Development of Hot Seamless Tube Wall Thickness Gauge by Electromagnetic Acoustic Transducer

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