Monitoring of the thickness of stainless steel pipelines at high temperatures using a coil-only EMAT

The ultrasonic monitoring of internal defects and residual wall thickness online measurement in pipeline welds and elbows under high-temperature and high-pressure conditions has substantial engineering significance. It tracks defect growth and thinning rates, enabling hazard warning and life prediction. This study establishes a finite element (FE) model of the examination process using a coil-only electromagnetic acoustic transducer (EMAT). It analyses the effects of brazed copper plates and thickness variations on EMAT excitation efficiency in stainless steel specimens, while simultaneously studying the variation laws of the amplitude of ultrasonic echo signals at different temperatures. Finally, an experimental system is constructed, with tests conducted on specimens with varying brazed copper plate thicknesses under different temperatures. Experimental results indicate that brazing a 0.1 mm-thick copper plate onto the specimen improves the signal-to-noise ratio (SNR) of ultrasonic waves by 12.04 dB. At 650°C, the SNR remains ≥7.91 dB, with a thickness measurement error of 0.57%. Furthermore, a linear array of coil-only EMATs successfully detects a flat-bottom hole in a stainless steel specimen without a brazed copper plate at 460°C, demonstrating the feasibility of full-scale stainless steel pipeline monitoring. The proposed method significantly enhances the applicability of coil-only EMATs in harsh industrial environments.