High temperature thickness measurements of stainless steel and low carbon steel using electromagnetic acoustic transducers

Thickness measurements were made on steel pipe at high temperatures using a non-contact watercooled EMAT (electromagnetic acoustic transducer) and also a laser-EMAT system where a portable Nd:YAG laser with a fibre optic cable was used to generate ultrasound on a sample, and a water-cooled coil-wound EMAT used to detect ultrasound. The set-up was designed so that the laser was fired through a hole in the centre of the EMAT, and a low pass 5 MHz filter employed to reduce the plasma noise. Back wall reflections were clearly visible at temperatures up to 900 °C on stainless and ferromagnetic low carbon steel, enabling wall thickness to be measured, taking into account thermal expansion of the sample. The water-cooled EMAT system can measure wall thickness on ferromagnetic low carbon steel at temperatures up to the Curie point; here, ultrasound generation is dependent on the magnetic state of the steel.