Fatigue tests were conducted under fully reversed axial loading (R = -1) in laboratory air and 3% NaCl solution using friction stir welded (FSW) joints of 7075-T6 aluminium alloy sheets. The FSW joint exhibited lower tensile strength than the parent metal. Heat input during FSW process re-dissolved strengthening precipitates, resulting in softening in the weld zone and lower tensile strength. In laboratory air, the fatigue strength of the FSW joint was comparable to that of the parent metal, which could be attributed to grain refinement in the stir zone (SZ) and dynamic aging during fatigue loading in the softened weld zone. In 3% NaCl solution, the fatigue strength of the FSW joint was lower than that of the parent metal. Corrosion pits were preferentially formed at the boundary between thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ), which led to premature crack initiation in the FSW joint. Such predominant formation of corrosion pits was due to the sensitization caused by heat history during FSW process.
In structural materials, it is investigated to make crystal grain size less than 1μm in order to improve strength and toughness. Authors reported that fine ferrites were obtained by laser irradiation in steel. However, it is not made clear that an effect of laser irradiation on the materials which do not have a transformation. In this study, we performed laser irradiation for austenite stainless steel SUS304 as the materials which did not have a transformation and investigated an effect of laser irradiation on grain refinement. Solution treatments were performed for SUS304 and their initial grain size became 23 and 68μm. Then they were given four kinds of strain ε and irradiated laser under some laser traveling speeds ν and some defocus distances ƒd. As the result, they became recrystallization grain and their size were obtained about 10 and 15μm, for initial grain size respectively, under the best condition (laser power=1.5kW, fd=60mm, v=350 mm/min, ε= 40%). Then a recrystallization grain size was caused by each factor of laser traveling speed (heating temperature, heating time), strain and defocus distance. Furthermore, as an electro-gas arc weld of SUS304, the grain size was 40μm in heat affected zone, was irradiated after shot peening, it was clarified that fine grain was obtained from 10 to 15μm at the heat affected zone.
The stress corrosion cracking (SCC) susceptibility of the SMAW metals for Inconel alloy 600 to which Cr was added to 14.8-21.4mass% has been investigated on the basis of CBB test in the pressurized hot water (corresponding to the service condition of BWR nuclear power plant), since the TIG weld metal of alloy 82 involving 18-22mass% Cr possesses much better resistance to SCC than the SMAW metal of alloy 182 (Cr content = 13-17mass%). When their Cr contents were increased to the same level as those of the alloy 82, the weld metals of alloy 182 sustained only slight SCCs in the as-welded state, and no crack was detected after the post weld heat treatment (SR+LTA) of stress relief annealing at 893 K followed by aging at 673 K. These results suggest that the higher Cr content of the alloy 82 is responsible for its higher resistance to SCC than that of the alloy 182. The Cr carbides precipitated at the grain boundary during the welding and the SR+LTA treatment were also changed from M7C3 type to M23C6 type with the increase in the Cr content. Though the Cr content at the grain boundary in weld metal containing 14.8mass%Cr subjected to the SR+LTA treatment was 3mass%, the Cr content of weld metal containing 18.5mass%Cr was not less than 10mass%. The addition of the Cr to the alloy 182 increased the Cr content in the grain boundary region, suggesting that the intergranular SCC can be suppressed when the Cr content at the grain boundary is not less than 10mass%. In addition to the carbide, Ni16(Mn, Cr)6Si7 (G phase) was precipitated at the grain boundary in the alloy 182 containing 18.5mass% Cr when the SR+LTA treatment was applied. TEM-EDS analyses suggested that the G phase was enriched in P, and so could decrease the P content in the grain boundary region. Probably, the decreased P content at the grain boundary due to the precipitation of G phase contributed to the enhancement of the SCC resistance of the Cr-added alloy 182 by the SR+LTA treatment.
The intergranular stress corrosion cracking (IGSCC) susceptibility of the shielded metal arc weld metals for Inconel alloy 600 has been investigated with particular reference to the influences of P and Nb contents on the basis of creviced bent beam (CBB) test in the pressurized hot water (corresponding to the service condition of boiling water reactor nuclear power plant). The IGSCC susceptibility of the weld metals in the as-welded state exhibited a tendency to increase with P content. When the post weld heat treatment (SR+LTA) of stress relief annealing at 893 K followed by aging at 673 K was applied, however, the influence of P content on the IGSCC susceptibility of weld metals depended on the Nb content; i.e., the IGSCC susceptibility of the weld metals containing 1.2mass%Nb was almost independent of P content, whereas the weld metal containing 4.4mass%Nb showed much decreased IGSCC susceptibility by increasing the P content from 0.012mass% to 0.031mass%. Since the double-loop electrochemical potentiokinetic reactivation (DL-EPR) test suggested that the grain boundary corrosion resistance of the weld metals of 4.4mass%Nb decreased with the rise in P content, the improved IGSCC susceptibility of the weld metals of 4.4mass%Nb and 0.031mass%P cannot be explained in terms of the corrosion resistance of the grain boundary. The weld metal containing 4.4mass%Nb was hardened significantly by the precipitation of γ' phase (Ni3Nb) in interdendritic areas during the SR+LTA treatment. The increase in the hardness brought about by the SR+LTA treatment was depressed significantly by the rise in P content probably owing to the reduction in the precipitation area of γ' phase. It can be considered that the decreased hardness contributes to the improvement in the IGSCC susceptibility with increasing the P content, since the tensile stress applied to the specimen during the CBB test increases with the hardness.
Fundamental objective of this study is to ensure safety of nuclear reactor. A few accidents of leak from welded zones at pipe penetration part of reactor vessel or at coolant pipe are reported at home or abroad. One of the main causes is welding residual stress. Therefore, it is very important to know the welding residual stress in order to maintain high safety of the plant, estimate plant life cycle and design effective maintenance plan. Welded joints of nuclear reactor vessel have complex shapes, and the welding residual stresses also have three-dimensional complex distributions. In this study, inherent strain-based theory and method are applied to measure the welding residual stresses. The inherent strain method is one of analytical method as inverse problem, using least squares method, based on finite element method. So the method gives most probable value and deviation of residual stress. Reliability of estimated result can be discussed. In this method, inherent strains are unknowns. When residual stresses are distributed complexly in 3-dimensional stress-state, the number of unknowns becomes very large. So, inherent strain distribution is expressed with appropriate function to decrease largely the number. A mock-up is idealized for a welded joint at pipe penetration part of actual reactor vessel. The inherent strain method is applied to measure residual stress of the joint. In this paper, applicability of inherent strain distribution function is diagnosed. 10 kinds of functions are applied to estimate the residual stress, and accuracy and reliability of analyzed results are judged from 3 points of view, that is, residuals, unbiased estimate of variance of errors and welding mechanics. Most suitable function is selected, which brings most reliable result.
The new process called L-SIP(outer surface irradiated Laser Stress Improvement Process) has been developed to improve the tensile residual stress to the compressive stress at the inner surface near pipes' butt welded joint. The characteristic of this process is to produce the plastic strain into the pipe by applying the temperature difference at the pipes' inner and outer surface without using water cooling method. In this paper, we have studied the stress and strain behavior in rapidly heating the pipes' outer surface. The temperature gradient occurs in the pipe thickness when heating the outer surface rapidly. By the thermal expansion difference between the inner and outer surface, the tensile thermal stress generates at the inner surface and the compressive thermal stress generates at the outer surface. Furthermore, the tensile plastic strain will be produced at the inner surface and the compressive plastic strain will be produced at the outer surface. The plane which balances between inner stress and outer stress moves toward inside, because the compressive strain of the outer surface is larger than that of the inner surface when the temperature becomes even in the pipes' thickness. The compressive residual stress occurs on the pipes' inner surface by this plastic deformation. This mechanism can also be applied to a circumstance when the heat penetrates to around 1/2 of the pipes' thickness and the temperature of the inner surface does not rise because of the short time heating. Therefore, water-cooling the inner surface is not necessary in this method. This mechanism and the effect of the stress improvement for austenitic stainless steel pipe (SUS316TP 4B×Sch160; O.D.=114.3mm thickness=13.5mm) are verified by the axisymmetric thermo-elastic-plastic finite-element method analysis.
The new process called L-SIP (outer surface irradiated Laser Stress Improvement Process) is developed to improve the tensile residual stress of the inner surface near the butt welded joints of pipes in the compression stress. The temperature gradient occurs in the thickness of pipes in heating the outer surface rapidly by laser beam. By the thermal expansion difference between the inner surface and the outer surface, the compression plastic strain generates near the outer surface and the tensile plastic strain generates near the inner surface of pipes. The compression stress occurs near the inner surface of pipes by the plastic deformation. In this paper, the theoretical equation which calculates residual stress distribution from the inherent strain distribution in the thickness of pipes is derived. And, the relation between the distribution of temperature and the residual stress in the thickness is examined for various pipes size. (1) By rapidly heating from the outer surface, the residual stress near the inner surface of the pipe is improved to the compression stress. (2) Pipes size hardly affects the distribution of the residual stress in the stainless steel pipes for piping (JISG3459). (3) The temperature rising area from the outside is smaller, the area of the compression residual stress near the inner surface becomes wider.
The new process called L-SIP(outer surface irradiated Laser Stress Improvement Process) is developed to improve the tensile residual stress of the inner surface near butt weld joint of pipes. In L-SIP, the pipe outside temperature rises locally at short time by a laser beam moving in the circumferential direction. If the heating time is short enough, the temperature of the pipe inner surface dose not rise, and a temperature difference occurs between the outer surface and the inner surface. By the thermal expansion difference between the outer surface and the inner surface, the thermal stress occurs in the pipe and the new plastic deformation is produced. After cooling, the compressive residual stress is produced near the inner surface of the pipe with the effect of this plastic deformation. In order to verify the stress improvement mechanism of L-SIP, thermo-elastic-plastic FEM analysis is conducted for austenitic stainless steel pipe (SUS304TP 4B×Sch160; O.D. =114.3mm, thickness=13.5mm). It is showed that the residual stress on the pipe inner surface change to the compressive stress like the axial-symmetry model. The experiment is done for butt weld joint of the same pipe as FEM analysis. The welding residual stress is improved from +200MPa (tensile) to less than -100MPa (compression) on the pipe inner surface. Material qualification tests of welding joint are done after L-SIP. It is verified that L-SIP has no bad influence on the welding joint of the stainless steel pipe.
The new process called L-SIP (outer surface irradiated Laser Stress Improvement Process) is developed to improve the tensile residual stress of the inner surface near the butt welded joints of pipes in the compression stress. The temperature gradient occurs in the thickness of pipes in heating the outer surface rapidly by laser beam. By the thermal expansion difference between the inner surface and the outer surface, the compression stress occurs near the inner surface of pipes. In this paper, the theoretical equation for the temperature distributions of pipes heated by moving rectangular Gauss distribution heat source on the outer surface is derived. The temperature histories of pipes calculated by theoretical equation agree well with FEM analysis results. According to the theoretical equation, the controlling parameters of temperature distributions and histories are q/2ay, vh, ax/h and ay/h, where q is total heat input, ay is heat source length in the axial direction, ax is Gaussian radius of heat source in the hoop direction, v is moving velocity, and h is thickness of the pipe. The essential variables for L-SIP, which are defined on the basis of the measured temperature histories on the outer surface of the pipe, are Tmax, F0=kτ0/h2, vh, WQ and LQ, where Tmax is maximum temperature on the monitor point of the outer surface, k is thermal diffusivity coefficient, τ0 is the temperature rise time from 100°C to maximum temperature on the monitor point of the outer surface, WQ is τ0×v, and LQ is the uniform temperature length in the axial direction. It is verified that the essential variables for L-SIP match the controlling parameters by the theoretical equation.
The new process called L-SIP (outer surface irradiated Laser Stress Improvement Process) is developed to improve the tensile residual stress of the inner surface near the butt welded joints of pipes in the compression stress. The temperature gradient occurs in the thickness of pipes in heating the outer surface rapidly by laser beam. By the thermal expansion difference between the inner surface and the outer surface, the compression stress occurs near the inner surface of pipes. In this paper, the essential variables for L-SIP is studied by experimental and FEM analysis. The range of the essential variables for L-SIP, which are defined by thermo-elastic FEM analysis, are Tmax=550–650°C, LQ/√rh ≥3, WQ/√rh ≥1.7, and, 0.04≤F0≤0.10 where Tmax is maximum temperature on the monitor point of the outer surface, F0 is k×τ0/h2, k is thermal diffusivity coefficient, τ0 is the temperature rise time from 100°C to maximum temperature on the monitor point of the outer surface, WQ is τ0×v,υ is moving velocity, LQ is the uniform temperature length in the axial direction, h is thickness of the pipe, and r is average radius of the pipe. It is showed by thermo-elastic-plastic FEM analysis that the residual stresses near the inner surface of pipes are improved in 4 different size pipes under the same essential variables. L-SIP is actually applied to welding joints of 4B×Sch160 and 2B×Sch80 SUS304 type stainless steel pipes within the defined range of the essential variables. The measured welding residual stresses on the inner surface near the welding joints are tensile. The residual stresses on the inner surface change to compression in all joints by L-SIP.
This study investigated the electrode life and the electrode degradation mechanism during alternate resistance spot welding of galvannealed steel sheets and bare steel sheets. As the result, it was found that the electrode life in this type of welding was extremely short in comparison to welding of only galvannealed steel sheets. The reason for this short life is explained by the sticking of the alloy layers formed on the electrode top to the bare steel sheet surface, so that the narrower and sharper shape as the original electrode top is the better to prolong the life. Although the ring-shape nuggets were not formed during alternate resistance DC welding, the electrode degradation mechanism is similar to that during the resistance DC welding of only galvannealed steel sheets.
In the friction stir welding of advanced high strength steel (AHSS) sheets with tensile strength grades between 590 and 1180 N/mm2, the proper welding condition range and the influence of the thermal history, varied by the welding conditions, on the microstructures and mechanical properties of the welds were investigated. It was verified that the proper welding conditions were present for the steel sheets up to 1180 N/mm2 grade, regarding the fact that an increase in the revolution pitch, ie, a decrease in heat input, tended to reduce the stir zone (SZ) and thus cause the incomplete consolidation at the bottom of the weld. In the regions of SZ and the heat-affected zone (HAZ) heated above Ac1 temperature, the higher tool rotation speed presumably resulted in the higher peak temperature, evidently increasing the fraction of newly transformed martensite and hardness. In the region of HAZ below Ac1, the original martensite in the base metal was tempered and the hardness was decreased, which was rather apparent with the steels with higher strength. The tensile strength of the weld joint was as high as that of the base metal for the steels up to 980 N/mm2 grade, while, for further higher grades, lower than that of the base metal because of the HAZ softening, which was effectively minimized by properly controlling the welding conditions.