In order to investigate effects of tungsten (W) content on the friction welding of copper-tungsten sintered alloy (Cu-W alloy) to oxygen free copper (OFC), the tensile strength of joints between Cu-50%, W alloy and OFC obtained under various welding conditions has been compared with that of the joint between Cu-70%W alloy and OFC which was reported in a previous paper. The strength of Cu-50%W/OFC joint and Cu-70%W/OFC joint was increased with rise in forge pressure P2, and saturated at P2 above 250 MPa. At P2 above 150 MPa, however, the tensile strength of the Cu-50%W/OFC joint was considerably lower than that of the Cu-70%W/OFC, though both the axial displacement and friction torque during the welding for the former were much larger than those of the latter. On the tensile test, fracture of the Cu-50%W/OFC joint welded at P2 above 150 MPa was initiated in the Cu-W alloy adjacent to the weld interface and propagated along the weld interface, while the Cu-70%oW/OFCC joint was fractured in the heat affected zone of the OFC. in the Cu-50%W/OFC Joint, the W content of the Cu-W alloy adjacent to the weld interface was increased significantly compared with that of the base metal, and cracks were formed in the region where the W content was increased. It is considered that these cracks were responsible for the initiation of fracture in the Cu-50%W alloy next to the weld interface and for the lower strength of the Cu-50%W/OFC joint than that of the Cu-70%W/OFC joint. The increase in the W content and the formation of the crack became more remarkable with the increase in axial displacement during the welding, suggesting that these were caused by the plastic deformation of the Cu-50% \V alloy durnig the welding. The formation of the crack was avoided by decreasing the amount of the plastic deformation of Cu-50%W alloy, i.e., by increasing the friction speed and decreasing friction and forge pressure.
The strength of the diffusion bonded joints of ductile cast irons having the strength of 360 (FCD 37) or 490 MPa (FCD 50) and the ductile cast iron to a mild steel had been investigated in the 1st report. In this report, the effects of the post bond heat treatments-annealing for ferritization and normalizing for structure changing of matrix-have been studied. The results obtained are as follows; (1) The tensile fracture of the joints which were annealed after that bonding carried out using the insert metals of Ni or Fe foils occurred in the base metal. However, both joints without a insert metal and with Cu foil fractured at the bonding interface. (2) When the ductile cast irons were directly bonded without a insert metal the joint strength after annealing depended on the volume fraction of graphites which precipitated on the bonding interface by decomposition of Fe3C during annealing, and abruptly deteriorated with increasing the fraction of these graphites. This graphite volume fraction increased in proportion to the pearlite volume fraction around bonding interface before annealing. (3) As to the joints of the ductile cast iron to the mild steel, the joint strength after annealing depended on the bondability between the ductile cast iron and the insert metal. Except the insert metal of Cu foil, the tensile fracture occurred at the mild steel side away from the insert metal. (4) The tensile strength of ductile-ductile joints increased as the volume fraction of pearlite formed in matrix by normalizing for structure changing incrased and was the same as that of the base metal up to 550 MPa. When the strength of the base metal was higher than 550 MPa the joint strength reached the plateau value of 85% of the base metal strength. (5) On the other hand, the strength of both joints without the insert metal and with Cu foil did not exceed 80% of the base metal strength of 430 to 1000 MPa.
The effects of coating configurations and hot-pressed strengthening treatment on the properties of ceramic-metal composite coatings were investigated. Alumina and Ni-base self fluxing alloy were plasma sprayed onto mild steel using recommended spray conditions for alumina. Hot-pressed treatments were carried out in vacuum. The adhesive strength of the coatings under static loading and the repeated thermal shock resistance were tested as well as the abrasive wear and electrochemical corrosion resistance. The porosity, coating thickness and microhardness were also measured. The results obtained are summarized as follows: 1) The mixed coatings were superior in wear resistance, whereas the composite coating proved to be more resistant to repeated thermal shock and corrosion. 2) Hot-pressed treatment showed excellent effects on the coatings properties. It tightened the substrate/coatnig interface which resulted in higher adhesive strength, and it also made the coatings to be denser. The mixed coatings showed improvements in their wear and thermal shock resistance, corrosion and thermal shock resistance were, on the other hand, improved in the case of composite coatings. However, over a certain hot-pressed treatment temperature, the composite coatings performance was reduced.
For the application of electron beam welding to nuclear and thermal power plant, electron beam weldability of carbon steel (SGV49, SB49) and Cr-Mo low alloy steel (SCMV3A) have been investigated. The results obtained are summarized as follows; (1) Tensile strength and 0.2% proof stress of the welded joint at room temperature and high temperature up to 350°C satisfy the base metal code and the MITI code. Test specimens fractured in the base metal. (2) Tensile properties of carbon steel weld metal are also good. (3) Reference nil ductility temperature (RTND-r) of weld metal in SGV49 steel is -30°C as derived from Charpy V-notch tests and drop weight tests. (4) The high cycle fatigue strength of the welded joint without reinforcement is similar to that of the base metal. The fatigue strength of the joint with reinforcement is lower than that of the base metal, and its fatigue strength reduction coefficient is 1.07 to 1.31 at 105 to 2×106 cycles.
Crack Tip Opening Displacement (CTOD) properties in the HAZ of 50 kgf/mm2 (490 MPa) class steels were investigated from the view point of fracture facet size and fraction of M-A constituent. The C-Mn microalloyed steels with carbon content of 0.025 to 0.19 wt. % were selected in this study. The results obtained were as follows: (1) The CTOD properties in the HAZ were strongly affected by the carbon content which varied both the fracture facet size and fraction of M-A constituent in the local brittle zone near fusion line. (2) The fracture facet size got small and the fraction of M-A constituent was increased by increasing the caron content. (3) The distributions of carbon content in the HAZ were analyzed by the EPMA. The peak points in carbon content corresponding to the M-A constituent were 1.07-1.32 wt.% in spite of that the carbon contents in the bulk of base metal were 0.025 to 0.19 wt.%. (4) The cementite as well as the M-A constituent were observed in the HAZ of the steel plate containing carbon more than 0.06 wt. %. (5) The mechanism of the M-A constituent formation was discussed in condensing the carbon atoms. (6) The CTOD properties, as compared with the Charpy impact properties, were greatly influenced by the M-A constituent.
In this paper, a model of repair welds is assumed for three-dimensional thermal elasto-plastic and two-dimensional plane-deformation thermal elasto-plastic analyses. The results of the analyses are compared in order to clear and to consider the mechanism necessary for replacing three-dimensional problems with plane-deformation problems. Thesummaryofthemainconclusionsisasfollows: Promtheresultsofthree-dimensionalthermalelasto-plasticanalyses; 1) In the portion which undergoes a quasi-steady state temperature history, whether there is temperature gradient along the weld line or not, the producing residual stresses and plastic strains are same at each cross section along weld line if the rigidity to thermal shrinkage is uniform. 2) The shorter of the length of repair weld and the slower of travel speed, the more stress component of perpendicular to weld line, σy, increases. Moreover, the plastification is delayed by slow travel speed, so that plastic strain component of welding direction, εpx decreases. From the comparison between the results of two-dimensional plane-deformation and three-dimensional analyses 3) Only when the weld length is long and there is no temperature gradient along the weld, three-dimensional problems can, in the strict sense of production mechanism, be replaced plane-deformation problems including the distribution and magnitude of transient and residual stresses and plastic strains. However, as residual stresses are controlled by the yield condition of a material, replacement of the three-dimensional problems with plane-deformation problems leads to estimation of the distribution and magnitude without large difference in the results. If the travel speed is decreased, it needs to be paid attention to underestimate σy, in plane-deformation analysis. Whereas, since the distribution and magnitude of plastic strains are largely affected by the mechanical boundary conditions, the estimation of plastic strains are less accurate than that of residual stresses in plane-deformation analysis. 4) The temperature distribution for thermal elasto-plastic stress analysis of plane-deformation can be given from two-dimensional analysis which assumes no thermal conduction from the cross section. 5) According to the model, if the travel speed is v≥0.0055 m/s in the portion which undergoes quasisteady state until it cools down to 400°C (the weld length; l≥0.2 m, at least the middle parts of model undergoes quasi-steady state), the distribution and magnitude of residual stresses and plastic strains can be estimated from plane-deformation analysis. When only residual stresses are needed to be known, ν≥0.0017 m/s may be enough for the plane-deformation analysis to work out.
In this paper, the mechanical behavior on SR (stress relief annealing) for the purpose of removing the residual stress produced by repair welding of thick plate is clarified. The obtained main result is as follows: From the results of three-dimensional thermal elasto-plastic stress and creep analyses: 1) At the early stage during SR-treatment, the creep strain tends to easily accumulate in HAZ (Heat Affected Zone) and weld metal in which has been existed larger welding residual stress than the base metal, so that the stress relaxation is remarkable in HAZ and weld metal. After the stresses in HAZ and weld metal reached to be almost same magnitude of those of base metal, the stress relaxation progresses same amount all over the plate. 2) The configuration of stress distribution is not changed during SR-treatment regardless of heating and holding stage, and then the plastic strain is not produced during SR-treatment. 3) If the stress relaxation characteristic is poor in the base metal, the creep strain can easily accumulate in the weld metal and HAZ. Comparing the results of three-dimensional thermal elasto-plastic creep analysis with that of two-dimensional plane-deformation one and one-dimensional approximate analysis: 4) The choosing of SR condition, i.e. heating rate, holding temperature and holding time etc., can be predicted by the results of one-dimensional approximate analysis. 5) In the case of replacing three-dimensional problem with two-dimensional plane-deformation problem for analyzing of the welding residual stress and SR, the accuracy of the creep strain and stress during and after SR depends upon how to actually represent the severity of mechanical conditions including the magnitude of welding residual stresses.
In this paper, a predicting method of welding residual stress by using the source of residual stress (inherent strain) is proposed. And the validity of the method is demonstrated by numerical experiments with the aid of the finite element method. Welding residual stress is produced as a result of thermal elasto-plastic behavior. Their source is composed of the thermal strain and/or plastic strain, etc., which is called here inherent strain. Taking a butt welding joint as an example, the sources of residual stress are estimated in different sizes manufactured on the same welding condition. It is found that the distributions of the source of residual stress are almost the same if the sizes of the joint are larger than that of a specific one, which is named a standard size. It is demonstrated that the residual stress produced in the joint of any different sizes can be predicted accurately by elastic analysis using the inherent strain in a standard size of the joint, unless the sizes are too small.
The Induction Heating Stress Improvement (IHSI) technique is regarded as one of the most effective remedies for intergranular stress corrosion cracking (IGSCC) cocuring in the heat affected zone (HAZ) of susceptible stainless steel in some boiling water reactor piping systems. In this process, cooling water must flow at a velocity high enough to keep the inside surface of the pipe relatively cool, creating a high temperature gradient through the pipe wall. However, nuclear plant pipings, where forced flowing during this process may be difficult, have been welded. This paper presents computer simulation analyses and experimental verifications of post-IHSI residual stress distribution of the welded portion of a nozzle with a thermal sleeve, where forced water cooling is difficult. From these analyses and experiments a new IHSI technique is developed. This technique consists of new heating method which has temperature grandient through both the pipe wall and longitudinal direction.
Supposing the case in which leakage takes place in tubing of steam generator, the explosive plugging method was developed by Japan Welding Engineering Society etc. It was indicated by the researchers that this method is effective to the purpose. It is not yet known whether the plugged parts keep the stable strength or not, during the long service period in the environment of steam generator. Under the above consideration, following three tests on plugged part are conducted. 1. Thermal Shock Test 2. Stress Corrosion Cracking Test 3. Liquid Sodium Corrosion Test Testing results are grained as follows. 1. Peeling occures immediately after beginning of thermal shook test. Cracking occures after 150-200 thermal cycles in SH, 200-400 thermal cycles in EV and 1000 thermal cycles in SG. 2. Residual stress in tube plate near the plug is compressive. SCC does not occur in the tube plate there. It occures on the inner surface of plug. 3. The susceptibility to SCC of explosive bonded boundary is lower than that of base metal due to martensite in bonded boundary. 4. Explosive bonded boundary is somewhat weaker than base metal in the resistance to the sodium attack under the testing condition employed in liquid sodium corrosion test.
Characteristics and behavior of tungsten arc and feasibility of arc welding of mild steels were investigated in the pressure range from 0 to 5MPa (gauge). As the cathode, 2% thoriated tungsten of 4 mm in diameter and of 30° in vertex angle was used. Results obtained are summarized as follows: (1) Arc voltage increases with increasing pressure. VA+VK is 11.5V in OMPa and it decreases with increasing pressure. It shows about 9.3V constantly over 0.5MPa. (2) Arc column concentrates and the diameter of arc decreases with increasing pressure. As the result, current density in centre of arc increases considerably. (3) The area of cathode spot decreases and therefore cathode mode of arc changes from normal mode to cathode spot mode with increasing pressure. As the result, the erosion of tungsten tips increases. Especially, the erosion in He atmosphere is much greater than that in Ar atmosphere. (4) The depth of penetration of mild steel plate increases with increasing pressure. Therefore, the optimum welding current for penetration welding of thin mild steel plate decreases considerably with pressure. (5) Sound penetration welds of mild steel plates of 3.2 mm in thickness can be obtained under pressure range up to 3MPa.
A mathematical formulation has been developed to represent the velocity field and the temperature field of DC welding arc. The formulation utilized an engineering approach in which the plasma was regarded as a continuum of known electrical properies, the behavior of which could be represented by using the MHD equations and the convective heat balance relationship including the concept of turbulent nature of flow. The calculations preceded by specifying the arc current and electric field distributions, and then governing equations were solved numerically, ultimately yielding the velocity and temperature profiles within the system under various condition of arc burning. The theoretical predictions of the temperature field and heat transfer characteristics are compared and discussed with experimental results.
Diffusion-welded joints were made under welding atmosphere such as vacuum at 4 and 4×10-3 Pa, and Ar, H2 and air at 1×105 Pa. Better welding atmosphere for diffusion welding was investigated from the estimation of mechanical properties of diffusion-welded joints, defects such as void and inclusion, and residual gas in voids of diffusion-welded joints. The materials used were SUS304 stainless steel and Ti-6Al-4V alloy, and the results obtained are as follows: 1) The best welding atmosphere is vacuum at 4×10-3 Pa for both materials in five kinds of welding atmosphere used. 2) For the good selection of welding atmosphere, it is necessary to consider three items as follows. a) Welding atmosphere which does not have influence on quality of base metal. b) Welding atmosphere which does not oxidize the welding surface. c) Welding atmosphere which does not remain in voids. 3) At diffusion-welded joints made under Ar atmosphere, Ar remains in voids of diffusion-welded joints. Ar does not diffuse into the base metal at all, and prevents the shrinkage of voids.
The effects of aluminum content in aluminum bronzes and bonding time on the joint strength of a commercial aluminum bronze and a stainless steel joints, which were bonded using the insert metal of Ni foil at the bonding temperature of 890°C, have been investigated. And also, Cu-Al binary alloys having various aluminum contents instead of the aluminum bronze were bonded to the stainless steel using Ni foil, in order to obtain the fundamental information of the diffusion layer, which affects the joint strength. The results obtained are as follows; (1) In case of Cu-Al binary alloys, the phase of the diffusion layer that was formed on the Cu-Al alloy side of bonded zone was a solid solution of the Ni, Cu and Al in case of 2.13 wt% Al-Cu alloy, and κ phase (Ni3Al) in case of 4.52 wt%Al. β phase (Cu3Al+NiAl) and β+κ in case of 7.62 wt%Al. (2) The joint strength depended on the aluminum content and the maximum joint strength of 427MPa was obtained in 3.51 wt%Al and the bonding time of 3.6ks, but the joint efficiency decreased as the aluminum content became higher. (3) When the aluminum content was below 2 wt%, the fracture in tensile test occurred in the bronze of base metal up to the bonding time of 30 ks. As the aluminum content increased over 2 wt%, the fracture occurred in the diffusion layer and also the maximum joint efficiency was gradually shifted towared short bonding time. (4) Kirkendall effect was recognized on the bonded interface between the bronze and the Ni foil, and the voids were formed in Kirkendall interface. These voids have acted as crack initiation sites. (5) The number and size of voids in the diffusion layer increased as increasing the aluminum content, and also its size grew with the increase of bonding time. (6) In case of 8.35 wt%Al, the fracture occurred from the void that was formed in the Ni foil instead of the void in the diffusion layer when bonding time was over 30 ks.
An experimental model of a reverse side bead width control system which employes a fiber optic thermal sensor and a microcomputer has been developed. The system is worked by monitoring infrared radiation emitted from the surface of the workpiece. In this system, a welding peak current of low frequency pulsed welding current is controlled in accordance with fluctuations in the surface temperature so as to ensure the optimum surface temperature is obtained for a high quality reverse side bead. Using this system, butt weld with groove and with gap were carried out. The fluctuations in the temperature and in the reverse side bead width were smaller when an operational gain in the welding peak current was 1 A/pulse than that was 3 A/pulse. The fluctuations of the reverse side bead width was less than ±0.5 mm at the butt weld with groove and with 0.5 mm gap.
The bonds in various electric devices are recently required high precision and high quality at bonds. To control the material deformation, thermo-characteristics by resistance heating while bonding are examined, and the optimum electrode materials and their constitution are selected as described in the report-1 and the report-2. To get high quality for bonds, examinations about defects at the interface are done, and then the vapor shield process for cleaning interface is newly developed. Moreover, the effects of the vapor shield process on preventing defect formation is successfully proved as described in the report-3 and in the report-4. In this study, to apply the above process named Vapor Shielded Pressure Bonding on mass-production line of contact-bars of electromagnetic switchgears, the monitoring process of metal expulsion which is conclusive fatter of bonding quality and inprocess quality control system are examined. The main results obtained are as follows; 1) Metal expulsion volumes fluctuate in continuous bonding of a number of contact-bars, because both the alterations of material properties and supplied power bring fluctuation of heat inputs. 2) In finding metal expulsion volume of each bonding by electrode displacement, the correct volume can be detected by employing forging stage before bonding. For, alteration of surface confiugration of contact materials which obstruct the correct detecting of expulsion can be effectively neglected by forging. 3) The beginning point of expulsion while bonding can be found accurately in real time by monitoring electrode displacement. 4) The inprocess control system by monitoring electrode displacement is successfully developed. The system can continuously produce the sound bonds of contactbars without any bad one.
Possibility of detecting the information on weld line or groove shape near molten pool in TIG arc welding process by combination of He-Ne laser and interference filter with narrow width at half maximum (WHM: 1.0 nm) is shown experimentally for in-process control of arc welding process. Relative light intensity ratio of laser light reflected or scattered from surface of object and light emitted from arc or molten pool can be easely calculated theoretically on the assumpution that transmittance of interference filter is expressed as Gauss's error distribution function with wave length, and then it is shown theoretically that relative light intensity ratio increase with decrease of half width (WHM) of the filter. On other hand, adoptig the interference filter with extrem narrow half width, or max transmittance of the filter is easely influenced by the change of temperature, humidity, incident angle of laser and so on. As the result, optimum half width on interference filter exists for improving the relative light intensity ratio. It is clarified experimentally that the light scattered from projected laser spot near molten pool can easely detected without disturbance of powerful arc light, and furthermore, it's method can be applied to get the information on behaviour of the molten pool in TIG arc welding process, by using combination of He-Ne laser and the interference filter with narrow band pass (WHM: 1.0nm.)
An author's previous paper on heat-treating of Type 316L weld metal revealed that the decomposition of δ ferrite in the weld metal (EWM) containing mainly an eutectic δ-ferrite occured rapidly as compared to that in the weld metal (PWM) containing mainly a primary δ ferrite and this reason was attributed to the difference in the lattice coherency at the δ/γ interface. In this paper, to clarify metallographically the results described above, the precipitation process of M23C6 and sigma phases was investigated by transmission electron microscopy using the weld metals heattreated at 923 or 1123K. In the EWM specimen, at both the heat-treating temperatures, the M23C6 phase precipitated initially at the δ/γ interface and then within δ ferrite grains and the precipitation of σ phase occured finally. However, in the PWM specimen, the precipitation of M23C6 could not be observed at all within δ ferrite grains. This suggests that M23C6 formed readily in the EWM specimen, of which the δ ferrite phase contained larger amounts of molybdenum than that in the PWM specimen. On the other hand, the M23C6 at δ/γ interface in the EWM specimen precipitated rapidly and in large amount, comparing with that in the PWM specimen. Particularly, as this tendency was remarkable at 1123K, the coherency of the δ/γ interface was considered to influence considerably the precipitation of M23C6.
The effect of electromagnetic stirring on the microstructure of fully austenitic stainless steel weld metal was investigated. The parameters examined to achieve the grain refinement were magnetic field intensity, the frequency of alternately stirring and relative distance from an electrode to magnetic field center. Bead-on-plate TIG welds were made in the conditions that welding current was 60A and travel speed was 3 cm/min. A significant decrease in the grain size of weld metal could be achieved when the electrode was located 1 to 2 cm apart from the magnetic field center to welding direction and the stirring frequency was 0.5 to 1 Hz. It was the most likely that remelting of a primary dendrite arm by stirred molten metal played a main role of the grain refinement of weld metal, and both the existence of fragments separated from a primary dendrite arm and the increase in constitutional supercooling ahead of solid-liquid interface due to molten-metal stirring assisted the weld metal in grain-refining.
This paper deals with the predictive equation for obtaining a maximum hardness in the heat-affectedzone. The estimating equation was derived using the kinetics of phase transformation and the theory of heat transfer. The material constants included in the equation were determined from hardness data of CCT-diagram by the stepwise multiple regression analyses. The predictive equation was proved to be more reliable than other formulae by comparing with data of hardness reported by many researchers.
It has been reported that clean steels with very low sulphur and oxygen contents may be more susceptible to hydrogen cracking than traditional grades. However, some investigations showed little effect of sulphur on weldability. In order to clarify the reason for this discrepancy, many steels with widely varied sulphur contents (0.001%-0.0037%) were laboratory-melted and their HAZ hardnesses were examined. Hydrogen diffusion rates were also examined for the base metals with varying contents of sulphur, and their HAZs to investigate the significance of inclusions in reducing the risk of hydrogen cracking through their roles on hydrogen trapping. The findings in the experiments are as follows: 1) In C-Si-Mn steels, MnS and MnS-Oxide inclusions act as ferrite nucleation sites. HAZ hardenability of these steels thus increases as the sulphur content decreases. 2) In C-Si-Mn-Nb steels, Nb suppresses the function of MnS as ferrite nucleation. Therefore, HAZ hardenability in this type of steel is not influenced by sulphur content. 3) Hydrogen is found to be temporarily trapped in inclusions. As steel contains less inclusions, diffusible hydrogen tends not to be trapped in base and thus hydrogen accumulation in HAZ is more likely. 4) Inclusions influence the susceptibility of steel to hydrogen HAZ cracking mostly through enhancement of hardenability and partly through hydrogen trapping.
The standard conditions for stress relief annealing (SR-treatment) of welded structures such as pressure vessels are indicated by JIS or ASME code. However, application of these conditions to high quality thick plates seems to be unreasonable, since they require for such plates to simply keep higher temperature for longer time than acutally needed. This is partially due to the lack of sufficient knowledge on the effect of SR-treatment over the reduction of welding residual stresses, especially for thick plates. This study is to develop some simple calculation method to accurately estimate residual stresses remained in a very thick plate after SR-treatment. In the 1st report, approximate equations were developed for the relaxation test both at changing and constant temperatures because of its similar stress relaxation phenomenon to that of SR-treatment. In this report, this method is further developed and applied to SR-treatment of welded joints of very thick plates in more general stress states and boundary conditions. Using the various relations between stresses and strains at high temperatures, estimating equations are formulated in order to simply calculate transient and residual stresses in welded joints during and after SR-treatment. The results are compared with the highly accurate analytical result based on the finite element method. The both results show such a good coincidence that the appropriateness of the new method is confirmed.
It is known that stress concentration is occured at root of welded joint with backing strip by misalignment, as the misalignment causes eccentricity to load. However, it is hard to avoid misalignment in field welded joints of steel deck plate. In this paper, fatigue behaviors were investigated on welded joints with backing strip which were fabricated by overhead position. The stress concentration factors of various root shapes were analyzed by the finite-element method. Estimating equation from root radius and root angle was derived based upon FEM results. The fatigue strengths for 2 million cycles were calculated from the fatigue test results, then the stress concentration factor for required fatigue strength was obtained. The results obtained in this reports are as follows; 1. The fatigue lives of specimens with misalignment 2.0 and 3.0 mm were inferior to that of specimens without misalignment, and the fatigue life was largely decreased at misalignment 4.0 mm for manual weled and CO2 welded joints. 2. Even slag inclusion and blow hole were existed in weld metal, they did not cause crack initiation. The fatigue cracks were mainly initiated at the weld root of specimen. 3. The stress concentration factor was mostly influenced by the misalignment. 4. Whether the root shape was convex or concave, the stress concentration factor was n ainly influenced by shape of toe at base metal side. 5. The stress concentration factor can be obtained by the estimating equation, which consists of eccentricity to load and shape of toe at the base metal side. 6. The stress concentration factor to satisfy required fatigue strength was given in relation to toe radius and flank angle.