Jin-Hyun Koh

Design and Fabrication of Remote Welding Equipment in a Hot-Cell

The remote welding equipment for nuclear fuel bundle fabrication in a hot-cell was designed and developed. To achieve this, a preliminary investigation of hands-on fuel fabrication outside a hot-cell was conducted with a consideration of the constraints caused by the welding in a hot-cell. Some basic experiments were also carried out to improve the end-plate welding process for nuclear fuel bundle fabrication. The resistance welding equipment using end-plate welding was also improved. It was found that the remote resistance welding was more suitable for joining an end-plate to end caps in a hot-cell. This paper presents an outline of the developed welding equipment for nuclear fuel bundle fabrication and reviews a conceptual design of remote welding equipment using a master-slave manipulator. Furthermore, the mechanical considerations and a mock-up simulation test were described. Finally, its performance test results were presented for a mock-up of the remote resistance welding equipment for nuclear fuel bundle fabrication.

Development of Zircaloy-4 Endplate Welding Technology for a DUPIC Fuel Bundle Assembly

This study was carried out to establish the remote resistance welding technology for a hot cell environment for Direct Use of spent PWR fuel In CANDU reactors (DUPIC) fuel fabrication. A preliminary investigation for a hands-on fuel fabrication outside a hot cell was conducted with consideration of the constraints caused by a welding in a hot cell. The optimum resistance welding parameters for the endplate welding process were obtained in terms of the welding current, electrode pressure, and welding time. The welds made on the basis of optimum welding parameters showed higher torque strength than those made by laser beam welding. The integrity of the welds fabricated by resistance welding was confirmed by the results of the torque test, an examination of the microstructure, and the fracture surface of the welds.

A Study on Characteristics of Dissimilar Welds between Super Duplex Stainless Steel UNS S32750 and Carbon Steel A516-70 with FCAW

The metallurgical and mechanical characteristics, toughness and corrosion resistance of dissimilar welds between super duplex stainless steel UNS S32750 and carbon steel ASTM A516Gr.70 have been evaluated. Three heat inputs of 21.12, 24.00, 26.88kJ/cm were employed to make joints of dissimilar metals with flux cored arc welding(FCAW). Based on microstructural examination, vermicular ferrite was formed in the first layer of weld at low heat input(21.12kJ/cm) and Creq/Nieq of 1.61 while acicular ferrite was formed in last layer of weld at high heat input(26.88kJ/cm) and Creq/Nieq of 1.72. Ferrite percentage in dissimilar welds was lowest in the first layer of weld regardless of heat inputs and it gradually increased in the second and third layers of weld. Heat affected zone showed higher hardness than the weld metal although reheated zone showed lower hardness than weld metal due to the formation of secondary austenite. Tensile strengths of dissimilar welds increased with heat input and there was 100MPa difference. The corrosion test by ferric chloride solution showed that carbon steel had poor corrosion resistance and pitting corrosion occurred in the first layer(root pass) of weld due to the presence of reheated zone where secondary austenite was formed. The salt spray test of carbon steel showed that the surface only corroded but the amount of weight loss was extremely low.

Heat Treatment Effect on Pitting Corrosion of Super Duplex Stainless Steel UNS S32750 FCA Welds

This study was carried out to investigate the influence of a sigma phase precipitation on the pitting corrosion resistance and microstructural change of super duplex stainless steels(UNS S32750). The welds made by FCAW (Flux Cored Arc Welding) were heat treated (, , ) and quenched. Based on the microstructural examination, it was found that the phase was formed in base metals and welds heat treated at while there were little phases formed in base metals and weld metal experienced the relatively fast cooling from and . On the other hand, the most weight loss due to pitting corrosion occurred in base and weld metals heat treated at . It was confirmed that the pitting corrosion occurred in the phase boundaries of ferrite/sigma and austenite/sigma. The pitting corrosion resistance decreased owing to an increase in Cr, Mo depleted areas adjacent to the intermetallic phases such as phases. The hardness was greatly increased due to the precipitation phases.

Effect of the Wear of Contact Tips on the Weld beads made by GMAW Process

The effect of the wear of contact tips on the weld beads made by a Gas Metal Arc Welding(GMAW) process was studied. In order to correlate between the wear of tips and beads, the droplet transfer was investigated by a high speed camera and waveform shapes, and the condition of beads was examined by a low magnification camera as well. It was found that the worn contact tip had caused to shift the contact point between the tip and the welding wire which resulted in nonuniform beads. In addition, the worn contact tip had changed the welding resistance and brought about the arc instability during welding.

Irradiation Behavior of Reactor Pressure Vessel SA508 class 3 Steel Weld Metals

Abstract Irradiation behavior of the reactor pressure vessel SA508 class 3 steel weld metals was examined by Charpy V Notch impact specimens. The specimens were exposed to a fluence of 2.8x1019 neutrons(n)/cm 2 (E>1 MeV) at 288℃. The irradiation damage of weld metal was evaluated by comparison between unirradiated and irradiated specimens in terms of absorbed energy and lateral expansion. The specimens for neutron irradiation were welded by submerged arc welding process at a heat input of 3.2 kJ/mm which showed good toughness in terms of weld microstructure, absorbed energy and lateral expansion. The post-irradiation Charpy V notch 41J and 68J transition temperature elevation were 65℃ and 70℃, respectively. This elevation was accompanied by a 20% reduction in Charpy V notch upper shelf energy level. The lateral expansion at 0.9㎜ irradiated Charpy specimens showed temperature elevation of 65℃ and was greatly decreased due to radiation damage. Key Words : Radiation damage, SA508 class 3, Submerged arc welding, Impact test, Absorbed energy, lateral expansion, Transition temperature elevation

Effect of Heat on the Soundness of Zircaloy-4 End Cap Closure Using a Resistance Upset Welding

The objective of this study is to investigate the effect of main heat on the soundness of Zircaloy-4 end cap welds and the process of weld line elimination in resistance upset welding for a Direct Use of spent PWR fuel In CANDU reactor (DUPIC) fuel fabrication. The weld current and amount of upset increased linearly with the main heat, from which two relations between them were derived. It was found that the threshold to obtain sound welds was 50% of the main heat in terms of upset size, mechanical strength, and weld line elimination. A microstructure of resistance upset welds of Zircaloy-4 comprised basketweave, Widmanstätten, and martensite depending on the alteration of the main heats. Dimples on the uniaxially fractured surface at the weld line in Zircaloy-4 welds were larger and deeper than those on a biaxially fractured surface. It was also found that the process of weld line elimination in the resistance upset weld of Zircaloy-4 could be divided into three stages in terms of the presence of many pores, their shrinkage and elimination, and the shrinkage of the original weld interface with increasing weld current.

Development of a Remote Welding Machine for a DUPIC Fuel Bundle Fabrication

A remote welding machine for a DUPIC (Direct Use of spent PWR fuel In CANDU reactors) bundle fabrication was designed to establish the optimum welding processes in a hot cell environment. An initial investigation for hands-on fabrication outside the hot cell was performed in the consideration of the constraints of welding machine in the hot cell conditions. Gas tungsten arc welding (GTAW), laser beam welding (LBW), friction welding (FW), and resistance welding (RW) process were assessed as candidates for this application. A preliminary welding investigation to improve the RW process was also performed. The RW process was determined to be the most suitable process in a hot cell environment for joining an endplate to an endcap. An advantage of RW would be a qualified process for overlapped plates welding for which there is extensive production experience were available. A preliminary investigation for a hands-on fuel fabrication outside the hot cell was conducted in the consideration of the constraints caused by a welding in a hot cell. The optimum resistance welding parameters for the endplate welding process were obtained in terms of the current, electrode pressure and welding cycle. This paper presents an outline of the developed RW machine for a DUPIC bundle fabrication and reviews the conceptual design of a remote RW welder by using a master-slave manipulator. The design of RW machine by using the 3D modeling method was also designed. Furthermore the integrity of the welds by the resistance welding was confirmed by the results of the torque test, an examination of the microstructure and the fracture surfaces of the welds.Copyright

The Study on Characteristics of Dissimilar Weld Metals between AA6061-T6 and Silafont-36 by Laser Welding

최근 유엔기후변화협약(COP21) 등 국제환경규제의 강도가 심해짐에 따라 자동차 산업에서는 배기가스 배 출의 감소를 위해 경량화와 친환경 자동차로 대응하고 있다. 차체의 경량화를 통한 연비 향상은 지속적으로 이루어져 왔지만 내연기관 자동차를 전기차로 대체하려 면 충분한 주행거리를 확보해야하기 때문에 추가적인 경량화가 필요하다. 이에 따라, 비강도와 내식성이 우수한 알루미늄 합금 이 경량 소재로 각광받고 있다. 알루미늄 합금이 차량 에 적용되기 위해서는 성형성이 우수하면서도 높은 강 도를 가져야 하는데, 이러한 조건을 만족하는 6000계 열 합금이 body structure와 outer panel 및 inner panel에 널리 사용되고 있으며 각종 프레임 등 복잡한 알루미늄 합금 AA6061-T6와 Silafont-36의 이종금속 레이저 용접부 특성에 관한 연구

The Effect of PWHT on the Characteristics of Duplex Stainless Steel(ASTM A240 UNS S31803) Weld Metals made with FCAW

The effect of post-weld heat treatment(PWHT) on microstructure, mechanical properties and pitting corrosion resistance of UNS S31803, duplex stainless steel(DSS), weld metals was investigated. Three heat inputs of 7.6, 12.5, 18.0kJ/cm were employed to make joints of DSS with E2209T1-1/4 wire and PWHT was conducted in three temperatures of 1070, 1100, 1130°C. Microstructure in weld metals after PWHT consisted of allotriomorphic rather than needle-shaped austenite. Secondary austenite was scarcely formed and intermetallic compounds were not formed. Ferrite content was increased with increasing PWHT temperatures because δ-ferrite was stable phase and ferrite can not be sufficiently transformed to austenite due to rapid cooling. The difference of Cr, Mo and Ni contents between δ-ferrite and austenite after PWHT increased compared to that before PWHT. Hardness of weld metals became lower than that of base metal after PWHT. Also, hardness of weld metals was increased while impact absorbed energy was decreased with increasing PWHT temperature because of increase of δ-ferrite content. In pitting corrosion test, the critical pitting corrosion temperature(CPT) after PWHT increased by approximately 10°C, and CPT of weld metals was between 35 and 40°C. Heat-treated specimens in 1070°C was corroded in base metal because ferrite content of base metal was quite high over 60%.