New automatic arc welding process has been developed for welding ship's shell plates upwards from the ground. In this process, the axis of the leading welding gun corresponds to that of a weld bead, therefore it is called ‘Horizontal Coaxial Arc Welding’ (HCAW). In order to keep the welding process under control, the simplified numerical model expressing the static balance of forces on the leading molten pool has been designed. The model enables to estimate the weld bead height and the bead formation rate. As a result of analysis by the model, the wire extension length will reach its equilibrium state spontaneously in the case of using a constant voltage characteristic power source, and controlling of the wire feeding speed is effective to reduce the change of the wire extension length during welding in various root opening.
The problem of emissivity change on metal surface complicates the temperature measurement of molten pools in infrared (IR) radiation thermometry. Therefore, an emissivity correction is one of the most important subjects in IR radiation thermometry. The objective of the present investigation is to develop an ultraviolet (UV) radiation thermometry, which is an emissivity correction free method. According to the Planck's law of radiation, it is evident that the effect of temperature on the spectral radiance is greater at the shorter wavelength, and radiation thermometry in UV range is expected to reduce the error caused by the emissivity change. Generally, the radiation energy in UV range is lower than that in IR range at the same temperature. A measurement system that is sensitive to the UV radiation has been developed in the present work. The system consists of an image intensifier, and multiple optical band-pass filters. In the present paper, two-dimensional temperature distribution on TIG weld pool has been measured by using the developed UV radiation thermometer. It is made clear that the luminance temperature distributions measured by IR and UV range are quite different, and the surface temperature on molten pool is affected by sulfur concentration in the material.
AC pulsed MIG welding is an appropriate process to join aluminium alloy sheets for its great gap tolerance and low heat input to avoid burn through. However, in case of very high speed welding the low heat input is no longer an advantage since lack of penetration becomes a problem. Irradiating laser beam to the vicinity of arc can solve this problem and adjustment of the laser power can control the penetration. As a result, thin aluminium alloy sheets can be jointed at a high speed of 4 m/min with sufficient gap tolerance. Moreover, investigation of the effect of laser beam diameter on the welding result of thin sheets indicates that the defocused laser beam with a diameter of several millimetres can further improve the bridging ability for joint gap and tolerance for torch aiming deviation. Consequently, high power diode laser with relatively thick beam waist is suitable for this application and a diode laser/AC pulsed MIG hybrid welding robot system has been developed.
In the present research, the laser robot system was set up for cladding and repairing using a pulsed YAG laser, a 6-axis robot, an optical fiber and a special nozzle for powder supply. With this system, SUS304, Colmonoy No 5 and FP160 alloy powders were cladded on SM400B steel plate. The shape of clad layer, microstructures and performances of the clad layer were investigated by changing the process parameters. A sound clad layer with fine microstructure and low dilution was obtained. Moreover, the study on technology of rapid prototyping and repairing were performed by multi-layers cladding with a computer programming.
NOx pollution in the environment has become of a remarkable concern in Japan. Photocatalytic property of TiO2 material is recognized to be effective for the elimination of NOx with low concentration in large quantity. Thermal spraying process is feasible in making a large area and thick coating on the surface of the large construction. Therefore, direct coating onto the surface of the existing infrastructures by thermal spraying seems to be advantageous. In this study, formation of photocatalyst TiO2 coating was tried by thermal spraying. In order to reduce the NO2 generation from NO in the oxidation stage, it is effective to add adsorption substances to TiO2. The effect of addition of various additives to TiO2 both on the elimination property of NOx and reduce property of NO2 was also investigated in this study. Various adsorption substances were added to TiO2 and the photocatalytic properties were investigated. Improvements both in the elimination ratio of NOx of agglomerated powders with Ca3(PO4)2, CaCO3 and diatomite additives and in the generation ratio of NO2 was performed. In the case of additives of Ca3(PO4)2 and nickel to TiO2, the elimination ratio of NOx of agglomerated powder improved by about 15%, and generation ratio of NO2 was reduced to one third with compared to the pure TiO2 powder. Photocatalytic properties of the coatings with various additives were also investigated. Compared to TiO2 coating, generation ratio of NO2 was reduced in that with any kinds of additives. However, in the coating with nickel addition, elimination ratio of NOx decreased. It is inferred that melted nickel during spraying covers the surrounding TiO2 particles and interferes the contact between TiO2 and NOx. A possibility was confirmed that the photocatalytic characteristic can be improved greatly by adding the appropreate additives.
A formula predicting hardness and tensile strength of weld metal has been proposed in which the effects of alloy elements are considered in such a menner that C, Si, Mn, Ni, Cr and Mo contribute to hardenablity and Cu, Nb, V, and Ti contribute to precipitation hardening. This predictive work has been based upon 955 data of weld metals of SMAW, SAW and GMAW with tensile strengths from a mild steel grade to a 1,000 MPa high strength grade. The welding conditions in these data vary from 1.3 to 13.7 kJ/mm in arc energy and up to 450°C in preheat and inter-pass temperatures. Satisfactory prediction by this formula has been recognized for almost all of 955 weld metals.
Hyper interfacial bonding process was developed as a new conceptual joining process for ultra fine-grained steels. This process is characterized by sequential bonding procedures, namely, the extremely rapid heating of surface by high frequency induction heating, removing the heating coil rapidly and pressure joining by applied stress. The average grain size of ultra fine-grained steel used was 1.50 μm. The width of HAZ in hyper interfacial bonded joint was approximately 3.5 mm. Bainite, ferrite and pearlite were formed in the higher temperature region of HAZ, and M-A constituents were formed in the lower temperature region of HAZ. Grain size adjacent to the bonding interface had grown to approximately 16 μm, and the maximum hardness of joint was about Hv250. Tensile strength of joint was 558 MPa at the heating time of 0.17 s. The elongation and the reduction area of joint were comparable to those of the base metal when the upset length was more than 0.1 mm.
Tensile properties and microstructure of weld metal of X80 plates were evaluated. X80 plates were welded using full-automatic Gas Metal Arc Welding (GMAW) under several welding conditions. To clarify effects of welding conditions on weld joint performance in X80 pipeline, strength, toughness and hardness of weld joints were evaluated. Grain size and microstructural component of the weld metal were measured. Overmatching for the specified minimum yield stress (SMYS) of X80 was obtained for all the welding conditions. If distribution of the yield stress of X80 base metal was taken into consideration, however, welding conditions to obtain overmatching were limited. The strength and hardness of the weld metal decreased as the heat input increased. This was due to the change of acicular ferrite lath width and percentage of grain boundary ferrite and ferrite side plate. On the other hand, Charpy impact energy was high, irrespective of the welding conditions.
The effect of weld thermal cycle on creep rupture strength and microstructural change at high temperature service was investigated to clarify the mechanism of deterioration of creep rupture strength in Heat Affected Zone (HAZ) of heat resistant ferritic steel. Simulated weld thermal cycles, a temperature range from 1103 to 1473 K, were applied to test specimens machined from 10% Cr-3% Co-3% W-V, Nb ferritic steel plate. After heat treatment at 1013 K for 1.8 ks, simulating Post Weld Heat Treatment (PWHT), creep rupture strength were evaluated at 923 K with the applied stress of 98 MPa, and various microstructural examinations were carried out. The experimental results showed that creep rupture strength was deteriorated in HAZ heated in the peak temperature range of 1173-1273 K and that HAZ after creep test had the lower density of fine particle mainly consisting of MX type carbo-nitride less than 0.1 μm in diameter effective for dispersion strengthening as compared in base metal. In the HAZ heated at that peak temperature range M23C6 type carbide was partially dissolved and in creep tested specimen of that HAZ, MX type carbo-nitride contained chromium especially. The growth of MX in that HAZ was confirmed in accordance with the Ostwald ripening dominated by chromium diffusion. In conclusion, the deterioration of creep rupture strength in HAZ was caused by the decrease of dispersion strengthening effect by higher growth rate of fine particle. The mechanism to explain the acceleration of growth of fine particle in the HAZ was introduced as the following. M23C6 type carbide partially dissolved during weld thermal cycle precipitated again as fine particles and these fine particles caused the Ostwald ripening of MX dominated by chromium diffusion resulting in higher growth rate compared with that dominated by vanadium diffusion in case of base metal.
The effect of chromium content on creep rupture strength and microstructural change at high temperature service was investigated to clarify the mechanism of deterioration of creep rupture strength in Heat Affected Zone (HAZ) of heat resistant ferritic steel. Simulated weld thermal cycles, a temperature range from 1103 to 1473 K, were applied to test specimens machined from 2% Cr-1.5% W-V, Nb ferritic steel plates. After heat treatment at 1013 K for 1.8 ks, simulating Post Weld Heat Treatment (PWHT), creep rupture strength were evaluated at 923 K with the applied stress of 98 MPa and various microstructural examinations were carried out. In 2% Cr steel as well as 10% Cr steel shown in previous report, creep rupture strength was deteriorated in HAZ heated in the peak temperature range of 1173-1273 K, and that HAZ after creep test had the lower density of fine particle mainly consisting of MX type carbo-nitride less than 0.1 μm in diameter effective for dispersion strengthening as compared in base metal. However, a degree of deterioration of creep rupture strength in HAZ was smaller in 2% Cr steel than in 10% Cr steel. The growth rate of fine particles such as M23C6 type carbide and MX type carbo-nitride was lower in HAZ of 2% Cr steel compared with in that of 10% Cr steel. The growth of fine MX in 2% Cr HAZ was confirmed in accordance with the Ostwald ripening dominated by chromium diffusion as well as 10% Cr HAZ. The growth rate in the Ostwald ripening was described as a function of chromium content which increased as the increase of chromium content. As results, the mechanism of deterioration of creep rupture strength suggested in HAZ of 10% Cr steel, which the partial dissolution of carbide during weld thermal cycle could accelerate Ostwald ripening, was confirmed to have the universal validity to that in HAZ of 2% Cr steel.
It is an important subject to clarify the mechanism of welding crack formation to eliminate defects in welds in structures. Thus, the authors developed a finite element method (FEM) using temperature dependent interface elements in order to analyze all the processes of the crack formation, propagation and its arrest. The proposed method is applied to the Trans-Varestraint hot cracking test and the influences of the parameters included in the interface element are clarified. In the proposed model of hot cracking, the scale parameter r0 and BTR (Brittleness Temperature Range) need to be determined. If these parameters for alloys with different chemical compositions are accumulated as a database, the formation and propagation of hot cracking can be predicted for wide range of materials used in real welded structures. The procedure to determine the values of these parameters is proposed. Further, the validity or the general applicability of the parameters as material constants is demonstrated through the cross examination between the Trans-Varestraint test and the Houldcroft test. From these results, it is found that the results of experiments and computations agree well with each other quantitatively for both of the hot cracking tests. This suggests that the formation and propagation of hot cracking can be predicted using the proposed method.
Welding filler metals that phase-transformation temperature is low have been developed recently at NIMS. In order to improve the reliability of welded joints by reducing welding residual stress, the developed welding filler metals are used. It is difficult to detect phase-transformation temperature (Ms : starting temperature of phase-transformation or Mf : finishing temperature of phase-transformation) of filler metals with low phase-transformation temperature by using Formaster because of the lack of its cooling power. As transformation behavior of filler metals to be used in welding is considerably different from that of single filler used with low phase-transformation temperature owing to dilution of the base metal, it should be detected at actual welding because it remains no more than speculation. The authors detected the phase-transformation behavior during cooling process in welding by using the laser speckle strain measurement. As base materials, 9% Ni steel and low carbon steel plates were used for the experiments. On the other hand, several kinds of filler metals with low phase-transformation temperature were supplied. The transformation range and expansion during phase-transformation were clearly detected even the range was near the room temperature.
Recently, materials with low transformation temperature have been investigated for improvement of reliability in welded joint. Due to the introduction of compressive residual stress by using the material, fatigue strength or resistance to cold cracking are improved. In this report the authors examined the effects of phase-transformational behavior on residual stress and their distribution using some basic models and some types of the behavior. Expansions and temperature range of phase-transformation detected in the previous report using the laser speckle strain measurement are also used in this report. Results show that reduction of residual stress from tensile to compressive occurs only with severe restraint condition and low transformation temperature, especially low Mf temperature. In the previous report, the authors suggested the possibility of superplasticity during phase-transformation. Experimental results of residual stress distribution indicated the necessity of consideration for superplasticity by using the low transformation temperature materials.
Flanged edge welded joints are often used to join the same edge planes of two thin plates in a ferritic steel structure. They have crack-like incomplete root penetration, where stress concentration is very high. When the structures are subjected to repeated stress which exceeds a certain fatigue limit, fatigue cracks will initiate and propagate from the flanged edge welded joints. However, the fatigue behavior of the welded joint has not been investigated. In this research, in order to investigate the weld residual stress near the weld root tip of the flanged edge welded joints, both the measurement using sectioning method and the thermo-elastoplastic analysis were carried out. In addition, fatigue tests and fatigue crack propagation analyses were also performed on the welded joint to clarify the fatigue strength quantitatively. The experimental and analytical results of the residual stress showed that there was high compressive residual stress near the weld root tips. The fatigue test results indicated that all fatigue cracks in eight test specimens initiated at the weld roots. Their fatigue strengths at 2×106 cycles for the weld throat section was approximately 1.9 MPa and the fatigue limit was about 1.2 MPa. The results of fatigue life analysis showed that when the main plate thickness varied from 1.0 mm to 4.0mm, the fatigue strength increased in proportion to approximately 0.7 times of the main plate thickness.
In order to evaluate the hydrogen embrittlement susceptibility of high strength steel independently of stress concentration factor Kt and specimen size, Local Approach method modified to evaluate the hydrogen content distribution in the specimen was used. In the method it was considered that the size of micro cracks depends on the hydrogen content. The stepwise hydrogen embrittlement tests were carried out to obtain the hydrogen embrittlement susceptibility of the material which was 1400 MPa grade JIS SCM440 (0.40C-0.24Si-0.81Mn-1.03Cr-0.16Mo (mass%)). Stress was applied to the specimen, which is circumferentially notched round-bar type with the stress concentration factor of 4.9, after hydrogen pre-charging and homogenization treatments. The applied stress on the first step was set at 702 MPa and the stress of 14 MPa was increased on and after the second step. The holding time of stress was over 12 h on the first step and over 2 h on and after the second step. The diffusible hydrogen content was measured by thermal desorption analysis immediately after fracture. The distribution of stress and hydrogen content near notch root was calculated by FE-analysis. In the modified Local Approach method the new parameter ∑w,cr was introduced including the stress factor and hydrogen content factor. The parameter ∑w,cr obtained by the stepwise test obeyed the Weibull distribution as the Weibull stress in the conventional Local Approach. The hydrogen embrittlement susceptibility of any specimens with various stress concentration factors was able to be evaluated uniquely by the Local Approach considering the effect of hydrogen content distribution.
Cast iron has been widely used for the parts of sliding wear such as cylinder bores of motor engines because of its high resistance, which is mainly due to the fine pearlite structure and the existence of graphite acting as a solid lubricant. These features are derived from the rich carbon content (3-4 mass%). However, under severe sliding conditions such as those in high-power motor engines or exhaust-gas recycle (EGR) engines, cast iron without any treatment is not sufficient for wear resistance. In this study, iron casting with Mo-alloyed and W-alloyed was presented. Effects of the alloying of these metals on the microstructure, the hardness and the wear resistance were studied. Furthermore, surface modification by laser melting was carried out to obtain a chilled microstructure, that is, a non-equilibrium phase. The difference between the equilibrium and the non-equilibrium phases was examined for the tribological characateristics. 3% Mo-alloyed, 8% Mo-alloyed and 8% W-alloyed cast iron with laser melting showed high wear resistance ; on the other hand 16% Mo-alloyed cast iron as cast showed high wear resistance.
Authors tried to butt-weld a mild steel plate to an aluminum alloy plate by the solid slate welding using a rotating pin. This study investigated the effects of a pin rotation speed, the position for the pin axis to be inserted and the pin diameter on the tensile strength and the microstructure of the joint. The main results obtained are as follows : Butt-welding of a steel plate to an aluminum alloy plate was easily and successfully achieved. The maximum tensile strength of the joint was about 86% of that of the aluminum alloy base metal. Many fragments of the steel were scattered in the aluminum alloy matrix, and fracture tended to occur along the interface between the fragment and the aluminum matrix. A small amount of intermetallic compounds was formed at the upper part of the steel/aluminum interface, while no intermetallic compounds were observed in the middle and bottom parts of the interface. A little intermetallic compound was also often formed at the interface between the steel fragments and the aluminum matrix. The regions where the intermetallic compounds formed seem to be fracture paths in a joint.
Bonding of high density graphite by brazing with filler metal and sinter bonding with adhesive resin using direct current heating and sufficient pressing was studied. Radiative heating and slight pressing were also tested for comparison. Al, Ag-Cu-Ti, Ni-Cr-B were used for filler metal and phenolic adhesive resin containing graphite powders and hardening material was used for adhesive resin. Joint properties were evaluated by microstructure and bending strength of joint. Main results obtained are as follows : (1) At Al, Ag-Cu-Ti, Ni-Cr-B brazing using radiative heating and slight pressing (1.5×10-3 MPa), wettability of filler metal to graphite was very poor and 4 point bending strength of joint was low extremely. At Al, Ag-Cu-Ti, Ni-Cr-B brazing using direct current heating and sufficient pressing (9.8 MPa), above mentioned wettability was improved and 4 point bending strength of joint increased. But, 4 point bending strength of the later brazing joint was lower than bending strength of base graphite (78 MPa). This difference is based on generation of void and cavity. (2) At sinter bonding with phenolic adhesive resin using radiative heating and slight pressing (1.5×10-3 MPa), voids generated in adhesive resin and 4 point bending strength of joint was low. At sinter bonding with phenolic adhesive resin using direct current heating and sufficient pressing (9.8 MPa), voids disappeared and 4 point bending strength of joint equivalent to bending strength of base graphite (78 MPa) was obtained. This difference is based on generation of void in phenolic adhesive resin. (3) It was confirmed by gas analysis test that H2O gas generated by condensation-polymerization reaction of phenolic adhesive resin at heating. Removal of H2O gas is important, because H2O gas causes voids in phenolic adhesive resin. Holding at 353–393 K and sufficient pressing at sinter bonding process are effective for removal of H2O gas in phenolic adhesive resin.
Bonding of glassy carbon composite by brazing with filler metal and sinter bonding with adhesive resin using direct current heating and sufficient pressing was studied after investigation of microstructure and component of composite. Al, Ag-Cu-Ti, Ni-Cr-B were used for filler metal and phenolic adhesive resin containing graphite powders and hardening material was used for adhesive resin. Joint properties were evaluated by microstructure and bending strength of joint. Main results obtained are as follows : (1) This carbon composite had piled structure of black color layer and white color layer containing black color layer locally. It was confirmed by XPS that black color layer was graphite powder rich layer containing glassy carbon and white color layer containing black color layer locally was glassy carbon rich layer containing graphite powder, carbon fiber. It was confirmed by microstructure observation and XPS that top surface of composite (surface of 1st black color layer) was the most glassy carbon rich layer. (2) Polishing of joint surface is necessary for obtaining of adequate roughness and flatness. Component of bonding surface varies with this polishing of joint surface. (3) Relationship between joint properties and various bonding surface was investigated. At Al, Ag-Cu-Ti, Ni-Cr-B brazing, wettability of filler metal to each bonding surface was very poor and 4 point bending strength of joint was zero in any bonding surface. At sinter bonding with phenolic adhesive resin, 4 point bending strength of black color layer joint was about 20 MPa and that of white color layer joint containing black color layer locally was about 10 MPa. These bending strength of joint were lower than bending strength of base carbon composite (124 MPa). Bonding did not advance at top surface of composite (surface of 1st black color layer : the most glassy carbon rich layer). It was assumed from these results that bending strength of joint decreased with increase of glassy carbon. Plasma discharge in H2O atmosphere for black color layer (polishing surface of 1st black color layer : polishing 5 μm) was effective to increase of bending strength of joint. 4 point bending strength of plasma discharged joint was about 35 MPa.
This study was carried out to develop the flux and filler metals that enabled brazing a magnesium alloy plate of AZ31B at the temperature lower than 490°C. The following results were obtained in this study. The newly developed flux that consisted of CaCl2, LiCl and NaCl containing Ca ion and Li ion could successfully removed the oxide film from the magnesium alloy surface at around 450°C, and enabled us to braze the magnesium alloy at the temperature lower than 490°C. In addition, we were successful in developing the brazing filler metal with the melting temperature lower than 480°C that consisted of magnesium and indium as a main component and a small amount of zinc to lower the melting temperature. A joint with high strength equivalent to the base metal one was achieved using the above flux and the filler metal with little zinc, and the joint strength decreased with the increase in the zinc content.