Fujio Yoshikubo

Mechanism of Compressive Residual Stress Introduction on Surfaces of Metal Materials by Water-Jet Peening

Water-jet peening (WJP) has been applied to several Japanese nuclear power plants as a method of preventive maintenance against stress corrosion cracking. WJP introduces compressive residual stress reaching hundreds of micrometers in depth, comparable with shot peening (SP), and much smaller plastic deformation at the processed surfaces than SP does. The causes of these features are investigated from the perspective of the impact processes on the surfaces. Pulse-load propagation simulation through elasto-plastic calculations using a finite-element method program was applied to analyze the effects of various parameters of the impact processes on the depth profiles of the residual stress and the amount of plastic deformation on the surface of austenitic stainless steels processed with either WJP or the SP. The calculated depth profiles of residual stress and plastic deformation were similar in some degree to the experimental results of an XRD residual-stress analysis and a plastic-strain analysis using both cross-sectional hardness measurements and EBSD analysis. The analysis reveals that the depth of the compressive residual stress tends to increase as the size of the loaded spot during impact increases. The average and maximum observed load spots using WJP were 0.25 and 0.95 mm in diameter, respectively. These diameters were respectively 1.3 and 4.8 times as large as the calculated diameter of a load spot using SP. The reason that the depth of the compressive residual stress using WJP is comparable with that using SP is considered to be the fact that the sizes of the load spots during the impact with WJP are in the same range as those with SP. Shots impact the surface during the SP process, while shock waves generated by the extinction of cavitations impact the surface during the WJP process. The analysis reveals that the shots deform the surface locally with much higher surface pressure in the early stages of the impact, while shock waves deform the surface evenly throughout the wave passage across the surface. It is inferred from these analyzed results that the media impacting the surface make a difference in the hardness and microstructure of the processed surface.© 2010 ASME

Evaluation for Nondestructive Crack Sizing Capability on Cracked Surface Treated by Water Jet Peening

Water Jet Peening (WJP) is an approved mechanical mitigation technique that has been widely applied in Japanese nuclear power plants since 1999. WJP suppresses stress corrosion cracking (SCC) initiation in SCC susceptible materials which have been widely used in the fabrication of reactor internals [1].Laboratory data shows that WJP is effective at mitigating or suppressing shallow crack propagation if the post application compression depth layer is deeper than the original crack depth and that WJP has no adverse effect on pre-existing cracks, such as crack propagation or acceleration of the crack propagation during and after WJP [2]. In addition, it has also been reported that WJP has no adverse effect on Ultrasonic testing (UT) sizing capability for pre-existing cracks in 304S.S. and Alloy 182 [3].Based on these evaluation test results, Hitachi-GE Nuclear Energy, Ltd (HGNE) successfully applied WJP on the cracked surfaces of reactor internals in an operating Japanese nuclear power plant in 2011 [4].Recently, to evaluate the applicability of this process in a PWR power plant, HGNE has performed an evaluation of Ultrasonic testing (UT) and Eddy Current Testing (ECT) sizing capabilities for surface cracks on a dissimilar metal weld coupon that had been mitigated utilizing the WJP process. No significant differences in their sizing capabilities were observed between the before and after WJP treatment test coupons. During this evaluation, it was also confirmed that surface residual stress on the coupon was improved from tensile to compressive (from max approximately 800MPa to min approximately −600MPa).As a result of this evaluation, it was determined that WJP has no adverse effect on UT and ECT sizing capabilities. In addition, HGNE has taken this knowledge and applied it to the development of a WJP tool for PWR Bottom Mounted Instrumentation (BMI) nozzles, which accommodates all the parameters used for this testing and enables HGNE to reduce the total application time as a result of the inclusion of advanced technological improvements.Copyright

Application of Water Jet Peening on Pre-Cracked Nickel Based Alloy and Stainless Steel

Water Jet Peening (WJP) has been widely applied to Japanese nuclear power plants since 1999 in order to mitigate stress corrosion cracking (SCC) generation on SCC susceptible material used their reactors as an approved mechanical mitigation technique against SCC [1].Laboratory data shows that WJP has an effect to mitigate or suppress shallow crack propagation if the achieved compression depth is deeper than the crack, and that WJP has no adverse effect on pre-existing cracks on the mitigated surface, such as crack propagation during WJP treatment or acceleration effect after its treatment [2]. However, there was no field experience of applying WJP on cracked surfaces of operating reactor internals since we removed the crack(s) as the practice in Japan was to remove the crack prior to WJP implementation.When observed crack can be left as is according to its profile, it is required to perform the crack sizing periodically by Ultrasonic Testing in order to monitor its propagation. In order to apply WJP on the cracked surface, additional testing was needed to clarify that no adverse effect on UT sizing capability after WJP will be observed. The Japan Atomic Power Company (JAPC) and Hitachi-GE Nuclear Energy, Ltd. (HGNE) have performed several tests about this UT crack sizing test before and after WJP treatment and other related test before its field application.Copyright

Mitigation of Stress Corrosion Cracking Based on Residual Stress Improvement by Water Jet Peening (WJP)

We have developed a practical peening technology using cavitating water jet. Water jet peening (WJP) is a preventive maintenance technology for nuclear power plants. WJP changes the residual stress on weld surfaces of reactor internals from tensile to compressive to mitigate the stress corrosion cracking (SCC). The operating conditions of WJP are controlled on the basis of ‘JSME Codes for Nuclear Power Generation Facilities.’ WJP has several advantages of operation, especially no foreign material is left in the reactor vessel since only water is injected, and wide range of the residual stress improvement is obtained since the cavitating flow spreads along the weld surface. We have also developed a prediction method of the residual stress improvement by WJP using a combination of a cavitating jet simulation and a residual stress simulation. We numerically simulated impulsive bubble pressure that varied in microseconds in the cavitating jet with ‘bubble flow model’. The bubble collapse energy was estimated by the bubble pressure. The residual stress simulation was conducted under the input conditions obtained from the bubble collapse energy. The residual stress distributions on and under the weld surface were predicted. The distributions were compared with measured data, and the result confirmed that the developed method for predicting the compressive residual stress after WJP was valid.Copyright