小口径突合せ溶接配管に対する残留応力改善方法の開発（デルタＴ工法の適用による残留応力の改善）

概要

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To prevent stress corrosion cracking (SCC) initiation at the inner surface of the butt-welding region of a small-diameter pipe, a residual stress improvement method has been developed. In this method, the butt-welding region is heated up to 325 degrees centigrade in air. After heating, a water quenching process is applied to the inner surface of the butt-welding region. During a water quenching process, a large thermal stress is generated by the temperature difference between inner and outer surfaces. When a thermal stress exceeds the yield stress of piping material, a tensile yield is generated at the inner surface of the butt-welding region. Because a tensile yield changes into a compressive residual stress, a compressive residual stress can be introduced at the inner surface of the butt-welding region after application of this method. In this study, residual stresses at the inner surface of the butt-welding region were measured. The measured residual stresses proved this method effectively reduces residual stresses and prevents SCC initiation.