本研究では、PWRプラント運転を想定した熱時効初期の残留応力緩和メカニズム解明の一環として、LP、WJP、バフ研磨の三工法および機械加工を施した600合金について、熱時効時の組織変化を明確にすることを目的とした。各工法の施工材についてEBSD解析を行うことにより、応力緩和過程に関与すると考えられる組織学的因子の検討を実施した。...
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Microstructural Investigation of Residual Stress Relaxation Process of Ni-based Alloy by EBSD Analysis

The solidification brittleness temperature range (BTR) and local strain in laser surface melted regions of Type 316L stainless steels were investigated in this study, to make clear the mechanism of solidification cracking in the region. The BTR in laser surface melted regions was estimated by the calculation of its upper and lower limits with using the dendrite growth model for Fe-Cr-Ni system and the thermo-dynamic analysis based on calculated phosphorus concentrations at dendrite boundaries in the melted...
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Theoretical Analysis of Hot Cracking Behavior in Melted Regions of Type 316L Stainless Steels by Laser Surface Melting Treatment for Maintenance to Prevent SCC

The relaxation behavior of the compressive stress in nickel-based alloy 600 introduced by WJP and buffing stress improving treatments has been investigated. Hardness reduction behavior in WJP and buffing processed samples during thermal aging has been found to follow the Johnson-Mehl-Avrami equation and the rate constant for hardness reduction to be expressed by the Arrhenius equation. EBSD crystal orientation analysis has confirmed that the strain is introduced mainly in the surface region. Positron lifeti...
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Mechanism for stress relaxation behavior of WJP and buffing stress improving treatments

In order to prevent primary water stress corrosion cracking (PWSCC), water jet peening (WJP) has been used on the welds of Ni-based alloys in pressurized water reactors (PWRs). However, the compressive residual stresses induced by WJP might be relaxed during the actual operation because of the elevated temperature environment. Therefore, the stability of the compressive residual stresses caused by WJP has been investigated during thermal aging. The result shows that the compressive residual stresses has...
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Relaxation behavior of residual stress generated by water jet peening under operation temperature environment ― Evaluation for long term stability of residual stress improving treatment on Ni base alloy (report 1) ―

Japanese government has recently addressed a policy to increase capacity factor of existing nuclear PPs to achieve the goal to decrease the emission of CO2. Numerous preventive measures have taken in nuclear power plants to minimize the risk of unexpected long shutdown. Newly developed mitigation measures or repair methods need to be qualified to satisfy regulatory standards, before it is implemented to nuclear power plants. The qualification process needs to comply regulatory standards though it may consum...
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R/D and implement of temper bead welding as newly developed maintenance technique in nuclear power plant

Abstract For the last couple of years, issue of defects in Alloy 600 weld lowered the capacity factor of nuclear power plants. To apply newly developed repair technique, like temper-bead welding that has not been prescribed in technical standards, it is necessary to evaluate its technical applicability to the requirement. In the previous cases, this process has been completed by issuance of no action letter for specific area and condition, utilizing implementation process, such as confirm testing by J...
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Study on implementing newly developed repair welding technique within a limited time frame for nuclear power plant

Stress corrosion cracking (SCC) is one of serious aging degradation problems for the Alloy 600 components of pressurized water reactors (PWRs). In order to prevent SCC, various methods such as water jet peening (WJP), laser peening (LP), surface polishing have been used to introduce compressive stresses at the surfaces of the PWR components. However, it has been reported that such compressive residual stress introduced by these methods might be relaxed during the practical operation, because of high te...
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Kinetic analysis on the long term stability of the effect obtained by various residual stress improvement methods ― Evaluation for long term stability of residual stress improving treatment on Ni base alloy (report 2) ―

In order to estimate the stability of the stress improving effect by water jet peening (WJP) residual stress improvement methods during the long-term operation of nuclear power plants, the residual stress in nickel-based alloy 600 has been studied through the kinetics of hardness reduction behavior in practical operation temperatures. Based on the relationship between the residual stress relaxation and the hardness reduction rates, it has been found that in actual operating condition, when the hardness decr...
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Prediction of Residual Stress Stability Introduced by Water Jet Peening Residual Stress Improvement Method to Ni-based Alloy during Long-Term Operation

In order to compare the stability of the stress improvement effect introduced by laser peening (LP), water jet peening (WJP) and buffing residual stress improvement methods during the long-term operation of nuclear power plants, the hardness in nickel-based alloy 600 has been studied through the kinetics of hardness reduction behavior in practical operation temperatures. The predicted hardness results show that in actual operating condition, the time required to reduce the hardness to 180HV is 1268 years fo...
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Long-Term Stability Comparison of LP, WJP and Buffing Residual Stress Improvement Methods under Actual Operating Condition

Abstract Various research projects have been conducted on the residual stress distribution in the pipe weld since it is important to evaluate SCC initiation, or to evaluate structural integrity. Our previous report noted the necessity of considering the influence of complicated residual stresses in multi-path weld joint in order to enhance the evaluation of long-term reliability of mitigated stress. This paper describes the evaluation result about the changes in residual stress caused by variations in...
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Evaluation of varying extent on the residual stress distribution by effect of varied welding application and repair welding