Recently, higher steam inlet temperature and pressure have been required for fossil power plants in order to increase the power generation efficiency. Therefore, boron bearing high chromium steels with higher long-term creep strength are applied to structure materials in these plants. Dissimilar (metal) weld joints between a boron bearing high chromium steel and a low alloy steel are applied for various parts of boiler and other equipment in the plants. Caution has to be paid to maintain the quality of these weld joints because boron is diffused to the low alloy steel weld deposits by dilution from the boron bearing high chromium base metal. However, the existence form of boron and the effect of boron on reheat cracking susceptibility during post weld heat treatment (PWHT) have not been surveyed in the previous literature. Therefore, using Cr-Mo low alloy steels with varied contents of B and Cr, reheat cracking susceptibility of the weld metal was evaluated, and the precipitation behavior of the carbides during PWHT was observed. >From these results, the existence form of boron in boron bearing Cr-Mo steel weld metals was discussed in this study. Reheat cracking susceptibility clearly increased with boron addition, whereas it decreased with increasing chromium content. It was shown by thermo-dynamic calculation that possible existence forms of boron were four types, namely BN, M2B, M23(C, B)6 and dissolved boron. The amount of M23C6 type carbides was increased with increasing chromium content. On the basis of these results, it was presumed that the amount of dissolved boron in boron bearing low chromium steel weld metals was larger than in those with high chromium. It was suggested that the larger amount of dissolved boron enhanced strength difference between the matrix and the grain boundary in low chromium steels, hence it led to the higher susceptibility to reheat cracking.
For predicting welding distortion and residual stress generated by laser-arc hybrid welding, a series of experiments and analyses were carried out. A bead-on-plate welding was performed on SM490 steel by using a fiber laser and CO2 arc welding with changing their heat input ratio variously. The experiment was simulated by the thermal elastic-plastic analysis with the proposed simulation model considering the penetration shape by laser and arc separately. By using this model, the experimental results could be simulated with high accuracy. Therefore, the validity and generality of the numerical simulation model could be verified. The tendency and magnitude of angular distortion varied with the heat input ratio of laser and arc. The results indicated the possibility of the ideal heat input ratio of laser and arc for controlling angular distortion generated by hybrid welding. On the other hand, it was confirmed that the heat input ratio of laser and arc did not affect residual stress generated by hybrid welding.
The laser brazing was carried out for dissimilar joining of sialon to the WC-Co alloy using the eutectic type Ag-Cu alloy filler metal with different Ti contents ranging from 0 to 2.8 mass% in order to investigate the effects of Ti as an active element in the filler metal on the interface structure of the joint and the joint strength. The filler metal sheet was sandwiched by the sialon block and the WC-Co alloy plate. The laser heating was carried out by irradiating laser beam selectively on the WC-Co alloy plate. The brazed joint was obtained using the filler metal containing over 0.3 mass%Ti, which was joined through the interfacial compound layer consisted of TiN, Ti5Si3 and Cu4Ti. The shear strength of the brazed joint increased with increasing Ti content in the filler metal in the range of 0.3 to 1.7 mass%, and reached 106 MPa in maximum value, but it decreased at a higher Ti content.
Numerical simulations such as Finite Element Method (FEM) are widely used as tool of structural analyses in both design and production. However, in the application of FEM to welding problems, the simulation scale is usually limited to the welding joint level. Only a few large-scale welding analyses are performed on existing research because welding is transient problem and show strong nonlinearity. In such cases, it is necessary to use static implicit FEM to achieve an accurate analysis, but the larger analysis scale requires larger memory consumption and computing time. Thus, we previously proposed Idealized Explicit FEM (IEFEM) to achieve shorter computing time and lower memory consumption. Since IEFEM is based on dynamic explicit FEM, it is possible to perform the calculation for each degree of freedom (DOF) and element. Such characteristic indicates that IEFEM is suitable for parallelization. Then, in this study, we developed parallelized IEFEM using a graphics processing unit (GPU). The usefulness and validity of the developed method are considered by analyzing a 3-dimensional multi-pass moving heat source problem, which is very difficult to analyze with commercial FEM software because of its analytical scale. As a result, it is found that parallelized IEFEM accelerated by a GPU can analyze a large-scale problem having over 1,000,000 DOFs on a single PC.
Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view in low carbon low alloy steel weld metals. The samples from ESW and SAW deposits with various cooling rates were prepared in this study. In those samples, intragranular acicular ferrite formation was observed from inclusions. The inclusions contributing to acicular ferrite formation were multi-phase type consisting of amorphous phase, spinel type and MnS. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction patterns and EDS analyses that the Ti-enriched layer was TiO. The acicular ferrite had Baker-Nutting orientation relationship (OR) with TiO layer on the inclusion surface. The misfit was 3.0% at the interface between the acicular ferrite and TiO. Therefore, it is considered that the TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite by small lattice misfit. However, the morphologies of ferrite growth which nucleated from inclusions were different in both samples. Whereas ferrites nucleated from TiO were enough grown up in ESW, the size of nucleated ferrite in SAW was a few hundred nm in sizes. In the early stage of nucleation from TiO, ferrite had small deviation from Kurdjumov-Sachs (K-S) OR in both ESW and SAW. However, there was the difference in the growth stage of ferrite. The ferrite orientations were gradually changed to fit to K-S OR in ESW. On the other hand, the nucleated ferrite in SAW stopped growth and the newly nucleated ferrite which had K-S OR to prior austenite was formed in adjoining because of large super cooling due to small heat input.
In order to clarify the nitrogen absorption mechanism in gas tungsten arc welding, the measurement of the weld metal nitrogen content under nitrogen mixture shielding gases, and the numerical analysis of plasma heat source characteristics in nitrogen dissociation phenomenon were conducted. The nitrogen content of weld metal produced by He arc reduces to approximately a half relative to that by Ar arc in the shielding gas condition of less than about 1% mixture ratio. Additionally, it is assumed that a decline in the plasma temperature in the vicinity of the molten pools due to the generation of metal vapor, accompanied by a reduction in atom-like nitrogen content, cause intense impact on the reduction mechanism of weld metal nitrogen content in a He arc.
Deep penetration is an important advantage in laser welding. This study was undertaken in order to investigate effect of reduced-pressure atmosphere from 0.1 kPa to ambient pressure (101 kPa) on partial penetration welding. A 16 kW disk laser of 1030 nm in wavelength was employed and Type 304 stainless steel and A5052 aluminum alloy was met-run welded in 17-mm/s welding speed. The penetration depths of the stainless steel and the aluminum alloy achieved 26 and 23 mm, respectively, at vacuum pressure of 10 kPa. The depths were over 1.6 times as deep as those obtained at 101 kPa. Decrease of boiling temperature calculated on the basis of the reduced-pressure atmosphere coincided with an increase of the penetration depth. At less than 1 kPa, however, the penetration depth in steel was kept 26 mm steady and it in aluminum decreased to 19 mm. Refraction angle caused by a laser-induced plume measured with a 532 nm-wavelength probe laser beam was drastically reduced corresponding to surrounding pressure of 101 to 0.1 kPa. According to observation of the keyhole inlet with high-speed video camera, its size in steel decreased and stabilized with the pressure reduction, which indicate that reduced-pressure atmosphere made an incident laser beam focused stably with little influence of the plume and led to the stable keyhole inlet. On the other hand, the keyhole inlet in aluminum expanded more than quadruple of the ambient-pressure inlet size at 1 and 0.1 kPa, which seems to make such an unstable process that the weld penetration was reduced.
To evaluate the influence of minor and impurity elements such as C, Mn, P and S on the solidification and ductility-dip cracking susceptibilities of extra high-purity type 310 stainless steels, the transverse-Varestraint test was conducted by using several type 310 stainless steels with different amounts of C, Mn, P and S. Two types of hot cracks occurred in these steels by Varestraint test; solidification and ductility-dip cracks. The solidification cracking susceptibility was significantly reduced as the amounts of C, P and S decreased. The ductility-dip cracking susceptibility also reduced with a decrease in P and S contents. It adversely, however, increased as the C content of the steels was reduced. Mn didn't greatly affect the hot cracking susceptibility of the extra high-purity steels. The characteristic influence on solidification cracking was the ratio of P:S:C=1:1.3:0.56, while Mn negligibly ameliorated solidification cracking in the extra low S (and P) steels. The numerical analysis on the solidification brittle temperature range (BTR) revealed that the reduced solidification cracking susceptibility with decreasing the amounts of C, P and S in steel could be attributed to the reduced BTR due to the suppression of solidification segregation of minor and impurity elements in the finally solidified liquid film between dendrites. On the other hand, a molecular orbital analysis to estimate the binding strength of the grain boundary suggested that the increased ductility-dip cracking susceptibility in extra high-purity steels was caused by grain boundary embrittlement due to the refining of beneficial elements for grain boundary strengthening such as C.
In this study, we ultrasonically bonded Cu and Ni sheets and evaluated the thermal reliability of the joints at 473 K by high-temperature testing. Furthermore, we observed the interfacial microstructures of the joint and evaluated their effects on the bondability of the joint. Cu and Ni sheets were metallurgically bonded under the optimum condition of the ultrasonic bonding. Cu base metal strongly deformed compared with deformation of Ni base metal during the ultrasonic bonding. The ultrasonically bonded interface of Cu and Ni sheets was composed of bonded region and unbonded region. The bonded region was composed of two types of micro structures. Cu and Ni were locally stirred and approximately 200-nm-thick of solid soluted region of Cu and Ni atoms was formed in the bonded region. The Cu grains were fined, moreover, the Ni grains became fined from the bonded interface to Ni side 1-2μm. These fine grains can be formed by deformation of bonded base metals during the ultrasonic bonding process, suggesting that plastic deformation was formed from the soft Cu base metal to the hard Ni base metal. The ultrasonically bonded joints showed good thermal reliability at 473 K through the holding time up to 1000 h. Some of the Cu grains became coarse in the Cu base metal. Phase separation of solid soluted region of Cu and Ni atoms may occur during the high-temperature test performed at 473 K.
The purpose of this study is joining main and branch gas pipes by friction welding without digging a paved road. By this development, only making a hole at vertical side of the paved road, the branch pipe can be connected to main pipe. In this study, an end face of branch pipe was welded to a side of main pipe. Furthermore, in consideration that gas main pipe is laid in the ground, an influence of mud on joining state is examined. The experimental results are as follows; 1) Branch pipe welding without digging a paved road was enabled by friction welding method. 2) Developed method does not require digging and repairing the paved road. Therefore, construction cost can be reduced, and traffic is not obstructed. 3) Tensile strength and elongation of branch pipe friction welded joint were 17.7∼17.8MPa and 520∼525%, respectively. These were nearly equal to those of the base polyethylene. 4) A large amount of mud was eliminated from faying surface by friction. Therefore, the influence of the mud on tensile strength and elongation was not recognized.
The influences of welding condition on the atmospheric nitrogen mixing into the arc plasma in helium GTA welding was analyzed by numerical simulations. In order to evaluate the effects of the convection flow and the diffusion on the nitrogen mixing phenomenon, the distributions of the Peclet number was used. Elongation of the electrode length has low impact on the decrease of shielding gas concentration because the convection flow becomes dominant in this area which indicates higher Peclet numbers. Meanwhile, the nitrogen diffusion increases in the plasma area with the temperature of about 10,000K, so that elongation of the arc length leads to a remarkable decrease of shielding gas concentration. Additionally, the impact of convection flow increases in the arc center area where high-velocity plasma jet exists, and the shielding gas concentration tends to rise owing to higher welding current in the condition of sufficient shielding gas flow rate.