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홍현욱 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.11
High-frequency electric resistance welded (ERW) pipes have been used increasingly in natural gas and petroleum services due to remarkable progress in coil manufacturing, forming, welding and other pipe-making techniques. The quality standards of ERW pipes have become more stringent to be suitable for harsh environments such as the artic region and areas that contain sour H2S gas. Hence, ERW pipes should have low temperature toughness, a high level of strength, and excellent resistance to hydrogen induced cracking (HIC). With regard to the improvement of HIC resistance at the bondline, the essential factors are the base metal, the welding operations, and the subsequent online heat treatment of the ERW area. In the present study, the optimization of ERW conditions was studied to improve the resistance to HIC at the bondline in small diameter API X60 ERW pipes fabricated with slit coils. The design of experimental (DOE) method was used. It is evident that HIC is initiated preferentially at the elongated Si, Mn and Al-rich oxide inclusions, normally known as a penetrator on the bondline. No evidence was found of any centerline segregation effect. To meet the requirement to the HIC resistance, the fraction of penetrators must be less than 0.03 % and most of the penetrators should be circular-shaped. The guideline for an optimum ERW condition highlight the special relationship of the power input, the welding speed, the upset force and the strip thickness.
홍현욱,김태영,김태훈,문준오,이창훈,정준호 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.11
Recently, there has been a tendency for the buildings to be constructed in the form of high-rise and multipurposes. In addition, to secure the safety design of structures from natural disasters such as earthquakes and fire, the building standards have become more stringent. The present study aimed to design low carbon ferritic and bainitic steels with different contents of Mo, Ti, and Nb for both seismic and fire-resistant qualifications. The hot-rolled steel containing 0.3 wt% Mo-0.02 wt% Nb (‘A’ hereinafter) was primarily strengthened by bainitic transformation, whilst the steel with 0.2 wt% Mo-0.13 wt% Ti (‘B’ hereinafter) consisted of ferrite with a high density of nano-sized (Ti,Mo)-rich MX precipitates. The bainitic microstructure (‘A’ steel) was quite favorable to high-temperature strength and thermal stability. The yield strength of ‘A’ steel at both room and 600 °C temperatures increased consistently with increasing thermal exposure time (600 °C/200-1000 h), since the precipitation of NbC particles occurred while maintaining bainitic ferrite platelets with a high density of dislocations during exposure. During low cycle fatigue tests at room temperature, the main different feature between the two steels is that the ‘A’ steel showed cyclic softening while cyclic hardening was evident in the ‘B’ steel. The bainitic microstructure showed a better fatigue life due to increased ductility manifested by cyclic softening, by which dislocation cell was developed.
액체로켓 연소기용 Inconel 718 주조 및 단조 합금의 전자빔 용접부 미세조직 및 극저온 특성
홍현욱,배상현,권순일,이재현,도정현,최백규,김인수,조창용,Hong, Hyun-Uk,Bae, Sang-Hyun,Kwon, Soon-Il,Lee, Je-Hyun,Do, Jeong-Hyeon,Choi, Baig-Gyu,Kim, In-Soo,Jo, Chang-Yong 대한용접접합학회 2013 대한용접·접합학회지 Vol.31 No.6
Characterization of microstructures and cryogenic mechanical properties of electro beam (EB) welds between cast and forged Inconel 718 superalloys has been investigated. Optimal EBW condition was found in the beam current range of 36~39 mA with the constant travel speed of 12 mm/s and arc voltage of 120 kV for 10 mm-thick specimens. Electron beam current lower than 25 mA caused to occur the liquation microfissuring in cast-side heat affected zone (HAZ) of EB welds. The HAZ liquation microfissure was found on the liquated grain boundaries with resolidified ${\gamma}/Laves$ and ${\gamma}/NbC$ eutectic constituents. EBW produced welds showing a fine dendritic structure with relatively discrete Laves phase due to fast cooling rate. After post weld aging treatment, blocky Laves phase and formation of ${\gamma}^{\prime}+{\gamma}^{{\prime}{\prime}}$ strengtheners were observed. Presence of primary strengthener and coarse Laves particles in PWHT weld may cause to reduce micro-plastic zone ahead of a crack, leading to a significant decrease in Charpy impact toughness at $-196^{\circ}C$. Fracture initiation and propagation induced by Charpy impact testing were discussed in terms of the dislocation structures ahead of crack arisen from the fractured Laves phase.
니켈기 초내열합금의 파형 결정립계 구조가 보론 편석과 재현 열영향부 액화균열거동에 미치는 영향
홍현욱,최준우,배상현,윤중근,김인수,최백규,김동진,조창용,Hong, Hyun-Uk,Choi, June-Woo,Bae, Sang-Hyun,Yoon, Joong-Geun,Kim, In-Soo,Choi, Baig-Gyu,Kim, Dong-Jin,Jo, Chang-Yong 대한용접접합학회 2013 대한용접·접합학회지 Vol.31 No.3
The transition of serrated grain boundary and its effect on liquation behavior in the simulated weld heat-affected zone (HAZ) have been investigated in a wrought Ni-based superalloy Alloy 263. Recently, the present authors have found that grain boundary serration occurs in the absence of adjacent coarse ${\gamma}^{\prime}$ particles or $M_{23}C_6$ carbides when a specimen is direct-aged with a combination of slow cooling from solution treatment temperature to aging temperature. The present study was initiated to determine the interdependence of the serration and HAZ property with a consideration of this serration as a potential for the use of a hot-cracking resistant microstructure. A crystallographic study indicated that the serration led to a change in grain boundary character as special boundary with a lower interfacial energy as those terminated by low-index {111} boundary planes. It was found that the serrated grain boundaries are highly resistant to boron enrichment, and suppress effectively grain coarsening in HAZ. Furthermore, the serrated grain boundaries showed a higher resistance to susceptibility of liquation cracking. These results was discussed in terms of a significant decrease in interfacial energy of grain boundary by the serration.