ニッケル基合金の溶接金属組織と超音波探傷による欠陥検出性の関係

Abstract

Ultrasonic Test (UT) method is the most appropriate method in many kinds of non-destructive test methods because of capabilities for detection and depth sizing of flaw. But it is generally said that the flaw detection in nickel base alloy welds is more difficult than that in ferritic steel welds because of dispersion and attenuation of the ultrasonic wave. Therefore, influences of welding conditions on flaw detectability and signal to noise ratio (SN ratio) of UT in nickel base alloy welds were investigated. Tested weld joints were prepared with multi-pass gas tungsten arc method. Partial weld joints were applied to weaving or magnetic stirring when that welded. Specimens having an electrical discharge machining slit in weld metal or in heat affected zone were tested by a longitudinal wave of 5 MHz in frequency and 45 degree in incident angle. Obtained results are as follows. Welded joints having columnar structures with the same growth direction showed high SN ratio that means good flaw detectability. Effective welding conditions, which make the columnar structure to be the same growth direction, were found to be the application of weaving or magnetic stirring. Same growth direction in columnar structures means same angle between the columnar structure growth direction and the ultrasonic wave propagating direction. Then, ultrasonic wave has almost same velocity in the weld metal, and the acoustic impedance will be almost same. High SN ratio may be caused by above phenomena. The SN ratio of weld joints by different base metal can be well explained by the difference of acoustic impedance at the boundary columnar structure and base metal.