspects: ferrite grain size, and different percentages of martensite. Then, eddy current testing was used to study the mentioned microstructural changes by detection of impedance variations. The obtained results show that microstructural changes have a noticeable effect on the magnetic properties of dual-phase steels. The results reveal that both magnetic methods have a high potential to be used as a reliable nondestructive tool to detect and monitor microstructural changes occurring during manufacturing of dual-phase steels.