Array coil-only EMAT for high-temperature defect monitoring in stainless steel

Conventional permanent-magnet electromagnetic acoustic transducers (EMATs) are limited by ferromagnetic-particle adsorption, strong magnetic attraction, bulky construction, and magnet demagnetisation during high-temperature operation, while increased lift-off further reduces the signal-to-noise ratio. To address these limitations, this study proposes a high-temperature ultrasonic testing method based on an array coil-only EMAT. The dependences of longitudinal-wave amplitude and radiated acoustic-field distribution on element number, element diameter, and centre-to-centre spacing were systematically analysed. A dedicated detection circuit was constructed, and experiments were performed to quantify the effects of these parameters on defect echo amplitude. Linear, planar, and heart-shaped coil arrangements were subsequently compared through high-temperature defect-detection experiments on stainless steel specimens. The results show that increasing element number and diameter and optimising element spacing enhance defect echo amplitude. Compared with a single-element coil, the four-element array increased defect echo amplitude by a factor of 3.91. The proposed system detected a Φ5 mm flat-bottom hole located 50 mm below the surface of a stainless steel specimen at 470 °C. These findings demonstrate the feasibility of magnet-free, couplant-free ultrasonic inspection of high-temperature stainless steel components and its potential applicability to curved or geometrically constrained configurations.