Papers by Author: Kazuyoshi Saida

Abstract: The hot cracking (solidification cracking) susceptibility in the weld metals of duplex stainless steels were quantitatively evaluated by Transverse-Varestraint test with gas tungsten arc welding (GTAW) and laser beam welding (LBW). Three kinds of duplex stainless steels (lean, standard and super duplex stainless steels) were used for evaluation. The solidification brittle temperature ranges (BTR) of duplex stainless steels were 58K, 60K and 76K for standard, lean and super duplex stainless steels, respectively, and were comparable to those of austenitic stainless steels with FA solidification mode. The BTRs in LBW were 10-15K lower than those in GTAW for any steels. In order to clarify the governing factors of solidification cracking in duplex stainless steels, the solidification segregation behaviours of alloying and impurity elements were numerically analysed during GTAW and LBW. Although the harmful elements to solidification cracking such as P, S and C were segregated in the residual liquid phase in any joints, the solidification segregation of P, S and C in LBW was inhibited compared with GTAW due to the rapid cooling rate in LBW. It followed that the decreased solidification cracking susceptibility of duplex stainless steels in LBW would be mainly attributed to the suppression of solidification segregation of P, S and C.

Abstract: Temper bead welding is one of effective repair welding methods in case that post weld heat treatment is not easily applied. In order to evaluate the effectiveness of temper bead welding, hardness in HAZ becomes important factor. The neural network-based hardness prediction system of HAZ in temper bead welding for A533B low-alloy steel has been constructed by the authors in the previous study. However, for HAZ hardness prediction of other steels, it is necessary to obtain hardness database for each steel which is time-cost consuming, if the same method is used. The present study has been conducted to develop the generalized hardness prediction method applicable for other steels by utilizing the hardness data-base of A533B steel assuming that the hardness in HAZ of steels after tempering have a linear relationship with LMP (Larson-Miller parameter). On using the newly proposed extended method, only a few hardness data-base for the other steels is needed to obtain. Hardness distribution in HAZ of temper bead welding for other steels was calculated by using the extended hardness prediction system. The thermal cycles used for calculation were numerically obtained by a finite element method. The experimental results have shown that the predicted hardness is in good accordance with the measured one for steels without secondary hardening. It follows that the currently proposed extended method is effective for estimating the tempering effect during temper bead welding for the steels without secondary hardening.

Abstract: The microstructure and mechanical properties of a joint produced by laser brazing between A5052 and AZ31 with AZ61, AZ91 and AZ125 filler metal was investigated. The effects of filler metals on joint characteristics are also discussed. Measurement of microstructural factors in the laser brazed joint revealed that increasing the laser power results in a decrease in the weld toe angle and an increase in the bead width, which indicates superior wettability. A high strength laser brazed joint can be achieved through the combination of good wettability and a thin intermetallic layer produced by a laser power of 590 W in a brazed joint with AZ125 filler metal Any further increase in power, however, results in a rapid increase in the thickness of the intermetallic compound (IMC) reaction layer. The superiority of the brazed joint with AZ125 filler metal is due to its lower melting point than that of AZ61 and AZ91 filler metal.

Abstract: Temper bead welding (TBW) is one effective repair welding method for the large-scale nuclear power plants. Consistent Layer (CSL) technique is the theoretically most authoritative method among the five temper bead welding techniques. However in the actual operation, CSL technique is difficult to perform, and non-CSL techniques (Controlled Deposition technique, Half Bead technique, et al) are mainly used in the actual repair process. The thermal cycles in heat affect zone (HAZ) of non-CSL technique are more complicated than that of CSL techniques. Through simplifying the complicated thermal cycles to 4 types of thermal cycles, the neural network-based hardness prediction system for non-CSL techniques has been constructed. The hardness distribution in HAZ of non-CSL techniques was calculated based on the thermal cycles numerically obtained by finite element method (FEM). The predicted hardness was in good accordance with the experimental results. It follows that the thermal cycle simplification methods are effective for estimating the tempering effect during temper bead welding of non-CSL techniques.

Abstract: A fusion reactor is expected as one of the new electric power sources in next generation. Reduced activation ferritic/martensitic steel F82H is planned to be used as a structural material for the blanket modules set on the inner wall of the reactor. However, especially in the case of laser beam welding (LBW), the weldability of the steel was not completely clarified. On the other hand, although post weld heat treatment (PWHT) should be conducted for the welds of the steel in accordance with general standards for chrome steels, the heat treatment conditions were uncertain. Therefore, adaptability of LBW as a joining method for the steel and the applicable PWHT conditions for the welded joints were investigated in this study. The effect of LBW conditions on weld penetration behavior were ascertained by observation of cross sections in the welds. The adequate PWHT conditions were confirmed in consideration of both hardness distributions measured in welds and ductile-brittle transition temperatures (DBTT) evaluated using Charpy impact test. Full penetration without weld defects such as hot cracking, porosity etc. was obtained for plates with the thickness of 4mm of the steel by control welding conditions. That means laser beam is one of useful welding heat sources to realize sound weld joints of the steel. In addition, due to select appropriate PWHT conditions, the hardness in welds was suppressed to the level of base metal and the toughness in the welded joints was improved to a practical level without the damage to base metal.

Abstract: A new bonding technique of titanium and zirconium conducted at low temperatures was developed utilizing the hydrogen-induced transformation. Hydrogen charge treatment of the faying surfaces of titanium and zirconium was conducted with varying the charging time between 3.6-700ks prior to diffusion bonding. Diffusion bonding of hydrogen-charged titanium and zirconium was carried out at 600-800°C for 0.6-1.8ks applying the bonding pressure of 5-10MPa in vacuum. Titanium and zirconium hydrides were formed at faying surfaces after hydrogen charge treatment. The β-transus temperature at faying surfaces of titanium and zirconium was reduced to approx. 450-550°C with hydrogen-charging. The bond layer was phase transformed to a bcc structure (β) at the bonding temperature due to the hydrogen diffusion during bonding process. Grain growth across the prior bond interface was observed in the joints bonded at 750-800°C after hydrogen-charging for 300-500ks. Tensile strength of titanium joints bonded at 800°C attained approx. 70% of the base metal strength (approx. 1.6 times as high as non-charged joints), and corrosion resistance of the joints was comparable to that of the base metal. Furthermore, tensile strength of zirconium joints bonded at 800°C was approx. 1.7 times as high as non-charged joints. It follows that the solid-state bondability of titanium and zirconium at low temperatures was improved compared to the conventional diffusion bonding (direct bonding without hydrogen-charging).

Abstract: The wetting and flowing behaviors of the filler metal during laser brazing process were analyzed by the computer simulation. Two situations of the wetting and flowing during laser brazing were modelled, i.e., the metled Au-18%Ni and Ag-10%Pd filler metals on the butt joint of Inconel 600, and the melted Cu-8%Sn filler metal on the dissimilar butt joint of type 304 stainless steel to Cu. The filler metal droplet wetted and spread on the base metals and simultaneously infiltrated into the joint gap with the lapse of time. The Au-Ni and Ag-Pd filler metal infiltrated into the 0.3mm wide joint gap at the completion of brazing even in the single beam brazing. The Au-Ni filler metal did not infiltrate into the joint gap completely at the brazing clearances of 0.1-0.2mm in the single beam brazing, however, it could be filled up in the joint gap in the tandem beam brazing. The Cu-Sn filler metal wetted on the both base metals of stainless steel and Cu and filled up the 0.3mm wide joint gap when the location of preheating beam deviated in 0.5mm to Cu substrate, however, it did not infiltrate into the joint gap completely at the deviation distance of preheating beam to Cu substrate being 1.0mm. It followed that the wetting and flowing behaviors of the filler metal during laser brazing process could be predicted by the computer simulation.

Abstract: Microstructure and crystallographic orientation in the overlay weld metal have been investigated using Ni-base single crystal superalloy CMSX-4. Laser power, laser scanning speed and wire feeding speed were varied. The microstructure and crystal orientation were analyzed by microscopy and electron backscattered diffraction pattern. Microstructural observation revealed that the overlay weld metal grew epitaxially on the substrate and stray crystal tended to be formed at the conditions of high power and high Vf/Vw (Vf: Wire feeding speed, Vw: Laser scanning speed). In addition, solidification morphology, dendrite growth direction and single-crystallized condition were evaluated by combining solidification model to heat conduction model. These predicted results agreed qualitatively with the experimental ones. Based on the above results, the singlecrystallized cladding with ten passes could be achieved.

Abstract: The occurrence of microcracks, especially ductility-dip crack in multipass weld metal during GTAW and laser overlay welding processes of Ni-base alloy 690 was predicted by the mechanical approach. The stress/strain analysis in multipass welds was conducted using the thermo elasto-plastic finite element method. The brittle temperature range for ductility-dip cracking (DTR) of the reheated weld metal was determined by the Varestraint test. Plastic strain in the weld metal accumulated with applying the weld thermal cycle in multipass welding. The plastic strain-temperature curve in the La free weld metal did not cross the DTR in the cooling stage of GTAW process, however, it crossed the DTR in the cooling stage of reheating process by subsequent welding. On the other hand, the plastic strain-temperature curves of any weld passes in the La added weld metal did not cross the DTR. Ductility-dip cracks occurred in the La free weld metal except for the final layer, however, any ductility-dip cracks did not occur in the La added weld metal during multipass welding. It could be understood that ductility-dip crack would occur during not only single-pass welding but also multipass welding when plastic strain intersected the DTR.