Abstract
Ultrasonic inspection of welds joining dissimilar metals in nuclear power plants has proven to be a challenge, because the ultrasonic waves are subject to diffraction, distortion, scattering, and noise. These perturbations are due to their interactions with coarse-grained microstructures having anisotropic and heterogeneous metallurgical properties that can promote ultrasonic attenuation. In this paper, to improve the reliability of ultrasonic testing for dissimilar-metal welds (DMWs), ultrasonic beam characteristics for DMWs with a buttering layer were investigated in order to analyze the beam distortion phenomenon caused by inhomogeneous anisotropic properties and coarse grains. Ultrasonic testing was performed on DMW specimens using single ultrasonic transducers to investigate the behavior of the ultrasonic beam in the welds. According to the anisotropic and heterogeneous properties, when passing through the weld and the buttering layer of the DMW, ultrasonic waves were distorted and attenuation was high. In particular, in the case of using angular incidence that passed through the weld and the buttering layer in turn, the received ultrasonic data did not contain accurate internal information. From this, it was verified that internal defects may be detected by transmitting ultrasonic waves in different directions. Finally, the existing limitations on the application of non-destructive ultrasonic testing to dissimilar-metal welds were verified, and a solution to the measurement method was proposed.
Highlights
Welding and joining techniques are very important in nuclear power systems
Test specimens of different-metal welds were fabricated, and the specimens were used in the analysis of ultrasonic wave characteristics using single ultrasonic probes
The acoustic attenuation coefficients were calculated to evaluate the physical properties of the structure according to the dissimilar-metal welds (DMWs)
Summary
Welding and joining techniques are very important in nuclear power systems. This is because nuclear installations consist mostly of welded structures involving a wide variety of interconnected materials, and because the welds and joints are usually the most vulnerable areas. PWSCC occurs in dissimilar-metal weldment applied to the nuclear power equipment and piping under operating conditions typical of pressurized water reactors This cracking is generated by defects arising from stress corrosion because of the cracking of hardened oxide layers or arising from environmental conditions such as selective corrosion and inclusion elution. In addition to these weld characteristics, more inaccurate assessments are caused in detecting defects in the buttering layer in DMWs as applied to nuclear power plants To overcome such a limitation, studies on the nondestructive evaluation of welds of dissimilar metals using ultrasonic waves have been continuously conducted in recent years. The physical properties of the DMWs were measured by calculating the attenuation coefficients for each part of the DMWs, and the characteristics of the reflected ultrasonic signals were analyzed to reveal the internal defects
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