Development of the Residual Stress Improvement Method for Small-Diameter Butt-Welding Pipe (Improvement of Residual Stress through Application of Delta-T Process)

Abstract

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. 1. 緒 言 近年，原子力発電プラントの高経年化に伴い，オーステナイト系ステンレス鋼製の高温純水配管溶接部におけ る応力腐食割れ(以下，SCC：Stress Corrosion Cracking)の発生が顕在化している (1) ．SCC は，高温純水の高酸 化環境，溶接部近傍の引張残留応力，熱鋭敏化等の材料劣化が重畳する条件下で発生することが知られており， 熱鋭敏化しにくい低炭素ステンレス鋼の場合でも，溶接構造物製造工程の機械加工や表面仕上げにより硬化する と，高温純水中における SCC 発生感受性が高まることが報告されている (2)(3)(4) ．このため，再循環系配管等の大 口径配管には SCC 発生抑制対策として，高周波誘導加熱応力緩和法 (5) (以下，IHSI：Induction Heating Stress Improvement)等の残留応力改善方法が適用されている．IHSI は配管内部に冷却水を流した状態で配管外面を加 熱することで配管内外面に定常状態の温度差を付与し， 温度差で生じる熱応力により配管内面を引張降伏させて， その後， 運転条件で圧縮に転じることにより残留応力を改善する方法である． 一方， 小口径配管 (公称外径 60.5mm 以下，公称厚さ 5.5mm 以下)については，定常状態で配管内外面に高い温度差を付与するのが困難なため，上述 の残留応力改善方法は適用されていない．しかしながら，小口径配管であっても，突合せ溶接部近傍内面の残留 応力は 300MPa を超える場合があることが報告されている (6) ．また，突合せ溶接部近傍の配管内面には機械加工 (開先加工)が施されており，機械加工面の硬化によって SCC 発生感受性が高まる可能性が考えられる．このた め小口径配管であっても，SCC 発生抑制の観点から残留応力を改善することが望ましい．

Behavior, Failure Analysis, and Effectiveness of Mechanical Stress Improvement Process in Residual Stress Relaxations in Butt-Welded Austenitic Piping Using a Numerical Simulation Approach

“…The following year, Ford et al (2012) [18] delved into the intricate realm of intergranular stress corrosion cracking (IGSCC) in BWRs, unraveling the complexities of this speci c form of corrosion within the nuclear domain. Transitioning into 2012, Aoike and Tsuruki's research [19]presents the development of the Residual Stress Improvement Method for Small-Diameter Butt-Welding Pipe, demonstrating the practical application of stress improvement techniques in small-scale, intricate welding scenarios.Sullivan and Anderson's assessment in 2013 of the MSIP for mitigating primary water stress corrosion cracking in nickel alloy butt welds within piping systems approved for leak-before-break signi es an ongoing commitment to nuclear safety [20]. The storyline progresses into 2014 with Bolognesi and their associates shedding light on the modeling and characterization of residual stresses in material processing, presenting a holistic perspective on residual stresses across various manufacturing processes [21].…”