@misc{oai:ir.soken.ac.jp:00000501,
 author = {許, 男鎭 and ホウ, ナムジン and HEO, Nam-Jin},
 month = {2016-02-17, 2016-02-17},
 note = {Vanadium alloys have been regarded as one of the promising candidate structural materials for fusion first wall/blanket applications, due to their low induced radioactivity, good compatibility with lithium, good resistance to neutron radiation damage and good strength at elevated temperature.  Some critical issues such as large heat fabrication technology, uniradiated and irradiated mechanical properties, chemical compatibility, and corrosion properties have been studied to apply the vanadium alloys for the structural components of fusion reactors.  Based on the previous studies, V-4Cr-4Ti alloy was selected as a leading candidate.<br />　　One of the key issues for the vanadium alloys is the welding process, which is the requisite for fabricating structures.  It is important to control the environment during the welding for preventing the contamination with the impurities such as oxygen, nitrogen and carbon, because it is well known that those impurities have various effects on the properties of vanadium alloys.  Some research efforts for the welding on vanadium alloys have been carried out with gas-tungsten arc, electron beam, laser and so on.  Laser welding has some unique characteristics when compared to other welding processes.  It does not require either a complete electrical circuit, or a vacuum chamber or shielding for X-ray.  It should be noted that the blankets for fusion reactor are mainly composed of thin plates, their morphology is very complex and it is necessary to repair them by remote control.  Thus, the YAG laser welding is a quite attractive technique for application to the fusion blanket because of its flexibility for field, lenient requirement for atmospheric control, capability of deep penetration with less input, and remote operation capability.  Some researches for the Baser welding on vanadium alloys have been carried out, but most of them failed to prevent the contamination with the impurities, resulting in the degradation on the properties of the weldments.<br />　　The purpose of this study is to develop a Baser welding technology for low activation V-4Cr-4Ti alloys.  For this purpose, an environmental control box was fabricated to eliminate the contamination with the interstitial impurities, and then the YAG laser welding was carried out inside it.  The effect of the welding conditions on the mechanical properties of the weld zone, such as hardness, bending, tensile and Charpy impact properties, and the impurity behavior in the weld zone by microstructural observation for the mechanistic understanding of the property change, based on the understanding of hardness and precipitation behavior by heat treatment were investigated.<br />　　As the basic study supporting the development of the welding technology, it is crucial to enhance the understanding on the impurity behavior under various thermomechanical treatment processes.  In the present study, the effect of impurity levels on hardness and precipitation behavior by various heat treatments was also investigated.  The previous studies on the impurity effects on vanadium alloys revealed that even if the initial impurity content is rather high, good properties can be acquired by optimal heat treatment resulting in the formation of the precipitates with the impurities.  Thus, the key issues for fabrication of vanadium alloys are to eliminate the pickup of the critical impurities from the atmosphere during the fabrication and working process and to determine the optimal heat treatment for vanadium alloys.  Therefore, this study will be also valuable for optimizing the fabrication and thermomechanical treatment conditions of the alloys, as well as for the welding development.<br /><br />　　To investigate the impurity behaviors by heat treatment, V-4Cr-4Ti model alloys doped with various levels of oxygen and nitrogen were fabricated by button arc melting.  Model alloys were machined into 10 mm thick slabs from the buttons.  All the sheets were cold-rolled to a reduction of > 50% in thickness, followed by annealing at 1373 K for an hour in a vacuum (<10-3 Pa).  The specimens were then heat-treated again from 873 to 1373 K for an hour.  Vickers hardness testing and microstructure observation were carried out after annealing.<br />　　Impurities existed in both the solid solution and the precipitates.  Two kinds of precipitates were observed in the specimens doped with oxygen:  large and small precipitates.  They were identified as Ti-rich cubic phases and Ti-C-O compounds, respectively.  In the specimens doped with nitrogen, however, only the large precipitates were observed.  The large precipitates were stable in all heat treatments to 1373 K, but the fine precipitates appeared at 973 K and vanished at 1373 K.  The density of the large precipitates increased with nitrogen level more than oxygen level.  The density of the fine precipitates increased with oxygen level.  <br />Most of the nitrogen impurities are included into the large precipitates and are stable during the heat treatments to 1373 K.  Some fraction of the oxygen impurities will be in large and fine precipitates.  The formation of the fine precipitates at 973 K resulted in the increase in hardness by the precipitation hardening.  The hardness then decreased with the increase in annealing temperature due to the decrease in the precipitates density.  The oxygen in the fine precipitates will be released into matrix by annealing at 1373 K by resolution of the precipitates, resulting in another increase in hardness by the solid solution hardening.<br />　　According to the present results, the annealing temperature will be optimized at 1273 K or below, taking the precipitation hardening and the solid solution hardening into account.  Heat treatment at above 1273 K will result in the degradation of the properties.<br /><br />　　In the welding experiments, high-purity V-4Cr-4Ti alloy (NIFS-HEAT-2) fabricated by National Institute for Fusion Science was used.  The specimen was prepared by annealing at 1273 K for 2 hours before laser welding.  In this study, the Baser welding technology has been developed for vanadium alloys eliminating the contamination.  To prevent contamination with interstitial impurities, such as hydrogen, oxygen, nitrogen, carbon, etc. during welding, the laser welding was carried out in an environmental control box capable of supplying argon gas and ventilating the fume.  Bead-on-plate welds were produced on the 4 mm-thick plate using 2.0 kW YAG Baser.  Chemical analysis of the weld metal was carried out to measure the contaminations.  The microstructure was also observed with optical microscope (CM), scanning electron microscope (SEM) and transmission electron microscope (TEM).<br />　　The hardening occurred in the weld zone.  This was quantitatively explained by the experiments on the impurity behaviors in the V-4Cr-4Ti alloys.  Increase in hardness of the weld zone was due to the dissolution of large and small precipitates.  In the weld metal, dissolution of both large and small precipitates resulted in remarkable hardening.  Only small precipitates disappeared at a distance from the weld metal, resulting in smaller hardening.<br />　　Soundness of the weld joint was confirmed by estimating mechanical properties.  In the tensile test of the Joint, decrease in tensile strength did not occur but total elongation decreased slightly in the weld zone.  The absorption energy of the weld metal produced by the low power density was as high as that of the base metal.  <br />Decrease in the absorption energy in the case of the high power density seems to be due to the increased width of the weld metal and the heat-affected zone (HAZ) and the increased grain size in the weld zone.  To Improve the absorption energy of the weld metal, it is effective to decrease the input power density for reducing the grain size and the width of the weld metal and the HAZ.<br /><br />　　In summary, this study has demonstrated that the laser welding of vanadium alloys maintaining their good properties is possible by a simple environmental control.  The basic understanding of the behavior of interstitial impurities such as oxygen, nitrogen and carbon, during various thermomechanical treatment derived in this study was applied to explaining the properties of the weldment and to improving the procedures of thermomechanical treatment and welding., 総研大甲第673号},
 title = {Impurity behavior and weld joint properties of low activation vanadium alloys for fusion reactors},
 year = {}
}