The electromagnetic acoustic transducer system has been developed for the measurement of hot seamless steel tubes and pipes. The system consists of tunnel type electromagnet which generates the strong axial magnetic field around the pipe, and six sensors including transmitting and receiving coils which are arranged between poles and around the pipe. The system can measure 6 circumferencial points wall thickness of the pipe at the rate of 250 Hz.
The ultrasonic properties in hot steel such as velocity and sensitivity have basically been investigated. The measuring accuracy of the system has also been investigated in the laboratory and at No. 1 tube making plant in Kainan Steel Tube Works of Sumitomo Metal Industries, Ltd. The basic experiments showed that ultrasonic velocity can be calculated by measuring the surface temperature, and that the sensitivity changes due to the changes of air-gap and surface temperature can be compensated by AGC (automatic gain control) circuit. The system was positioned at the outlet of the finishing stand of the stretch reducer mill, and pipes of various sizes (42.7 mm-177.8 mm dia.×3.2-25 mm thickness) were produced to measure the wall thickness. The results showed that the measuring accuracy was less than±0.1 mm.
