Makoto Udagawa

Bio: Makoto Udagawa is an academic researcher from Japan Atomic Energy Agency. The author has contributed to research in topics: Residual stress & Reactor pressure vessel. The author has an hindex of 5, co-authored 16 publications receiving 71 citations.

Assessment of Residual Stress Due to Overlay-Welded Cladding and Structural Integrity of a Reactor Pressure Vessel

Abstract: In this study, the residual stresses generated within the overlay-welded cladding and base material of reactor pressure vessel (RPV) steel were measured for as-welded and postwelded heat-treated conditions using the sectioning and deep-hole-drilling (DHD) techniques. In addition, thermo–elastic–plastic creep analyses considering the phase transformation in the heat-affected zone using the finite element method (FEM) were performed to evaluate the weld residual stress produced by overlay-welding and postweld heat treatment (PWHT). By comparing the analytical results with the experimentally determined values, we found a good agreement for the residual stress distribution within the cladding and the base material. The tensile residual stress in the cladding is largely due to the difference in the thermal expansion of the cladding and the base material. It was also shown that considering phase transformation during welding was important for improving the accuracy of the weld residual stress analysis. Using the calculated residual stress distribution, we performed fracture mechanics analyses for a vessel model with a postulated flaw during pressurized thermal shock (PTS) events. The effect of the weld residual stress on the structural integrity of RPVs was evaluated through some case studies. The results indicated that consideration of the weld residual stress produced by overlay-welding and PWHT is important for assessing the structural integrity of RPVs.

Effect of neutron irradiation on the mechanical properties of weld overlay cladding for reactor pressure vessel

Abstract: This study investigates the effects of high fluence neutron irradiation on the mechanical properties of two types of cladding materials fabricated using the submerged-arc welding and electroslag welding methods The tensile tests, Charpy impact tests, and fracture toughness tests were conducted before and after the neutron irradiation with a fluence of 1 × 1024 n/m2 at 290 °C With neutron irradiation, we could observe an increase in the yield strength and ultimate strength, and a decrease in the total elongation All cladding materials exhibited ductile-to-brittle transition behavior during the Charpy impact tests A reduction in the Charpy upper-shelf energy and an increase in the ductile-to-brittle transition temperature was observed with neutron irradiation There was no obvious decrease in the elastic–plastic fracture toughness (JIc) of the cladding materials upon irradiation with high neutron fluence The tearing modulus was found to decrease with neutron irradiation; the submerged-arc-welded cladding materials exhibited low JIc values at high temperatures

Effects of Residual Stress by Weld Overlay Cladding and PWHT on the Structural Integrity of RPV During PTS

Abstract: In order to assess the structural integrity of a reactor pressure vessel (RPV), it is assumed that a surface crack resides through the cladding at the inner surface of the vessel. It is, therefore, important to precisely evaluate stress intensity factor (SIF) under the residual stress field due to weld overlay cladding and post-weld heat treatment (PWHT). In this work, numerical simulation based on thermal-elastic-plastic-creep analysis using finite element method was performed to evaluate residual stress distribution near the cladding layer produced by weld overlay cladding and PWHT. The tensile residual stress of about 400 MPa occurs in the cladding at room temperature after the PWHT. The residual stress distributions under the normal operating conditions (system pressure and temperature) of RPV were also evaluated. The effect of residual stress and evaluation methods on SIF behavior for various crack size were studied under typical pressurized thermal shock (PTS) conditions such as small break loss of coolant accident (SBLOCA), main steam line break (MSLB) and large break loss of coolant accident (LBLOCA). It is clarified from comparison of this weld simulation with the other simple methods that SIF is affected by residual stress by weld overlay cladding and PWHT.Copyright © 2007 by ASME

Assessments of Residual Stress Due to Weld-Overlay Cladding and Structural Integrity of Reactor Pressure Vessel

Abstract: Austenitic stainless steel is cladded on the inner surface of ferritic low alloy steel of reactor pressure vessels (RPVs) for protecting the vessel walls against the corrosion. After the manufacturing process of the RPVs including weld-overlay cladding and post-weld heat treatments (PWHT), the residual stress still remain in such dissimilar welds. The residual stresses generated within the cladding and base material were measured as-welded and PWHT conditions using the sectioning and deep-hole-drilling (DHD) techniques. Thermal-elastic-plastic-creep analyses considering the phase transformation in heat affected zone using finite element method were also performed to evaluate the weld residual stress produced by weld overlay cladding and PWHT. By comparing analytical results with those measured ones, it was shown that there was a good agreement of residual stress distribution within the cladding and base material. Tensile residual stress in cladding is mostly due to the difference between the thermal expansions of cladding and base materials. It was also shown that taking the phase transformation during welding into account is important to improve the accuracy of weld residual stress analysis. Using the calculated residual stress distribution, fracture mechanics analysis for a postulated flaw during pressurized thermal shock (PTS) events have been performed. The effect of weld residual stress on the structural integrity of RPV was evaluated through some case studies. The result indicates that consideration of weld residual stress produced by weld-overlay cladding and PWHT is important for assessing the structural integrity of RPVs.Copyright © 2010 by ASME

Failure behavior analyses of piping system under dynamic seismic loading

Abstract: Some nuclear power plants in Japan experienced severe seismic ground motions that exceed their structural design specifications. Therefore, it is important to assess the margin of structural design against failure of piping systems under the severe seismic loading. The purpose of this study is to establish an analytical approach by which the failure behavior of piping systems can be simulated under severe dynamic seismic loading considering plastic effects. A series of dynamic-elastic-plastic analyses was performed focusing on a dynamic excitation test of a three dimensional piping system. The analytical results agree with the experimental data in terms of natural frequency, natural vibration mode, response accelerations, elbow opening-closing displacements, strain histories, failure position and low-cycle fatigue failure life.

Residual stress in laser cladded rail

Abstract: To improve the fatigue life of components subject to loads with high surface strain gradients, it is possible to coat them with an alloy of higher durability. The present study focuses on the effect of cladding high value track components, made of a standard rail steel UIC 900A/grade 260, with a layer of a premium martensitic stainless steel to reduce wear and fatigue. The laser cladding process inevitably generates residual stresses in the clad and parent metal, which could be detrimental to the integrity of the component. Therefore, measurements to determine the residual stress state of cladded rail were performed using semi-destructive centre-hole and deep hole drilling and non-destructive neutron diffraction techniques. Subsequently, the effects of cycling loading and wear, representative of typical service loads, on the redistribution of the residual stress field were investigated. It was observed that laser cladding causes a triaxial compressive residual stress field in the clad and near the interface and a tensile stress field in the parent material. The stress field is shown to change when the first cycle of load is applied but reaches a steady state after only 10 cycles: After the 10th cycle there is no evidence that the clad continues accumulating strain which could indicate that there is low risk of ratcheting. Wear effect on residual stress redistribution was found to be local on the surface of the specimen only.

Assessment of Residual Stress Due to Overlay-Welded Cladding and Structural Integrity of a Reactor Pressure Vessel

Abstract: In this study, the residual stresses generated within the overlay-welded cladding and base material of reactor pressure vessel (RPV) steel were measured for as-welded and postwelded heat-treated conditions using the sectioning and deep-hole-drilling (DHD) techniques. In addition, thermo–elastic–plastic creep analyses considering the phase transformation in the heat-affected zone using the finite element method (FEM) were performed to evaluate the weld residual stress produced by overlay-welding and postweld heat treatment (PWHT). By comparing the analytical results with the experimentally determined values, we found a good agreement for the residual stress distribution within the cladding and the base material. The tensile residual stress in the cladding is largely due to the difference in the thermal expansion of the cladding and the base material. It was also shown that considering phase transformation during welding was important for improving the accuracy of the weld residual stress analysis. Using the calculated residual stress distribution, we performed fracture mechanics analyses for a vessel model with a postulated flaw during pressurized thermal shock (PTS) events. The effect of the weld residual stress on the structural integrity of RPVs was evaluated through some case studies. The results indicated that consideration of the weld residual stress produced by overlay-welding and PWHT is important for assessing the structural integrity of RPVs.

Effect of creep constitutive equation on simulated stress mitigation behavior of alloy steel pipe during post-weld heat treatment

Abstract: Post-weld heat treatment (PWHT) is usually conducted to reduce the residual stress and to improve the mechanical properties of the welded region. The thermal elasto-plastic creep analysis is used to estimate the mitigation of the residual stress in the welded region analytically. The stress mitigation is caused by effect of the creep relaxation behavior during PWHT. Thus, creep constitutive equation is used to estimate the residual stress mitigation. The material properties of Norton's law and the Norton-Bailey law for alloy steel pipe JIS STPA23 (equivalent to ASME SA335P11 seamless steel tubes) were experimentally investigated. Creep tests were conducted at 400, 500, 600, and 700 °C. Specimens were subjected to certain stresses, and creep strain was measured. The material properties of Norton's law and the Norton-Bailey law were calculated from the measured creep strain data. Five bead-on curved plate specimens were fabricated in order to verify the thermal elasto-plastic creep analysis done with these creep constitutive equations. The curved plate was made from a pipe cut in the hoop direction. The specimens were welded without filler metal by gas tungsten arc weld (GTAW) on both surfaces in the center of the curved plate. The residual stress in one specimen was measured without PWHT. The other specimens were measured under different PWHT conditions. The results of analysis with the two creep constitutive equations agreed well with the experimentally measured results. In addition, no differences in the creep constitutive equations were observed at high PWHT temperature. Accordingly, these analyses conducted with both types of creep constitutive equation were verified to be effective. The effect of creep constitutive equation on simulated stress mitigation behavior during PWHT has been clarified for analytically estimating residual stress in the welded region.

Failure criteria of a carbon steel pipe elbow for low-cycle fatigue using the damage index

Abstract: A seismic isolation system installed in a nuclear power plant will respond to the load caused by an earthquake. As a result, a larger displacement may occur in the presence of the system, compared to its absence. This increased displacement may involve higher seismic risks for some facilities. Probabilistic seismic fragility analysis is conducted to assess the seismic safety of the major elements and systems of a nuclear power plant that may be at increased seismic risk. The installation of piping systems that connect seismic-isolated and general structures may involve increased seismic risks because they are expected to have a large displacement. For the reliable analysis of the seismic fragility of piping systems, the failure modes and criteria that can represent actual failure must be defined. Therefore, in this study, the ultimate state of the elbow, the vulnerable part of piping systems, was defined as leakage, and an in-plane cyclic loading test was conducted. Moreover, an attempt was made to quantify the failure criteria for a 3-inch carbon steel pipe elbow using a damage index, which was based on the dissipated energy that used the moment-deformation angle relationship.

A comparative study on microstructure and properties of Inconel 52M overlays deposited by laser beam and GTA cladding

Abstract: The current study investigates the overlays deposited by laser beam (LB) and gas tungsten arc (GTA) cladding with Inconel 52M filler wire in nuclear power plants. The effects of the deposition methods on the cross section profile, microstructures, and mechanical properties of both overlays are studied using optical microscope, scanning electron microscope (SEM), tensile and impact test and microhardness measurements. Experimental results show that LB cladding with higher wire feed rate improves the deposition efficiency as compared with GTA cladding. The microstructure of the LB clads consists of cellular and columnar dendrites, and Nb-rich spherical particles are precipitated in the interdendritic regions, while the equiaxed dendrites are dominant in the GTA clads. The microhardness of the GTA clads is 20–30 HV higher than that of the LB clads. Furthermore, the tensile strength, toughness, and elongation of the LB clads decrease in comparison to the GTA clads. The deep penetration of the LB clads resulting in a wave shape of each pass and high dilution does not benefit the mechanical properties as compared with the GTA clads.