The development of non-contact laser and EMAT ultrasound measurement systems for hot steel
Non-contact laser and EMAT ultrasound systems have been developed for ultrasound measurements on hot steel. The pulsed lasers are capable of generating wide bandwidth ultrasound pulses and the transduction is entirely remote. The EMATs operate in close vicinity to the metal testpiece and can be used for both generation and detection of ultrasound. EMATs require no special surface preparation of the steel samples and the performance may even be enhanced by the presence of oxides on the surface. The EMAT can be operated either using a permanent magnet (P-EMAT) or a pulsed field electromagnet (E-EMAT); the latter is capable of producing a magnetic field over 1Tesla and would allow the easy removal of the ferromagnetic debris from the EMAT face. The EMATs were water cooled to allow operation at elevated temperatures. The EMATs were used to generate shear waves and as the detector they were sensitive to both the shear and longitudinal waves. Both types of EMAT are capable of making ultrasound measurements on steel up to a temperature of about 800°C in pulse echo mode when the same EMAT was used for both generation and detection. A water cooled E-EMAT was used as the receiver of laser generated ultrasound. The system operated in send-receive arrangement where the EMAT is positioned on the generation side and measurement can be made from a single side has been made up to a temperature of lOOO°C. The measurement was carried out by using a Nd: VAG laser as the generator. The E-EMAT also has been used to receive ultrasound generated by a pulsed CO2 laser and could make ultrasound measurements up to a temperature of about 800°C. Besides; the laser-EMAT system was also used for epicentral measurements at elevated temperatures. Ultrasound measurements by a shear wave EMAT on ferromagnetic steel have been made using the shear wave arrivals in a range of temperature up to about 750°C where the EMAT is sensitive to both shear and longitudinal wave. Above this temperature however, ultrasound measurement was based on longitudinal wave arrivals as the EMAT is no longer sensitive to shear wave. At a temperature close to the Curie point, Te, of the sample, enhancement of longitudinal wave occurs. This may correspond to the reorientation and redistribution of the magnetic flux in a thin ferromagnetic layer on the bulk paramagnetic sample due to the cooling effect caused by the water-cooled EMAT or may be due to volume magnetostriction. We have monitored the magnetisation of the sample at elevated temperature and noted that the magnetisation of the sample reduces rapidly to zero as the temperature increases and approaches the Te' The cooling effect on hot samples caused by momentary contact with the water cooled EMATs was measured and shown to be minimal and superficial. The laser-EMAT ultrasound system could be used for thickness measurement on hot steels provided the velocity is known as a function of temperature. A series of measurements was carried out on steel samples in the laboratory to determine this data. The technique produces no significant damage to steel and has the potential of being rapidly scanned to test hot moving metals on the production line.