Gas pressure welding method is often applicable for joining of rails and reinforcing steel bars. However, if bonding work is inferior, a large quantity of oxide inclusions remain at the weld interface and those inclusions will cause the weld defects. In the previous study, the morphologies of oxide inclusions were estimated by the observation of the transition behavior for inclusions on each welding stage during the gas pressure welding. In this study, we traced the morphological change of oxide inclusions and base material in pressure welding process to clarify the transition behavior of oxide inclusions. In the first stage, Fe-O type oxide inclusions were mainly deoxidized by carbon, which exists in base material, and the degree of decomposition was influenced by temperature. Furthermore, oxide inclusions, which contain Si and Mn, emerged from outside region of Fe-O type oxide inclusions due to participation of Si and Mn, which exist in base material. In the final stage, almost of oxide inclusions existed on the weld interface were Si-Mn-O type, and the Si content of these oxide inclusions gradually increased as well as downsizing of oxide inclusions as welding process advanced.
This study investigated the electrode life and the electrode degradation characteristics during resistance spot welding of aluminium alloy sheets. When a long welding time and the DR type electrode shape were used, the electrode life was much longer than that with a short welding time and the R type electrode shape. The degradation mechanism of electrode tip was explained in the following way; firstly, Al-Cu alloy layers were formed at the electrode top during consecutive welding, and then the layers stuck easily to the surfaces of aluminium alloy sheets so that notable wear took place and the surface profile of electrode tip became flat. In order to eliminate the direct contact between the electrode and the sheet, a special Cu-foil insert device for resistance spot welding of aluminium alloy sheets was developed and achieved an extremely long electrode life.
In order to evaluate the crack propagation behavior in FSM-monitoring, which is one of the non destructive inspection methods based on the electric potential difference method, a series of experiments and analyses were carried out. It was confirmed that the change of crack propagation direction or the initiation and propagation of cracks could be monitored by the variation of FC value corresponding to the potential difference change. It was elucidated that the initial potential difference largely affected FC value. The results indicated that arrangement of pairs and current direction should be noted for removing the effect of initial potential difference in monitoring of crack initiation and propagation. When the effect cannot be removed, not only FC value but also the potential difference change should be used as the measure for crack propagation in FSM-monitoring.
Stress corrosion cracking (SCC), which affects the structural integrity of reactor component, has been observed at some piping joints made by austenitic stainless steel in BWR plants. It is well known that the SCC behavior is significantly scattered depending upon the various conditions such as materials, piping geometry, crack growth rate, weld residual stress, and so on. Since probabilistic fracture mechanics (PFM) analysis method treats such scatter and uncertainties in the structural integrity evaluation, it is, therefore, useful to apply the PFM analysis to the evaluation of the piping integrity. In JAEA, the PFM analysis code of PASCAL-SP for aged piping has been developed based on Monte Carlo method as described in our previous paper1). Among the conditions related to SCC behavior, weld residual stress near the welded joint is one of the most important factors to assess the structural integrity of piping because the tensile residual stress becomes a driving force of a SCC. Welding conditions such as heat input, welding speed and piping geometry affect weld residual stress distribution at the welded joint of piping. Effect of the welding conditions on the weld residual stress distribution has not yet been evaluated quantitatively. Hence, in this study, an effect of uncertainty of welding conditions, such as scatters of heat input and welding speed during welding, on weld residual stress at the piping butt-welds was evaluated using the simulation method by varying the welding conditions. Probabilistic fracture mechanics analysis using PASCAL-SP was also performed to evaluate the effect of uncertainty of weld residual stress on the break probability of piping. It was clarified that the break probability increased with increasing the uncertainties of residual stress.
A friction stir welding (FSW) tool with good properties at elevated temperature is needed to perform the friction stir welding of high temperature materials. One of the reasons for delaying the application of FSW to high temperature metals is the absence of such a tool. The purpose of this study was to develop a welding tool for the FSW of high temperature materials. As a result, it has been clarified that iridium (Ir) is almost unaffected by oxidation at elevated temperatures. The Ir alloy increases its recrystallization temperature, high-temperature strength and high-temperature hardness by the addition of rhenium (Re). Although SUS304 stainless steel was friction stir welded by an Ir-10at%Re welding tool, the tool was hardly deformed.
The objective of this study is to ensure safety of nuclear reactors. A few accidents of leaks from welded zones at pipe penetration parts of reactor vessel or in coolant pipes have been reported around the world. One of the main causes of such leaks is welding residual stress. It is therefore very important to know the welding residual stress in order to maintain the safety of the plants, estimate plant life cycle and design an effective maintenance plan. Welded joints in nuclear reactors have complex shapes, and the welding residual stresses have complex three-dimensional distributions. The inherent strain method is an analytical method that solves an inverse problem using a least squares finite element method. The method gives the most probable value and the deviation of the residual stress, allowing the reliability of the estimated results to be discussed. In this method, the inherent strains are unknowns. When residual stresses have a complex three-dimensional distribution, the number of unknowns becomes very large. The inherent strain distribution is therefore expressed with an appropriate function, significantly decreasing the number of unknowns. In this study, inherent strain theory and method are applied to measure the welding residual stresses for a mock-up of a welded joint at a pipe penetration of reactor vessel. In this paper, applicability of strain measurement region is diagnosed. 5 kinds of regions are applied to estimate the residual stress, and accuracy and reliability of analyzed results are judged from 4 points of view, that is, residuals, unbiased estimate of variance of errors, welding mechanics and economy. Most reliable and economical measurement region is selected, which brings most reasonable result.
In order to clarify the behaviour and mechanism of the hydrogen embrittlement in SUS304ULC/Ta/Zr explosive bonded joint, the hydrogen embrittlement cracking at Ta/Zr bond interface was characterised. Cracks occurred in the Zr substrate along the wavy interface of the hydrogen-charged Ta/Zr joint. The cracking susceptibility increased drastically when the potential of specimen during hydrogen-charging was reduced below the redox potential of hydrogen. γ-ZrH and δ-ZrH were precipitated in the hydrogen-charged Zr and the precipitated γ-ZrH possessed a (0002)α-Zr//(11-1)γ-ZrH, [21-1-0]α-Zr//γ-ZrH crystallographic relationship. An in-situ observation of the hydrogen embrittlement cracking with SEM and TEM revealed that cracks were initiated in zirconium hydrides and propagated preferentially along zirconium hydrides. These results suggested that the hydrogen embrittlement mechanism of the Zr base metal was caused by the precipitation of zirconium hydrides and the brittle fracture of them.
In order to clarify the hydrogen embrittlement mechanism at Ta/Zr explosive bonded interface, the effects of the Ta content and microstructure on changes in hardness and toughness during hydrogen charging were investigated by using Zr-Ta binary alloys. Zirconium hydrides were precipitated after hydrogen charging in Zr-Ta alloys with the specific crystallographic relationship to α-Zr substrate, and there was no significant difference in the precipitation behaviour of zirconium hydrides with varying the Ta content and microstructure (α, α' and ωphase structures). The impact absorbed energies of Zr-Ta alloys were drastically reduced with an increase in hydrogen charging time, while their hardness slightly increased with hydrogen charging. A pure Zr was highly sensitive to hydrogen embrittlement, whereas the toughness value after hydrogen-charging was lowest when alloys involved ω phase structure, and it decreased with an increase in the Ta content of the alloy. Furthermore, the sensitivity of hydrogen embrittlement of the Zr substrate was heightened with an increase in the degree of cold working (rolling reduction). The hydrogen embrittlement crack occurring in Zr adjacent to the Ta/Zr bond interface would be attributed to the mechanism that cracks were preferentially initiated and propagated in the precipitated zirconium hydrides which were precipitated on the deformation texture of α-Zr (0001) formed during explosive bonding.
Hydrogen embrittlement cracking behaviours of SUS304/Ta/Zr explosive bonded joint during underwater polishing were investigated. Hydrogen embrittlement cracks occurred in the Zr substrate adjacent to the Ta/Zr bond interface during underwater polishing. The open circuit potential of Zr during underwater polishing was drastically reduced immediately after mechanical polishing (within a fraction of a second). The hydrogen yields of Zr-Ta alloys and cold-worked Zr during underwater polishing were estimated from the corrosion current determined by the Tafel extrapolation method. The hydrogen yield increased with a decrease in the Ta content of Zr-Ta alloy, and with an increase in the degree of working (rolling reduction) of Zr. It was deduced that the mechanical grinding in water removing the passive oxide film on the Zr substrate led to the hydrogen absorption into the Zr substrate and the precipitation of zirconium hydrides. Accordingly, hydrogen embrittlement cracks occurred in the deformation layer of Zr around the Ta/Zr bond interface due to the tensile residual stress in the explosive bonded joint.
The concentration of hydrogen and precipitation of zirconium hydrides in Ta/Zr explosive bonded joint were analysed by computer simulation. Numerical model of hydride precipitation under hydrogen diffusion was simplified by the alternate model coupled the macroscopic hydrogen diffusion with the microscopic hydride precipitation. Effects of the initial hydrogen content in Ta, working degree of Zr and post-bond heat treatment on the hydrogen diffusion and hydride precipitation were investigated. Hydrogen was rapidly diffused from Ta substrate into Zr after explosive bonding and temporarily concentrated at Ta/Zr bond interface. Zirconium hydrides were precipitated and grew at Ta/Zr bond interface, and the precipitation zone of hydrides was enlarged with the lapse of time. The precipitation of zirconium hydrides was promoted when the initial hydrogen content in Ta and working degree of Zr were increased. The concentration of hydrogen and precipitation of hydrides at the bond interface were reduced and diminished by post-bond heat treatment at 373K. It was deduced that hydrogen embrittlement in Ta/Zr explosive bonded joint was caused by the precipitation of zirconium hydrides and concentration of hydrogen at Ta/Zr bond interface during the diffusion of hydrogen containing in Ta substrate.
The occurrence condition of hydrogen embrittlement cracking at Ta/Zr bond interface was investigated with respect to the hydrogen content and applied stress in order to propose a guideline for the explosive bonding procedure to prevention of hydrogen embrittlement. Hydrogen charging test was conducted for SUS304ULC/Ta/Zr explosive bonded joints applied the different flexural strains. A hydrogen embrittlement crack occurred in the Zr substrate at Ta/Zr bond interface after hydrogen charging, and it was initiated at shorter charging times when the augmented strain was increased. The occurrence condition of hydrogen embrittlement cracking at Ta/Zr bond interface was shifted to lower stress and hydrogen content with an increase in the amount of explosive during bonding. It was suggested that hydrogen embrittlement in Ta/Zr explosive bonded joint could be inhibited by reducing the initial hydrogen content in Ta substrate less than approx. 5ppm.
Authors tried to butt-joint weld commercially pure titanium plate to aluminum alloy plate by solid state welding using a rotating probe, and investigated the effects of welding conditions and intermetallic compound formed at the weld interface on the properties of the joint. The following results were obtained. The titanium plate and the aluminum alloy plate were successfully butt-joint welded using a rotating probe. The tensile strength of the joint made under the optimum conditions was so high that the joint efficiency was about 97%. The joint strength varied with the probe rotation speed and the offset. Fracture of the joint made under the optimum conditions occurred at the position in aluminum alloy, where the hardness was lower, 5–7mm away from the weld interface as well as at the interface between aluminum matrix and titanium fragments which were scattered around the weld interface. By heating at 550°C for 2h, intermetallic compound obviously appeared at the weld interface of the joint made under the optimum conditions. Fracture of the heated joint occurred at the weld interface, and the tensile strength and the elongation extremely decreased.