Authors tried to develop a new nickel-based brazing filler metal containing no boron and phosphorous to suppress the formation of brittle phases in a brazed layer. The properties of a joint brazed using the new filler metal were investigated and compared with those of a brazed joint using a conventional nickel-based filler metal of BNi-5. The following results were obtained in this study. A new nickel-based brazing filler metal was successfully developed, which contains a small amount of Si and Mn and shows the melting temperature lower than about 1090°C. The hardness of the new filler metal was about 40% that of BNi-5 filler metal containing about 10% Si. In brazing of SUS304 stainless steel, using BNi-5 filler metal, a hard and brittle finally-solidified region appeared in the brazed layers with joint clearance over 100μm. However, using the new filler metal, the finally-solidified region appeared in the brazed layers with joint clearance over 200μm. The hardness of the finally-solidified region in the brazed layer using the new filler metal was about 54% that using BNi-5 filler metal. The tensile strength of a SUS304 joint brazed using BNi-5 filler metal extremely decreased at joint clearance over 100μm. On the other hand, the tensile strength of a joint brazed using the new filler metal decreased at joint clearance over 200μm. It seems that the brittle finally-solidified region formed in a brazed layer causes the joint to decrease the tensile strength. It was shown that the newly developed filler metal is superior to the conventional BNi-5 filler metal in hardness and in tensile strength of joints with wider joint clearance.
The use of thin steel components that can be joined by one-layer-one-pass laser welding or laser-arc hybrid welding has been increasing with the trend of reducing the plate thickness by using higher strength steels. This has enhanced the demand for laser-arc hybrid welding of HT780 steels in the bridge and industrial machinery applications. Taking this trend into account, the authors have researched the suitable welding parameters and consumables for the hybrid welding process that combines laser and MAG arc welding for joining HT780 steel sheets. As a result the following welding parameters have been proven to be most appropriate for better root gap resistance and lower spatter: MAG arc for leading (kept at the right angle)/laser for trailing (kept at the pushing angle); laser-arc distance of 3-5 mm; laser's focus distance of ± 0 mm; and shielding gas of Ar-20%CO2. As to the welding wire for laser-arc hybrid welding of HT780 steel, the HT590-class arc welding wire of Cr-Mo-Ti type with a low amount of sol. titanium has been revealed to be suitable to obtain sufficient weld joint tensile strength and toughness. Applied plate thickness range of the most relevant wire is from 6 mm to 12 mm.
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, and the influence of the constituents of base material on the transition behavior was clarified. In this study, we examined the influence of welding conditions on the transition behavior of oxide inclusions to find out the combustion and upset pattern for decreasing the oxide inclusions efficiently. As a result, it was found that the use of oxy/acetylene reduction flame and the low of welding pressure in the initial stage of welding process were effective to decrease the oxide inclusions. Moreover, it was confirmed that the proposal welding condition based on the examination was able to raise the quality of welds than the traditional welding condition.
A series of experiments under cyclic loads was carried out to propose the newly repair welding procedure under cyclic loads. It was confirmed that the sound welded joints could be obtained by gouging hot crack from the reverse side and by performing multi-pass welding on it even if hot crack occurred in the first-pass of the multi-pass welding from the surface. The soundness of welded joints could be confirmed from the results of the fatigue test, too. From the results of the fractography, the cellular dendrite which is the characteristic of hot crack was observed on the first pass of the weld metal. And striations which are the characteristics of fatigue crack were observed after the second pass and at the root of the welds. It was confirmed that fatigue crack was generated from the starting point of hot crack and the root of welds and it was propagated to the thickness direction by cyclic loads. The size of hot crack was influenced by welded metal and the magnitude of Δδb (root gap displacement before welding). As the measure of monitoring the propagation of fatigue crack after welding, Δδa (root gap displacement after welding) was proposed and its usefulness was verified by comparing with macrograph of welded joints. As to propagation of crack due to cyclic loads, it was effective that Δδb was made to be small. From the result of the fatigue test, soundness of the welded joints repaired by the proposed procedure was confirmed.
Temperature measurement of the arc plasma is so important regarding the realization of the physical property that spectroscopic techniques have been used to measure the temperature distribution in free-burning arcs in argon gas. These methods, however, assume that the arc plasma is axially symmetry, so they can be applied to only the symmetrical arc plasma. In the point of production, since the arc welding is applied to various joint geometries and the welding torch moves along the welding line, most of actual arc plasma phenomena are not axially symmetric. Therefore it is required to develop a measurement method of the non-axial symmetrical arc plasma. This study proposes a three dimensional temperature measurement method with use of the image reconstruction. This method could obtain much the same temperature distribution as previously reported methods and be applied to the non-axial symmetrical arc plasma.
It has developed the numerical simulation method of hydrogen diffusion for multi-layer weld on Cr-Mo-V steel. The developed simulation method can estimate the distribution of hydrogen concentration on weld metal and predict reduction of hydrogen by various dehydrogenation heat treatment conditions. The accuracy of calculation is practically enough for selecting the condition to prevent a cold cracking. In order to improve the accuracy of the calculation of hydrogen diffusion on weld, it is necessary to input the actual thermal history and initial hydrogen concentration into the calculation model. Also, it is important to model the hydrogen diffusion corresponding to the real phenomenon. It has found that the hydrogen degassing rate of Cr-Mo-V steel was slower than the conventional steel from measured data by gas chromatography. The hydrogen concentration was the maximum at the position of slightly inside from surface of last weld side. If it was compared the reduction of hydrogen concentration of 623K 4 hours and 553K 6 hours, the 623K 4 hours is much effected. However, when the longer time at 553K DHT is applied, it is expected further reduction of hydrogen concentration.
The objective of this study is to ensure the safety of pressure vessels. It is very important to know the welding residual stress in order to maintain the safety of welded components and design an effective maintenance plan, including the fabrication process. Very thick welded joint is studied in this research, so the welding residual stress has complex distribution in the section. The inherent strain method combined with Finite Element Method (FEM) is applied to measure the welding residual stresses accurately. One more subject is to examine the effect of stress-relief annealing (SR treatment) for the thick welded joint. So, two same mock-ups for a welded joint of pressure vessel were prepared for the study. As a practical measuring procedure of the inherent strain method, TL-method had presented. However, the method gives the determinate value of the residual stress, and does not give the most probable value and the deviation which are estimated by the original inherent strain method. So, the TL-method is improved to give them, allowing the reliability of the estimated results to be discussed. The deviations of the estimated results are small enough for the most probable values. It assures the high reliability of the results. The residual stresses after the SR treatment are small enough, compared with the welding residual stresses. Sufficient effect of the SR treatment is confirmed.
The RESA-1 neutron engineering diffractometer in the JRR-3 (Japan Research Reactor No.3) at the Japan Atomic Energy Agency, which is used for stress measurements, was upgraded to realize residual stress measurements of large scaled mechanical components. A series of residual stress measurements was made to obtain through-thickness residual stress distributions in a Type 304 stainless steel butt-welded pipe of 500A-sch.80 using the upgraded RESA-1 diffractometer. We evaluated effects of crack propagation such as stress corrosion cracking (SCC) and a part-circumference repair weld on the residual stress distributions induced by girth welding. Measured residual stress distributions near original girth weld revealed good agreement with typical results shown in some previous works using finite element method, deep hole drilling as well as neutron diffraction. After introducing a mock crack with 10 mm depth in the heat affected zone on the inside wall of the pipe by electro discharge machining, the axial residual stresses were found to be released in the part of the mock crack. However, changes in the through-wall bending stress component and the self-equilibrated stress component were negligible and hence the axial residual stress distribution in the ligament was remained in the original residual stresses near girth weld without the mock crack. Furthermore, changes in hoop and radial residual stress were also small. The residual stress distributions after a part repair welding on the outer circumference of the girth weld were significantly different from residual stress distributions near the original girth weld. The through-thickness average axial residual stress was increased due to increase of the tensile membrane stress and mitigation of the bending stress after repair welding. Throughout above studies, we evidenced that the neutron diffraction technique is useful and powerful tool for measuring residual stress distributions in large as well as thick mechanical components.
In order to investigate the durability of an Ir-based alloy tool, the FSW of 304 stainless steel plates was performed under various welding conditions, and the mechanical properties of the joints were measured. Defect-free joints are possible under certain conditions and the mechanical properties of the joints are the same as the base material. When the rotation pitch is about 0.3, the degree of tool wear of the Ir-based alloy tool with a 1.4-mm probe height, which was used for the FSW of a 304 stainless steel plate with a 2-mm thickness was less than or equal to that of the PCBN tool with a 2-mm probe height, which was used for the FSW of 304 stainless steel plate with a 9-mm thickness. The result of the tool life test in this study showed that the Ir-based alloy tool enabled the FSW of 304 stainless steel over a 75-m length.
Three commercially available welding consumables were surface welded to mild steel plate. Typical microstructures of their deposited metals were, hypoeutectic (Consumable A), hypereutectic (Consumable B), and hypereutectic with MC type carbides (Consumable C), respectively. The welding parameters were chosen so that the chemical composition of the deposited metals were kept about the same. When the welding was carried out in longitudinal direction with a bead-pitch of 8 mm, the deposited metal cooled rapidly. In contrast, when the welding was carried out reciprocating welding bead in transverse direction with a small bead-pitch of 3 mm, thermal cycle became shorter and the temperature of the welding zone was raised by the effect of multiple thermal cycles, resulting in a slow cooling of the deposited metals. Besides the thermal cycles, the thickness of the base plate was changed from 25 to 12 mm to reduce the cooling rate of the deposited metal. Abrasion resistance of each deposited metal was evaluated by the rubber wheel test. Main results obtained were as follows; (1) By the preheating effect of multiple thermal cycles, the cracking of the high Cr cast-iron deposited metals was reduced remarkably. (2) In general, the wear loss of the deposited metal was decreased as the hardness increased. (3) The wear loss of the consumable A deposited metal was decreased when cooled slowly and larger austenitic dendrite and inter-dendritic eutectic carbides were formed. (4) The wear loss of the consumable B deposited metal was decreased when cooled rapidly and primary carbides were surrounded by austenitic eutectic matrixes. (5) The consumable C deposited metals, in which MC type carbides and primary-like carbides were dispersed in austenitic matrixes, showed the best abrasion resistance among the consumables investigated.
The purpose of this study is prevention of generation of polyethylene dust when pipe is jointed by friction welding. The joining specimen is changed from I butt welding to taper faying surface, polyethylene is softened rapidly in the initial friction by wedge effect. Furthermore, an effect of a mud on joining phenomena is examined, when large amount of the mud is adhered to faying surface since the mud adhere to faying surface during gas and water piping works.; The experimental results are as follows; 1) Polyethylene of faying surface was softened very rapidly (about 0.1s). This main reason was wedge effect for taper faying surface. 2) Time that dust was generated was remarkably short, and quantity of dust was little. This dust remained at the tip and inside of faying surface, subsequently it was melted with molten polyethylene of faying surface and the dust was not observed inside of the pipe. 3) In the case of taper angle was 5°, arrival time to initial torque was extremely early. Steady torque was increased slightly as the taper angle was decreased. 4) A large amount of the mud was pushed out from faying surface. This mud was shaved by faying surface when it combined rotation and fixed side before friction welding. Remaining mud was mixed with molten polyethylene so that inside of the pipe was not dirty. 5) Tensile strength and elongation of the welded joint that mud was adhered were 24.0MPa and 503%, respectively. These were nearly equal to those of the clean welded joint. Therefore, rapid and low cost joining without dust was enabled by using taper faying surface.
The occurrence of ductility-dip crack in the laser overlay welds of alloy 690 to type 316L stainless steel was predicted by the mechanical and metallurgical approaches. Ductility-dip temperature ranges (DTRs) of alloy 690 laser overlay welds were estimated by Varestraint test during GTA welding. The grain boundary segregation of impurity elements such as P and S was numerically analyzed based on the non-equilibrium cosegregation theory when the welding speed and the amounts of P and S in the weld metal were varied. In accordance with the repression approximation between the DTR and the calculated grain boundary concentrations of P and S, the DTRs of alloy 690 were computed in laser overlay welding. The estimated DTR in laser overlay welds was reduced with an increase in welding speed and with a decrease in the amounts of P and S in the weld metal. Ductility-dip cracking in laser overlay welds was predicted by the plastic strain-temperature curve intersected the DTR. The plastic strain in laser overlay welding was numerically analyzed using the thermo elasto-plastic finite element method. The plastic strain-temperature curve in laser overlay welds intersected the DTR at decreased welding speed and increased (P+S) content in the weld metal. The predicted results of ductility-dip cracking in laser overlay welds were approximately consistent with experiment results. It follows that ductility-dip cracking in laser overlay welds could be successfully predicted based on the estimated DTR from grain boundary segregation analysis combined with the computed plastic strain by FEM analysis.
The objective of this study is to ensure the safety of nuclear reactors. A number of accidents caused by leaks from welded zones at pipe penetration points of the reactor vessel or in coolant pipes have been reported at sites 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. In this study, the inherent strain method combined with Finite Element Method (FEM) is applied to measure the welding residual stress accurately. A mock-up for a welded joint at a coolant pipe of an actual nuclear reactor was manufactured for the study. The inherent strain method is used to measure the three-dimensional residual stress distribution. In this method, the inherent strains are unknowns. When the 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. 10 kinds of inherent strain distribution functions are applied to estimate the residual stress distribution of the joint. Applicability of each function is diagnosed. Accuracy and reliability of analyzed results are judged from three points of view, that is, residuals, unbiased estimate of variance of errors and necessary conditions from welding mechanics. Most suitable function is selected, which brings most reliable result. The characteristics of the three-dimensional residual stress distribution are cleared. The circumferential stress and axial stress are important, related to stress corrosion cracking (SCC). The circumferential stress is estimated to be large tension near the outer surface of welding finish region. The large tensile axial stress is produced near the outer surface, widely in the circumferential direction. Near the inner surface where SCC may occur, both stresses are small tension or compression.
Effect of REM addition to alloy 690 filler metal on microcracking prevention was verified in laser clad welding. Laser clad welding on alloy 132 weld metal or type 316L stainless steel was conducted using the five different filler metals of alloy 690 varying the La content. Ductility-dip crack occurred in laser clad welding when La-free alloy 690 filler metal was applied. Solidification and liquation cracks occurred contrarily in the laser cladding weld metal when the 0.07mass%La containing filler metal was applied. In case of laser clad welding on alloy 132 weld metal and type 316L stainless steel, the ductility-dip cracking susceptibility decreased, and solidification/liquation cracking susceptibilities increased with increasing the La content in the weld metal. The relation among the microcracking susceptibility, the (P+S) and La contents in every weld pass of the laser clad welding was investigated. Ductility-dip cracks occurred in the compositional range (atomic ratio) of La/(P+S) <0.21(on alloy 132 weld metal), <0.10(on type 316L stainless steel). Solidification/liquation cracks occurred in that of La/(P+S) >0.99(on alloy 132 weld metal), >0.90 (on type 316L stainless steel), while any cracks did not occur at La/(P+S) being between 0.21-0.99 (on alloy 132 weld metal) 0.10-0.90 (on type 316L stainless steel). Laser clad welding test on type 316L stainless steel using alloy 690 filler metal containing the optimum La content verified that any microcracks did not occurred in the laser clad welding metal.
Friction stir welding (FSW) by using the self-reacting tool offers a solution to conventional FSW problem by using the fixed tool. Self-reacting tool is composed of upper and lower shoulder. To weld distorted material without inner pores, controlled pinch force between upper and lower shoulder succeeded in self-reacting FSW. The optimum welding condition of self-reacting tool was not wider than that of fixed-tool. Mechanical property of weld section by using the self-reacting tool was as strong as by fixed tool.
Generally, cemented carbides (WC-Co) that consist of tungsten carbides and cobalt are used for cutting tools. But since cemented carbides are made of import dependent rare metal, reuse and recycling are required. If cylindrical shanks that the cutting edges are cut off can be combined, it is possible to reuse those shanks as new cutting tools. But in this case, the joining strength of brazing method tends to be rather weak for being used as cutting tools. Instead of the brazing method, the diffusion bonding will be expected with the high strength and the small deformation. Many studies on the diffusion bonding were reported, but there were few studies on the diffusion bonding of cemented carbides. In this study, a potential for applying the diffusion bonding of carbides was investigated. So the effect of the bonding condition on the bending strength, the microstructure of the bonding line and the distribution of tungsten and cobalt were examined. As a result, the strength of the diffusion bonding was higher than that of the brazing at higher than 1100°C, and at 1200°C, it was almost the same as the base metal's strength. Cemented carbides were successfully joined by the diffusion bonding.