Improved Methods of Flaw Detection in the Walls of Small-Diameter, Thin-Wall Seamless Tubes Using Ultrasonic Immersion Test Techniques
Using the standard pulse-echo techniques of ultrasonic immersion testing for the detection of discontinuities in the walls of tubes for critical applications, difficulty may be experienced in the detection of certain types of discontinuities that do not provide a good reflector for the ultrasound. Unacceptable defects in the tube-wall may remain undetected, leading to potential in-service failure of the tube.
Successful results had been obtained for some years in detection of such defects in thick-wall tubes of 16 mm OD and greater using focused immersion transducers in the pitch-catch technique. Jigs located in tube- follower units have been used to maintain the essential probe-to-tube relationship. Arithmetic processing of the received signal has been used to enhance discontinuity detection and to suppress signals from such effects as tube eccentricity.
By careful control of the tube position and by positioning of the transducers in fixed manipulators, it has been possible to maintain the probe-to-tube relationship to a degree of precision sufficient for the application of the through-transmission technique to tube sizes in the OD range 6 mm to 15 mm. Re-positioning of the transducers to detect the smaller discontinuities encountered has been successfully achieved.
Further work needs to be undertaken to develop applicable standards for the range of tube sizes to be tested. However, repeat-ability of set-up sensitivity achieved so far indicates that the technique is applicable to the testing of small diameter tubes for critical applications.