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The coupling of a hot tensile test with a novel in situ observation technique using a high-speed camera was investigated as a high-accuracy quantitative evaluation method for ductility-dip cracking (DDC) susceptibility. Several types of Alloy 690 filler wire were tested in this study owing to its susceptibility to DDC. The developed test method was used to directly measure the critical strain for DDC and high temperature ductility curves with a gauge length of 0.5 mm. Minimum critical strains of 1.3%, 4.0%, and 3.9% were obtained for ERNiCrFe-7, ERNiCrFe-13, and ERNiCrFe-15, respectively. The DDC susceptibilities of ERNiCrFe-13 and ERNiCrFe-15 were nearly the same and quite low compared with that of ERNiCrFe-7. This was likely caused by the tortuosity of the grain boundaries arising from the niobium content of around 2.5% in the former samples. Besides, ERNiCrFe-13 and ERNiCrFe-15 indicated higher minimum critical strains even though these specimens include higher content of sulfur and phosphorus than ERNiCrFe-7. Thus, containing niobium must be more effective to improve the susceptibility compared to sulfur and phosphorous in the alloy system.

Surface melting by gas tungsten arc (GTA) welding and overlaying by cold metal transfer (CMT) brazing using low melting point filler wire were investigated to develop a repair process for cracks in worn cast steel of steam turbine cases. Cr-Mo-V cast steel, operated for 188,500 h at 566 °C, was used as the base material. Silver and gold brazing filler wires were used as overlaying materials to decrease the heat input into the base metal and the peak temperature during the welding thermal cycle. Microstructural analysis revealed that the worn cast steel test samples contained ferrite phases with intragranular precipitates of Cr7C3, Mo2C, and CrSi2 and grain boundary precipitates of Cr23C6 and Mo2C. CMT brazing using low melting point filler wire was found to decrease the heat input and peak temperature during the thermal cycle of the process compared with those during GTA surface melting. Thus, the process helped to inhibit the formation of hardened phases such as intermetallics and martensite in the heat affected zone (HAZ). Additionally, in the case of CMT brazing using BAg-8, the change in the hardness of the HAZ was negligible even though other processes such as GTA surface melting cause significant changes. The creep-fatigue properties of weldments produced by CMT brazing with BAg-8 were the highest, and nearly the same as those of the base metal owing to the prevention of hardened phase formation. The number of fracture cycles using GTA surface melting and CMT brazing with BAu-4 was also quite small. Therefore, CMT brazing using low melting point filler wire such as BAg-8 is a promising candidate method for repairing steam turbine cases. However, it is necessary to take alloy segregation during turbine operation into account to design a suitable filler wire for practical use.

Solidification cracking occurs as a result of thermal strain and solidification behavior that is determined by welding conditions, especially welding speed. Brittle temperature range (BTR), an important index for evaluating solidification cracking, is used to quantity the effect of welding speed on susceptibility to solidification cracking. However, measurement of BTR using Trans-Varestraint test during laser welding has rarely been studied. The purpose of this work is to investigate the effect of welding speed on solidification cracking susceptibility in type 310S stainless steel using Trans-Varestraint test during laser welding. Solidification cracking and ductility-dip cracking were distinguished by observing fracture surfaces. Compared with traditional Trans-Varestraint test carried out for gas tungsten arc welding, the number of solidification cracks and total crack lengths during laser welding were lower; however, the number density of solidification cracks and total crack length per bead width during laser welding were higher. Both of these values had a tendency to first increase and then decrease slightly with increasing welding speed: the maximum values occurred at approximately 1.5 m/min. Temperature profiles at 0.2 and 1.0 m/min during laser welding were measured by an optical-fiber radiation thermometer combined with in-situ observation using a high-speed camera. BTR was measured using the center crack length along the heat flow in the welding direction. BTR at 1.0 m/min was less than that at 0.2 m/min during laser welding because the maximum crack length appeared at the side of molten pool at welding speeds greater than 1.0 m/min.

Shop primer is a predominant cause of pits and blowholes during fillet welding using a conventional CO2 arc welding process. The hot-wire laser welding process for fillet welds employs a reflected laser beam incident on a molten pool for melting the base metal, and produces a weld bead with an extremely low dilution of base metal. In this study, weld-defect prevention in hot-wire laser welding as applied to steel plates coated with shop primer was investigated by examining the vaporization of the shop primer at the molten pool front and beneath the stiffener at the root region. A melting technique employing a reflected laser beam incident on the molten pool surface can efficiently remove the shop primer at the molten pool front during welding. The quantity of several gases and metallic vapors emitted during decomposition of the shop primer at a root was reduced markedly during hot-wire laser welding under extremely low base metal dilution. The above features of the hot-wire laser welding process achieved sound fillet welds without pits and blowholes.

Solidification cracking during welding is very serious problem for practical use. Therefore, there are so many reports concerning solidification cracking. Normally, solidification cracking susceptibility of material is quantitatively evaluated using Trans-Varestraint test. On the other hand, local solidification cracking strain was tried to measure precisely using in-situ observation method, called MISO method about 30 years ago. Recently, digital high-speed video camera develops very fast and its image quality is very high. Therefore, we have started to observe solidification crack using in site observation method. In this paper, the local critical strain of a solidification crack was measured and the high temperature ductility curves of weld metals having different dilution ratios and different grain sizes to evaluate quantitatively the effects of dilution ratio and grain size on solidification cracking susceptibility by using an improved in situ observation method.

Hot-wire laser welding with a narrow gap groove was applied to ASTM A213 grade T91 steel in the application of ultra super critical boilers. A filler wire according to AWS A 5.28: ER90S-G was used. The effect of the depth-to-width ratio (D/W) of a weld shape on the susceptibility to solidification cracking was investigated. With a groove width of 3 mm, solidification cracking did not occur with a D/W ratio of less than 0.6 and more than 1.2. The hot-wire laser welding method with a narrow gap groove seemed to be efficient for practical use, when the higher D/W condition was used without solidification cracking. The high temperature strain, which occurred during solidification, was computationally calculated in order to make clear the effect of D/W ratios on the susceptibility of solidification cracking in theory. From in situ observation using a high-speed camera of the hot-wire laser welding phenomena, a three-dimensional Finite Element Method (FEM) was simulated to estimate an equivalent plastic strain in the weld metal during welding. An approach to the moving heat source consistent with such phenomenon was proposed and validated based on experimental measurements. The simulation results presented that a moving heat source by hot-wire laser welding was able to produce a particular weld shape at a higher D/W ratio with a lower strain rate, which corresponded with the relationship between D/W ratios and solidification cracking in experiments. Therefore, such a hybrid process is a very interesting alternative process to reduce the solidification cracking in narrow gap welding, especially in modified 9Cr1Mo steel.

The purpose of this work is to develop hot-wire laser welding for narrow-gap joints. Test pieces of ASTM A 304 stainless steel with a gap width of 3 mm were used. With welding by tentative filler rod made of Inconel 600 alloy, melting phenomena of the molten pool and the weld bead formation during welding were investigated by in situ observation using a high-speed camera. Variations in the main welding parameters such as wire current, wire feeding position, laser irradiation angle, and wire feeding angle were investigated to determine appropriate conditions. Experiments showed that under the optimum welding conditions, hot-wire laser welding was able to produce complete weld deposition with very low dilution of the base metal. An experiment on laser beam reflection indicated that the laser beam reflected from the molten pool was a crucial phenomenon to explain the formation of the weld bead, especially in terms of melting the side groove wall of the base metal. Welding parameters as mentioned above significantly affected weld quality. Furthermore, in the case of welding with filler wire ER NiCrCoMo-1, the Vickers hardness distribution along the transverse welding direction was examined to evaluate weld properties and the bonding strength at the fusion interface was examined in a tensile test, and the results showed that there was no great difference in the hardness profile throughout the cross section of the weld plate. Additionally, metallurgical examination of the fracture surface revealed the rupture occurred at the weld metal, probably because the bonding at the fusion interface was stronger than the weld metal. Therefore, the development of laser welding with hot wire can advance the multi-pass weld for an ultra-narrow-gap joint.

The aim of this paper is to develop the high efficiency and high quality fillet welding technique using the combination of hot wire system and Laser welding. The melting of filler wire depends on wire current and there is an adequate vale of the current in each wire feeding speed. The leg length increases by the addition of filler wire compared with the non filler welding. The length is also larger than the laser irradiated area. This must be caused by the laser beam reflection on the molten pool surface. The results reveal that the parameter optimization leads to the stationary welding phenomenon. Besides, obtained welds have attractive properties, namely low heat input and low dilution.

In this study, the effect of grain size on solidification cracking susceptibility of Type 347 stainless steel was investigated by using U-type hot cracking test with developed in-situ observation method and the effect of grain size of the weld metal on critical strain for solidification cracking was evaluated quantitatively. The grain size of the weld metal varied from about 70 to 210μm by changing in the grain size of the base metal. Consequently, it was concluded that according to the measurement of CST (Critical Strain Temperature), solidification cracking susceptibility increased with an increase in grain size of weld metal. Moreover, the area of the grain boundary for each unit area decreased when the grain size increased, and then strain which was applied for grain boundary increased and then critical strain for solidification cracking of grain boundary decreased with grain size. Therefore, it was guessed that the solidification cracking susceptibility increased with an increase in grain size.

Material flow during friction stir spot welding is investigated by experimental approaches in this study. Different kinds of fine wires and WC powder are used as trace material and disposed in the interface between upper material and lower material. After spot welding, detailed microscopic observations are carried out in the various sections. Continuous distributions of tracer materials are investigated during friction stir spot welding. Three-dimensional material flow model is proposed by the combination of the results, which are obtained in the present study and previous studies.

Effect of microstructures on liquation cracking is investigated. Melted eutectic formation and cracking are observed in the stir zone region of friction stir spot welds made in squeeze cast and in incompletely solutionised AZ91 sections containing Mg17Al12 particles. Crack susceptibility in the stir zone region is dramatically decreased in AZ91 base material which has been forged and solution treated, and is completely free of Mg17Al12 particles. It is suggested that rapid heating during the tool penetration stage in friction stir spot welding creates undissolved Mg17Al12 particles, which facilitate eutectic melting when the temperature in the stir zone reaches 437°C, the (α-Mg+Mg17Al12) eutectic temperature in the binary Mg–Al equilibrium phase diagram.

This study is aimed at developing a systematic method for evaluation and prediction of solidification cracking in laser dissimilar welded joints (laser DWJ). The initiation of solidification cracking was observed directly by using a high speed camera during U-type hot cracking testing while laser welding. The high temperature ductility curves were obtained based on the local critical strains of the solidification cracks, measured by investigating each frame of the observation film. The distribution of residual liquid metal during solidification was investigated by using both high magnification and in-situ observation. Moreover, material properties at the elevated temperatures were measured by using a developed tensile test. The weld strain during laser welding was calculated by 3D finite element analysis by using the material properties obtained. Consequently, all the results helped to provide a comprehensive understanding of solidification cracking in laser DWJ.

This study is purposed to develop an evaluation method of solidification cracking susceptibility for laser DWJ of different dilution ratios. By using U-type hot cracking test with in-situ observation, the critical strain for the initiation of solidification cracking was measured locally near the crack rather than the macro strain measured in most other hot cracking tests. Meanwhile, the temperature history during laser welding at the trailing region of the molten pool was measured experimentally. The high temperature ductility curve was achieved based on the above test results of the critical strain and the temperature history. The critical strain rate of temperature drop (CST) was used to evaluate the solidification cracking susceptibility. Moreover, the residual liquid metal at the solidification front during laser welding was observed directly by using in-situ observation with an optical microscope lens. Consequently, all the results showed that solidification cracking susceptibility is the highest when the ratio of Inconel600 takes up roughly 40% in the weld bead among the used Inconel600/SUS347 laser DWJ.

The purpose of this study is to develop the ultra-high-speed GTA welding process with hot-wire system using pulsed current to heat filler wire. For precise control of the ultra-high-speed (over 5 m/min) GTA welding, it is necessary to understand deeply the ultra-high-speed welding phenomenon. The melting phenomenon of filler wire was observed in detail by a high-speed camera and filler wire temperature was measured precisely by a radiation thermometer. As a result, precise temperature distribution of filler wire and clearly visualized wire melting phenomenon was obtained.

The purpose of this study was to develop the remote laser welding technique for galvannealed steel sheets using the pre-deforming process by laser heating and to develop in-line inspection system for remote laser welding system using visible images. It was concluded that the weldabillity of lap welding joint of galvannealed steel sheets could be improved by making the sheet gap of more than 0.04mm using the laser pre-deforming process before laser welding and moreover the quality of lap joints could be evaluated by using the proposed in-situ monitoring system.

This study is purposed to develop a systematic method for prediction of solidification cracking during laser welding by accurately obtaining the high temperature ductility and weld strain of weld metal. The U-type hot cracking test was carried out under various initial loads; solidification cracking behavior was in-situ observed by a high speed camera so that the critical strain, for the initiation of solidification crack, was measured precisely. With the critical strain and temperature history measured at the solidification crack, the high temperature ductility curve was achieved. Meanwhile, a three dimensional FEM-analysis was performed to obtain the detailed strain history at the crack initiation point during laser welding. In order to get the accurate strain history, it is necessary to obtain the accurate material properties of weld metal at the elevated temperature up to solidification temperature range. The FEM-analysis was carried out by using the obtained high temperature material properties, which were measured by a developed high temperature tensile test with in-situ observation. Therefore, it became possible to predict the occurrence of solidification cracking during laser welding accurately by using both of the detailed high temperature ductility curve measured experimentally and the strain history obtained by the 3D FEM-analysis.

Thermodynamic and kinetic modeling were conducted to simulate Cr depletion profiles near grain boundaries in Inconel 182 during heat treatments and laser surface melting (LSM) using Thermo-Calc and DICTRA code. The effect of Nb addition was also considered in the modeling. Based on the good agreement with Cr concentration distributions during the heat treatments measured experimentally, Cr depletion profiles adjacent to grain boundaries during the heat treatments and the LSM process were modeled. The Cr depletion profiles were evaluated using the Cr depletion area below the critical Cr concentration for intergranular cracking/intergranular stress corrosion cracking (IGC/IGSCC) susceptibility (12 mass%). Compared with the result of the Streicher test, the calculated Cr depletion areas showed good agreement with IGC/IGSCC susceptibilities. The sample after stress relief (SR) treatment had the largest Cr depletion area and showed the poorest IGC/IGSCC resistance. Cr depletion showed some recovery during subsequent low temperature sensitization (LTS). The sample after the LSM process had the smallest Cr depletion area and showed the best IGC/IGSCC resistance.

Cracking during dissimilar friction stir spot welding of thixomoulded AM60 and AZ91 sheet materials is investigated. Liquation cracking occurs in the thermomechanically affected zone (TMAZ) region during friction stir spot welding when AZ91 material is the lower sheet in the dissimilar AM60/AZ91 sandwich. A TMAZ microstructure comprising elongated α-Mg grains and aligned Mg17Al12 is created during the tool penetration stage in spot welding and melted eutectic film formation is facilitated since the temperature in the stir zone (504°C) is much higher than the α-Mg+Mg17Al12 eutectic temperature (437°C).There is no evidence of liquation cracking in the stir zone or the TMAZ region of AM60 sheet when it is the lower sheet in the dissimilar sandwich. However, liquation cracking is observed in the upper sheet material (AZ91) in the location beneath the tool shoulder close to its periphery.

Although it is generally assumed that friction stir seam welds and friction stir spot welds are free of many of the defect formation issues commonly associated with fusion welding, liquid penetration induced (LPI) cracking in the stir zone have been recently found in friction stir spot welds of AZ91. In the present study, cracking during friction stir spot welding of Mg-alloy (AZ91, AM60 and AZ31) sections is examined. Both liquation cracking and liquid penetration induced (LPI) cracking are observed in Mg-alloy friction stir welded joints. Local melting and cracking is also apparent in Al 7075-T6 friction stir spot welds produced with the precise objective of limiting dissolution of melted eutectic films in the high temperature stir zone and when spot welds cool to room temperature. Based on these test results there is no need to assume that the stir zone temperature during friction stir spot welding is less than that required for formation of melted eutectic films or for spontaneous melting of second-phase particles contained in the as-received base material.

This study was carried out on the development of the evaluation method for solidification cracking susceptibility of Inconel600/SUS347 dissimilar weld metals during laser welding. Some dissimilar weld metals which have different ratios of Inconel600/SUS347 were prepared by TIG welding and then were remelted on the U-type hot cracking tester by laser. Solidification cracking behavior during hot cracking test was observed by a high speed camera and the dynamic strain, close to the solidification crack, was evaluated. It appeared that local critical strain, for the initiation of solidification crack, was obtained by this strain measurement method. So the solidification cracking susceptibility could be directly evaluated based on the critical strain for different dissimilar joint. By using this method, it was discovered that solidification cracking occurred most easily when the ratio of Inconel600/SUS347 is 40%/60%, in the case of the Inconel600/SUS347 dissimilar laser welded joints.

Material flow and intermixing during dissimilar friction stir spot welding and friction stir seam welding are investigated. During friction stir spot welding, a ribbon of contiguous dissimilar lamellae is produced during each rotation of the tool and the number of intermingled lamellae contained in the intermixed region is determined by the tool rotational speed setting and the dwell time applied. When the rotating tool moves across the component, the ribbon of dissimilar contiguous lamellae continues to be produced and the linear distance in the traversing direction between dissimilar lamellae corresponds with the pitch distance [the travel speed (mm s−1) divided by the tool rotational speed (Hz)]. The material flow pattern produced when a threaded tool moves across a component is therefore a variant of that produced during the touch down period when the rotating tool is held stationary. It is suggested that the onion ring structures observed in similar and dissimilar friction stir seam welds mad...

The formation of local melted films during friction stir spot welding of as-cast AZ91D and thixomolded AZ91 material is investigated. The average temperatures close to the tip of the rotating pin vary from 438 to 454 °C during the dwell period in friction stir spot welding. These measured temperature values are higher than the melting temperature of α-Mg + Mg17Al12 eutectic (437 °C). It is suggested that the temperature in the stir zone during the dwell period is determined by the relative proportions of α-Mg and (α-Mg + Mg17Al12) eutectic material, which are incorporated during friction stir spot welding. Based on the stir zone temperature measurements and a detailed examination of material located at the root of the pin thread it is suggested that material is moved downwards via the pin thread and into the stir zone during the dwell period in friction stir spot welding. Evidence of local melted film formation is observed in the stir zone of AZ91 spot welds. It is suggested that melted films are retained since their dissolution rate is much slower in the high temperature stir zone than it is when melted films is formed in the stir zone during Al 7075-T6 friction stir spot welding. The spontaneous melting temperature, solute diffusion rate and the thermodynamic driving force for droplet dissolution are much higher during Al 7075-T6 friction stir spot welding.

Local melting and tool slippage during friction stir spot welding of different Al-alloy base materials is examined using a combination of detailed microscopy and temperature measurement. The stir zone peak temperature during welding is limited by either the solidus of the alloy in question or by spontaneous melting of intermetallic particles contained in the as-received base material. When spontaneous melting occurs this facilitates tool slippage at the contact interface. Accurate stir zone temperature and grain size measurements are essential elements when estimating the strain rate using the Zener–Hollomon relation. In Al 2024 and Al 7075 spot welds spontaneous melting of second-phase particles produces a drastic reduction in strain rate values. In Al 5754 and Al 6061 spot welds there is a strong correlation between tool rotational speed and estimated strain values. Local melted films dissolve rapidly in the high temperature stir zone and when the spot weld cools to room temperature following welding. Evidence of local melting is observed in Al 7075 friction stir spot welded joints made using a combination of rapid quenching, high plunge rates, and extremely short dwell time settings.

Liquid penetration induced (LPI) cracking is investigated during friction stir spot weld of AZ91, AZ31 and AM60 magnesium alloys. A combination of stir zone temperature measurement and detailed metallography has revealed differences in the cracking tendencies of different magnesium alloys when the dwell time during spot welding is varied. LPI cracking in AZ91 spot welds involves the following sequence of events: the formation of \(\alpha-\hbox{Mg}\,+\,\hbox{Mg}_{17}\hbox{Al}_{12}\) eutectic films in the thermo-mechanically affected zone (TMAZ) region immediately adjacent to the stir zone extremity, engulfment of melted eutectic films as the stir zone width increases during the dwell period, penetration of α−Mg grain boundaries and crack propagation when torque is applied by the rotating tool. Cracking occurs early in the dwell period during AZ91 spot welding and almost the entire stir zone is removed when the rotating tool is withdrawn. However, crack-free AZ31 and AM60 spot welds are produced when a dwell time of 4 s is used since the stir zone temperatures are much higher than the α-Mg + Mg17Al12 eutectic temperature (437 °C) and melted eutectic films dissolve rapidly following their engulfment by the growing stir zone. In contrast, the temperature during the dwell period in AZ91 spot welding is close to 437 °C and melted eutectic films are not completely dissolved so that spot welds produced using a dwell time of 4 s exhibit LPI cracking.

Local melting of eutectic films and cracking is found in Al 2024 and Al 7075 alloy friction stir spot welds. Dissolution of melted films removes all evidence melted film formation in spot welds made using typical welding parameter settings. For this reason friction stir spot welding is carried out at a rapid plunge rate of 10 mm s−1 and an extremely short dwell time of 0·05 s and after tool retraction, the welded samples are rapidly quenched using a mixture of methanol and liquid nitrogen at a temperature of −80°C. Eutectic films rich in Zn and Cu are formed in Al 7075 spot welds while melted Al2CuMg particles promote the formation of α-Al+Al2CuMg eutectic films in Al 2024 spot welds. Melted eutectic formation and cracking is also observed beneath the tip of the rotating pin during Al 7075 friction stir spot welding and is consistent with the occurrence of melt wear in this location.

The stir zone temperature and microstructures are compared in friction stir spot welds produced in Al 5754 and Al 6061 alloys. Electron backscattered diffraction was used to determine the relationship between tool rotation speed during welding and final stir zone grain size. Comparison of the grain sizes in rapidly quenched welds with those in air-cooled joints confirmed that grain growth occurred only in Al 6061 spot welds. There was no evidence of abnormal grain growth in the stir zones of Al 6061 welds; the final grain size could be represented using an Arrhenius equation. The strain rates during welding were determined by incorporating the stir zone temperature and average subgrain sizes in quenched spot welds in the Zener–Hollomon relation. When the tool rotation speed increased from 750 to 3000 RPM, the strain rate values ranged from 180 to 497 s−1 in Al 5754 spot welds and from 55 to 395 s−1 in Al 6061 spot welds. It is suggested that a no-slip boundary condition may be appropriate during numerical modeling of Al 5754 and 6061 friction stir spot welding. This is not the case during Al 7075, Al 2024, and Mg-alloy AZ91 spot welding because spontaneous melting facilitates slippage at the tool contact interface.

The stir zone microstructure, crystallographic texture, temperature and strain rate in the stir zones produced during Al 2024 spot welding using different tool rotational speed settings are investigated. The calculated strain rate during spot welding decreases from 1600 to 0.6 s−1 when the tool rotational speed increases from 750 to 3000 rpm. The low strain rate values are associated with tool slippage resulting from spontaneous melting of S phase particles at temperatures ≥490 °C. However, the calculated strain rate is 1600 s−1 in Al 2024 spot welds made using tool rotational speed of 750 rpm since the temperature never reaches 490 °C. Material transfers downwards via that pin thread during the dwell period in Al 2024 spot welding. It is proposed that this downward transfer of material provides a continuous supply of undissolved S phase particles, which melt spontaneously when the welding parameter settings produce stir zone temperatures ≥490 °C. A weak crystallographic texture where the {100} planes are oriented at about 45° to the θ-direction exists in the stir zones of spot welds made using different tool rotational speeds (from 750 to 3000 rpm). Another crystallographic texture where the {100} planes are parallel to the Z-direction (to the tool axis) is stronger in spot welds made using higher tool rotational speed settings. Also, material located at the root of the pin thread has a quite different crystallographic texture from that in the bulk of the stir zone.

The mechanism of liquid penetration induced (LPI) cracking is investigated in AZ91 friction stir spot welds. Liquid penetration induced cracking results from the following sequence of events: melted eutectic film formation in the periphery of the stir zone region, engulfment of melted eutectic films when the stir zone grows in width during the dwell period, penetration of α-Mg grain boundaries in the stir zone extremity and crack propagation when torque is applied by the rotating tool. Liquid penetration induced cracking occurs early in the dwell period during AZ91 spot welding and almost the entire stir zone is removed when the rotating tool is withdrawn. However, tool withdrawal does not provide the driving force for cracking during AZ91 spot welding; the upwards axial movement of the rotating tool at the end of the spot welding operation merely separates sections which are already cracked. The cracking tendency (the amount of the stir zone material produced during spot welding, which is removed...

A model based on the thermodynamic and kinetic was conducted to simulate the Cr depletion profiles near the grain boundary in Inconel 600 during the heat treatments and laser surface melting (LSM) process using Thermo-Calc and Dictra code. Based on the good agreement of Cr concentration distribution during heat treatments measured by experiments, the microsegregation of Cr induced by cellular microstructure formed during the LSM process was also modeled. The Cr depletion profile was evaluated using the Cr depletion area below the critical Cr concentration for intergranular cracking/intergranular stress corrosion cracking (IGC/IGSCC) susceptibility (8 mass%). Comparing with the result of Streicher test, the Cr depletion area calculated showed good coherence with the IGC/IGSCC susceptibility. The sample after SR + LTS treatment with the largest Cr depletion area showed the worst IGC/IGSCC resistance, while, the sample after LSM process with the smaller Cr depletion area showed the excellent IGC/IGSCC resistance.

The final purpose of this study is to establish the technology which repairs stress corrosion cracking (SCC) of overlaying of Inconel 182, which has been used as the material of the nuclear power plants and chemical plants by laser surface melting (LSM). In this study, the intergranular cracking/intergranular stress corrosion cracking (IGC/IGSCC) sensitivity was evaluated by the Streicher test and the Open Bend Beam (OBB) test. The parameters of LSM were laser power, traveling speed, defocus length and lapping rate of weld bead. The SCC, which occurred on the surface of overlaying of Inconel 182, was sealed by LSM and also the SCC sensitivity in remelted area remarkably improved in low heat input. LSM was considered as a useful method for repairing SCC in overlaying of Inconel 182.

Long term liquid zinc corrosion characteristics, up to 3000 h at 450°C, were investigated for the base metal and for multipass all weld metal deposits and weld joints produced via the gas shielded metal arc welding (GMAW and submerged arc welding (SAW) processes. The purpose of the present work was to determine the possibility of replacing the present shielded metal arc welding (SMAW) technique with the former two processes since they can offer demonstrably higher productivity than conventional SMAW in the fabrication process of hot dip galvanising baths. Regression analyses were also attempted to determine the corrosion rate quantitatively and thereby to establish a reasonable life prediction technique for such baths. It was found that the corrosion rate of the weld metal depends on the chemical composition, particularly on silicon concentration. Selection of low silicon (less than about 2.0 mass%) containing welding materials is thus vital to suppress the liquid zinc corrosion. Iron loss from th...

In this study, fatigue crack propagation properties in sea water are investigated, when considering the usage for hull structural steels. The rate of the optimum ferrite-bainite (F/B) steels shows superior resistance even in a sea water.Corrosion fatigue properties in sea water are investigated for both conventional ferrite-pearlite (F/P) steels and the F/B steels. It is clarified that fatigue strength of the F/B steels is higher than that of the F/P steels. As to corrosion fatigue crack propagation rates, the F/B steel originally has much superior resistance in normal conditions, and acceleration factors of corrosion crack propagation rate in sea water to that of air is almost constant regardless of microstructure in steels. These two characters brings superior propagation resistance in sea water. As to corrosion fatigue strength, the F/B steel has the advantage of not only a long fatigue crack propagation life, but also long fatigue crack initiation life. This extended initiation life in the F/B steel comes from the small number of corrosion pits, which might be corrosion fatigue crack initiation sites. This difference of pit number may depend on homogeneity to general corrosion. In case of the F/P steels, the microstructure is not so homogeneous, therefore, general corrosion is not uniform. In case of the F/B steels, the general corrosion is uniform because of fine and homogeneous microstructures.

The purpose of this study is to develop thermal and dynamic three dimensional strain analysis method for the laser weldment in order to obtain the plastic strain at elevated temperature in HAZ of the laser weldment, and to develop the prediction method of liquation crack initiation in HAZ of laser weldment. The U-type hot cracking test was performed as an experimental evaluation method for liquation cracking susceptibility of laser weldments of Inconel 718. At the same time, thermal and dynamic three dimensional elastplastic strain analyses were performed by FEM for U-type hot cracking test specimens. Heat transfer analysis and elastplastic strain analysis for laser weldments were confirmed by experiments. From this analysis, it becomes clear that the plastic strain at elevated temperature affects liquation crack initiation in HAZ, and the critical strain at elevated temperature, which controlled liquation crack initiation, can be calculated precisely by using this proposed analysis method.

Solidification cracking occurs easily at high welding speeds, and should therefore occur more easily during laser welding. Both the solidification behavior and thermal strain change depend on the welding speed, and therefore, the critical strain for solidification cracking must be measured to clarify the factors influencing the solidification cracking susceptibility. However, the critical strain required for solidification cracking under high welding speed conditions has not yet been determined. The aim of this work was to investigate the effect of welding speed on the solidification cracking susceptibility of Type 310S stainless steel. U-type hot cracking tests were conducted using a developed in-situ observation technique with high-speed camera, and the critical strain for solidification cracking was evaluated quantitatively. The critical strain for solidification crack initiation decreased with increasing welding speed. The distribution of residual liquid depended on the microstructure, and the morphological distribution of the residual liquid changed from a droplet to a thin film with increasing welding speed. The transition in distribution morphology of the residual liquid implies the material is susceptible to solidification cracking.

The purpose of this study was to investigate the melting phenomenon of filler wire in detail and to obtain the precise temperature distribution of filler wire during GTA welding under the ultra-high welding speed condition in order to develop the ultra-high-speed GTA welding process with the pulse-heated hot-wire system by using three kinds of materials. The melting phenomenon of filler wire was observed using a high-speed camera and the temperature distribution of filler wire was measured using a radiation thermometer. From the above result, the adequate welding conditions of each material to make the GTA welding process with the ultra-high welding speed could be obtained. The ultra-high-speed GTA welding process needed the adequate wire current in order to obtain the adequate temperature distribution and the adequate melting position of filler wire. Moreover, the temperature distributions of three kinds of filler wire could be estimated by using the proposed simple estimation method.