New peening technology is proposed to improve the fatigue strength of welded joints. By using this technology, compressive residual stress is introduced at weld toe by the developed peening procedure which plastic deformation is only applied to the base material near the weld toe. In this study, improvement mechanism of fatigue strength of weld joints by hammer peening on base metal was clarified by FEA. It was clarified that increasing of stress at weld toe is controlled by depression formed near the weld toe. Not only compressive residual stress at weld toe but also decreasing stress concentration at weld toe by plastic deformation on base metal was indicated as the factor of improving fatigue strength of weld joints.
For gas metal arc welding, the effect of CO2 mixture in a shielding gas on a metal transfer process was investigated through the observation of the plasma characteristics and dynamic behavior at the droplet's growth-separation-transfer by the temperature measurement methods which were suitable respectively to the argon plasma region and the metal plasma region. At the present experimental conditions, the metal transfer process was a spray transfer type with 100%Ar shielding gas. On the other hand, with 85%Ar+15%CO2 shielding gas, the metal transfer process was a globular transfer type in which the arc length was shorter, the width was narrower, and the time interval of the droplet separation was longer. For both shielding gases, the metal plasma region near the arc central axis exhibited 6500-7500 K which was lower than the argon plasma region. With 85%Ar+15%CO2 shielding gas, when the metal droplet grew below the electrode wire, the region below the droplet has a high plasma temperature and a high concentration of iron vapor which surrounded the droplet. The region also exhibited a remarkably high electron number density. At the spray transfer process, the argon plasma region had the electron number density twice higher than the metal plasma region. Meanwhile, at the globular transfer process, the metal plasma region had a higher electron number density than the argon plasma region, which corresponded to a higher electrical conductivity near the arc axis. This means that the electric current goes through the arc axis easier than the spray transfer process. This condition increases the temperature below the droplet. The thermal expansion increases the force preventing the droplet from falling down. In consequence, the metal transfer takes the globular transfer type.
The radial stainless steel plates (RPs) used for Toroidal field (TF) coil in ITER are 13 m long, 9 m wide and 10 cm thick, which are quite large. Even though they are very large structures, high manufacturing tolerances and high mechanical strength at 4 K are required. It is also required that each RP should be fabricated every three weeks. Therefore, the authors intend to develop efficient manufacturing methods for ITER TF coil RP. The laser welding is then selected as a welding method for RP. Especially, the development of high technology laser welding is necessary to prevent hot cracking in the material used for the RP, namely, fully austenitic stainless steel with high nitrogen content. The authors carried out trial laser welding experiments aiming at its application to RP. As a result, it was effective to make the angle of back inclination of the weld head at the uniform welding speed. It also seemed that the sensitivity of hot cracking could be reduced by optimizing the chemical compositions of material used for RP. The base material and the welded joints satisfied mechanical properties in 4K. The application of the laser welding technology to the fully austenitic stainless steel was therefore demonstrated.
In order to achieve lighter and stronger car bodies by applying high strength steel sheets, one of key technologies is enhancement of joints strength. In this study, we investigated dependence of strength and fracture behavior on chemical compositions of the steels in spot-welded L-type joints in detail. Consequently, the following experimental results were obtained: 1) Maximum load of the joint decreased with increase of carbon (C) and phosphorous (P). The maximum load was decreased by 0.4 to 0.7kN with increase of 0.1% in C, with C content ranging from 0.03% to 0.5%, and 0.5kN with increase of 0.01% in P, with P content ranging from 0% to 0.03%, 2) Fracture portion changed from the outside to the inside of weld metal with increase of C and P. 3) Fracture path was estimated to accord with solidification segregation site in the weld metal, in case of a steel of 0.2%C-0.03%P. 4) By implementation of an appropriate post heat during spot welding process for the steel of 0.2%C-0.03%P, the degree of solidification segregation was clearly reduced and the maximum load of the joins was improved by 70%.
Hot stamping spot welding tailored blank technology is a process to produce spot welded automotive body parts by the following process: Spot welding steel sheets in lap configuration → Hot stamping (Heating to about 900°C → Quenching and forming in water-cooled die→ Shot blasting to remove scale). This process has the advantage of producing high strength lap welded automotive body parts without increasing the number of forming dies. In this study, the mechanical properties of the hot stamped spot weld (spot welding →hot stamping) and conventional spot weld (hot stamping →spot welding) of the 1500MPa class uncoated boron steel sheets were compared. The obtained results are as follows. The tensile shear strength (TSS) of the hot stamped spot weld and conventional spot weld were comparable and the fracture modes were the same. On the other hand, the cross tension strength (CTS) of hot stamped spot weld was significantly higher than that of the conventional spot weld. The fracture position of the hot stamped spot weld was outside the nugget and conventional spot weld was inside the nugget. The high CTS of the hot stamped spot welds might be caused by the improvement of the fracture toughness of the nugget, which was caused by reduction of the solidification segregation of the phosphorus. It is assumed that the heating process after spot welding leads to the reduction of the solidification segregation. For the tension test, because there was no HAZ softening in the hot stamped spot weld, no fracture was observed in HAZ and a higher elongation was obtained.
Shielded metal arc weld metal for type 600 nickel base alloy (alloy 182) is used for weld components in nuclear power plants. To evaluate the intergranular corrosion resistance of alloy 182 after application of shot peening and subsequent thermal aging treatment at 593-793 K, we conducted the corrosion test (immersed in boiled 16% sulfuric acid + 5.7% copper sulfate aqueous solution at 57.6 ks) using specimens of alloy 182. The results show that the intergranular corrosion resistance of alloy 182 subjected to heat treatment at 893 K for 72 ks was improved by shot peening. Also, the intergranular corrosion resistance was not changed by thermal aging treatment at 593-793 K subsequent to shot peening. Because remaining chromium depletion layers along grain boundaries were still observed by transmission electron microscope (TEM) after shot peening, disappearance of chromium depletion layers cannot be a factor in the improvement of the intergranular corrosion resistance. The results of measurement of surface residual stress by the X-ray diffraction method show that the compressive residual stress introduced by shot peening still remained on the surface of the specimens. Based on these observations, we assumed that chromium depletion layers along grain boundaries near the surface were dissolved by the environment of the corrosion test, the dissolved regions were closed by the compressive residual stress on the surface, and then the remaining chromium depletion layers were protected from the corrosive environment. This assumption explains why the intergranular corrosion resistance was improved although chromium depletion layers remained.