Takashi Furukawa

The Influence of structures of Nickel Base Alloy Weld Metals on Flaw Detectability by Ultrasonic Test

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.

Study of a methodology of identifying important research problems by the PIRT process

In this paper, we propose a new methodology of identifying important research problems to be solved to improve the performance of some specific scientific technologies by the phenomena identification and ranking table (PIRT) process which has been used as a methodology for demonstrating the validity of the best estimate simulation codes in US Nuclear Regulatory Commission (USNRC) licensing of nuclear power plants. The new methodology makes it possible to identify important factors affecting the performance of the technologies from the viewpoint of the figure of merit and problems associated with them while it keeps the fundamental concepts of the original PIRT process. Also in this paper, we demonstrate the effectiveness of the new methodology by applying it to a task of extracting research problems for improving an inspection accuracy of ultrasonic testing or eddy current testing in the inspection of objects having cracks due to fatigue or stress corrosion cracking.

Efficient defect detections of elbow pipes using propagation characteristics of guided waves

This paper proposes an efficient method of defect detection at elbow part of piping. The comparison between the simulation and experimental results shown in the paper implies regions with high sensitivity correspond to high-amplitude regions. The simulation results show that the locations of high-amplitude regions can be controlled by the center frequency of the input signal. Therefore, efficient defect detection at elbow part can be realized by using a suitable frequency for the target area.

Influence of shot peening and thermal ageing treatment on resistance to intergranular corrosion in shielded metal arc weld metal for type 600 nickel base alloy

Abstract Shielded metal arc weld metal for type 600 nickel base alloy (alloy 182) is used for weld components in nuclear power plants. To evaluate the intergranular corrosion resistance of alloy 182 after application of shot peening and subsequent thermal ageing treatment at 593–793 K, we conducted the corrosion test (immersed in boiled 16% sulphuric acid +5.7% copper sulphate aqueous solution at 57.6 ks) using specimens of alloy 182. The results show that the intergranular corrosion resistance of alloy 182 subjected to heat treatment at 893 K for 72 ks was improved by shot peening. Also, the intergranular corrosion resistance was not changed by thermal ageing treatment at 593–793 K subsequent to shot peening. Because remaining chromium depletion layers along grain boundaries were still observed by transmission electron microscope (TEM) after shot peening, disappearance of chromium depletion layers cannot be a factor in the improvement of the intergranular corrosion resistance. The results of measurement of surface residual stress by the X-ray diffraction method show that the compressive residual stress introduced by shot peening still remained on the surface of the specimens. Based on these observations, we assumed that chromium depletion layers along grain boundaries near the surface were dissolved by the environment of the corrosion test, the dissolved regions were closed by the compressive residual stress on the surface, and then the remaining chromium depletion layers were protected from the corrosive environment. This assumption explains why the intergranular corrosion resistance was improved although chromium depletion layers remained.