Jinya Katsuyama

Study of Residual Stress Distribution at Start-Finish Point of Circumferential Welding Studied by 3D-Fem Analysis

Recently, the stress corrosion cracking (SCC) of internal cores and/or recirculation pipes of low-carbon austenitic stainless steel has become actualised. The SCC is considered to occur and progress near the welding zone because of the tensile residual stress due to welding. In the present work, the thermo-elastic-plastic analysis of the residual stress was performed in order to clarify the distribution of residual stress in the steady and unsteady welding regions by circumferential welding. The target was a butt-welded joint of SUS316L-pipes. The residual stress has been calculated using a three dimensional model and the results have been compared and discussed in detail.

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

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.

Fermi surface of a shape memory alloy of TiNi

Direct experimental information about the geometry of the Fermi surface of Ti 48.5 Ni 51.5 whose martensitic transformation temperature is 170 K is presented for the first time. The present results have shown four sheets of the Fermi surface with the same topology, but with different shapes and sizes, as those predicted previously by band structure computations. From geometrical features of the Fermi surface, new possible nesting features of the electron surface at R and the hole surface at Γ are suggested for future theoretical computations of the generalized susceptibility and phonon spectra of this alloy system.

Effect of Welding Conditions on Residual Stress and SCC Behavior at Butt-Welding Joints of Recirculation Pipes

Stress corrosion cracking (SCC) in recirculation pipes made of low carbon austenitic stainless steel (Type 316L) has been observed near butt-welding joints. The recent SCC grows near the welding zone mainly due to high tensile residual stress by welding since the effect of the other contributing factors of material and environment decreases due to the countermeasures. Therefore, the residual stress analysis due to welding of austenitic stainless piping is important and has been already conducted by many researchers. In present work, scatters of welding conditions such as heat input and welding speed were measured experimentally by producing a series of butt-weld specimens of Type 316L pipes. Distribution and its scattering of residual stress were also measured by non-destructive and destructive methods. The effects of welding conditions on residual stress have been evaluated by parametric FEM analyses considering the variation of some parameters based on the welding experiments. The effects of welding conditions on crack growth behavior have been also evaluated by SCC growth simulations using calculated residual stress distributions and a procedure in the fitness-for-service code. Welding parameters such as heat input and welding speed have a strong influence on crack growth rate since residual stress is also affected by scatter of these welding parameters.Copyright

Effect of Welding Conditions on Residual Stress and Stress Corrosion Cracking Behavior at Butt-Welding Joints of Stainless Steel Pipes

Stress corrosion cracking (SCC) in Type 316 L low-carbon austenitic stainless steel recirculation pipes have been observed near butt-welding joints. These SCC in Type 316 L stainless steel grow near the welding zone mainly because of the high tensile residual stress caused by welding. Therefore, for SCC growth analysis, it is important to assess the residual stress caused by welding of stainless steel piping. In the present study, which is focused on the scatters of welding parameters such as heat input and welding speed, these values were measured experimentally by fabricating a series of butt-welded specimens of Type 316 pipes. The distribution and scatter of residual stress were also measured by stress relief and X-ray diffraction methods. The effects of welding parameters on residual stress distribution have been evaluated through welding simulations based on finite-element analysis using three-dimensional and axisymmetric models. Parametric analyses were also performed, while taking into account the variation of some welding parameters based on the experiments. SCC growth behavior was calculated using simulated residual stress distributions and applying a procedure in the fitness-for-service code. It was clearly shown that the uncertainties on welding heat input and speed have strong influences on SCC growth behavior because residual stress is also affected by the scatter of these welding parameters.

Investigation on Evaluation Method Based on J Integral for Retardation of Crack Growth Due to Excessive Loading Beyond Small Scale Yielding Condition

Niigata-ken Chuetsu-Oki earthquake occurred in July 2007, whose magnitude was beyond the assumed one provided in seismic design. Therefore it becomes an important issue to evaluate the effect of excessive loading, in particular, for the components with existing crack. Fatigue crack growth rate is usually expressed by Paris’s law using the range of stress intensity factor (ΔK). However, applicability of the model to loading conditions beyond the small scale yielding remains as an issue since ΔK is inappropriate in such a high loading level. In this study, the fatigue crack growth behaviors after applying the excessive loads were investigated using austenitic stainless steel and carbon steel. Instead of ΔK, J-integral value for crack growth evaluation due to cyclic loading has been applied based on the experimental data to treat the excessive loading beyond small scale yielding. The finite element method (FEM) analyses were conducted to evaluate the stress distribution and plastic zone size for the excessive loading condition. The modified Wheeler model using J-integral range, ΔJ, has been proposed for the prediction of retardation effect on crack growth after excessive loading. It was indicated that retardation effect by excessive loading beyond small-scale yielding could be quantitatively evaluated using the J-Wheeler model.Copyright

A Probabilistic Evaluation Model for Welding Residual Stress Distribution at Piping Joint in Probabilistic Fracture Mechanics Analysis

Stress corrosion cracking (SCC) has been observed at some piping joints made by Austenitic stainless steel in BWR plants. In JAEA, we have been developing probabilistic fracture mechanics (PFM) analysis methods for aged piping based on latest aging knowledge and an analytical code, PASCAL-SP. PASCAL-SP evaluates the failure probability of piping at aged welded joints under SCC by a Monte Carlo method. We proposes a simplified probabilistic model which can be applied to the failure probability analysis based on PFM for welded joint of piping considering the uncertainty of welding residual stress. And the probabilistic evaluation model is introduced to PASCAL-SP. A parametric PFM analysis concerning uncertainties of residual stress distribution using PASCAL-SP was performed. The PFM analysis showed that the uncertainties of residual stress distribution largely influenced break probability. The break probability increased with increasing the uncertainties of residual stress.Copyright

Effect of Geometric and Welding Conditions on Through-Thickness Residual Stress

Recently, stress corrosion cracking (SCC) of core internals and/or recirculation pipes of austenite stainless steel (SUS316L) has been observed. SCC is considered to occur and progress at near the inner surface of the welding zone in butt-welded pipes, because of the tensile residual stress introduced by welding. In present work, three-dimensional and axisymmetric thermo-elastic-plastic finite element analysis have been carried out, in order to clarify the effect of geometric and welding conditions in circumferential welding zone on the residual stress. In particular, butt-welding joints of SUS316L-pipes have been examined. The residual stress was simulated by three-dimensional and axially symmetric models and the results were compared and discussed in detail.Copyright

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

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

Effect of Surface-Machining and Butt-Welding on Residual Stress and Hardness of Type 316L Stainless Steel Pipes

Stress corrosion cracking (SCC) near the welded zone of core internals and recirculation pipes of Type 316L stainless steel have been observed to initiate at inner surface due to tensile residual stress and hardening. Residual stress and hardening should be induced by not only welding but also surface-machining in these regions. Surface-machining is conducted before and after piping butt-welding to match the ID of pipes and to provide a smooth surface finish. Therefore, we experimentally evaluated Vickers hardness distribution and proposed a simulation method to estimate residual stress by surface-machining using a local micron scale finite element method (FEM) model in previous paper (PVP2006) [1]. In present work, residual stress and Vickers hardness distributions for thickness direction were evaluated by experimental measurement. Residual stress simulation by surface-machining and welding was carried out by proposed simulation method applied to simulate the butt-welding of full scale FEM model for weld specimen. Redistribution behavior of residual stress by welding after surface-machining was discussed based on experimental and analytical results. Estimation of Vickers hardness distribution by surface-machining was performed based on strained region map observed by Field emission scanning electron microscope (FE-SEM) equipped with electron backscatter diffraction (EBSD) and compared with experimental measurements. The effect of surface-machining before/after welding on distribution of residual stress and hardness are presented.Copyright