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극저온용 Fe-36%Ni강 자동 용접부의 White spot 생성 원인 분석
이희근 대한금속·재료학회 2019 대한금속·재료학회지 Vol.57 No.12
Fe-36%Ni steels have a much lower thermal expansion coefficient than stainless steels and aluminum alloys for low temperature. They also have moderately high strength and good toughness at cryogenic temperatures because of its stable austenite phase. These excellent low temperature characteristics make this alloy attractive for cryogenic applications such as the cargo containment system in liquefied natural gas carriers, pipes for low temperature and airplane rudder units. Generally, Fe-36%Ni steels are welded using Tungsten Inert Gas (TIG) Welding, Plasma Arc Welding (PAW) and Resistance Seam Welding. Lately white spots have been observed on the autogenous TIG welds of cryogenic Fe-36%Ni steels without any flux and wire in the cargo containment of LNG carriers. But white spot formation has not been studied yet. This paper covers the analysis results of the white spots formation on the autogenous TIG welds of cryogenic Fe- 36%Ni steels. The thickness of the white spots was approximately 6 μm and they were just stuck on the surface of the weld. Based on microstructural and qualitative analysis, the white spots were oxide aggregates (Al-Ca-Mg-Si-O), formed as a net structure in the center of TIG weld surface. The formation of white spots is caused by the excessive addition of minor elements such as Ca, Al, Mg in cryogenic Fe-36%Ni steels. The white spots on the TIG weld of cryogenic Fe-36%Ni steels were bigger than the weld spots on the TIG weld of stainless steel 316L, because of the lower solubility of Ca and Al in the austenite in the TIG Welds.
Miftakhur Rohmah,Gusti Umindya Nur Tajalla,Gilang Ramadhan,Yunita Triana,Efendi Mabruri 한국부식방식학회 2024 Corrosion Science and Technology Vol.23 No.1
A Fe-Ni Bainitic steel as a weathering steel application was developed by combining its excellent mechanical properties and corrosion resistance in maritime environments. Nickel concentration (0.4-3 wt%) and inverted austempering multi-step (IAM) process were primary determinants of the microstructure of the Fe-Ni Bainitic steel. The initial austempering steel was performed at 300 o C for 600 seconds to obtain a partly bainitic transformation. The steel was heated again for 1800 s at 450 o C. The microstructure was comprised of ferrite, a blocky martensite/austenite island, and a homogeneous lath-shape bainite structure with widths ranging from 4.67 to 6.89 µm. The maximum strength, 1480 MPa, was obtained with 3 wt% nickel. In this study, corrosion behavior was investigated utilizing potentiodynamic and electrochemical impedance spectroscopy (EIS) tests. A higher nickel content in Fe-Ni Bainitic steel refined the grain size, improved the bainite fraction, lowered the corrosion rate to 0.0257 mmpy, and increased the charge transfer of film resistance to 1369 Ω.
Fe계 합금 용가재가 적용된 9% Ni강 용접 조인트의 미세조직과 기계적 특성에 미치는 용접 후 열처리의 영향
최광수,정효연 대한금속·재료학회 2023 대한금속·재료학회지 Vol.61 No.9
A Fe-based filler material with the composition of Fe-20Ni-5Co-2.5Mn-0.2C was used for welding9% Ni steel, and the effect of post-weld heat treatment on the microstructure and mechanical properties ofthe welded joints was investigated. Post-weld heat treatment of the welded joint was performed in thetemperature range of 200 to 600 oC. After the experimental selection, the microstructure changes of the weldmetal and base metal were investigated in detail at heat treatment temperatures of 350 and 600 oC. Grainmorphology, size, and the location of precipitated cementite were studied using SEM and EBSD phase maps. The mechanical properties of the weld joint were characterized by tensile properties and micro-hardness. Based on the experimental results, this study provides key insights into the applicability of post-weld heattreatment to weld joints of 9% Ni steel prepared using Fe-based filler material to improve the mechanicalproperties, toughness and ductility of the weld joints.
이동복(Dong Bok Lee),정재옥(Jae Ok Jung),박순용(Soon Yong Park),조규철(Gyu Chul Cho),Xiao Xiao,김민정(Min Jung Kim) 한국표면공학회 2018 한국표면공학회지 Vol.51 No.2
Fe-2 wt.%Ni alloys were fabricated by metal powder injection molding, and their oxidation behavior at 600-700oC for 30 h in air was studied in order to find the effect of the small addition of Ni in the iron matrix on the high-temperature oxidation. Oxide scales that formed after oxidation consisted primarily of Fe₂O₃, where microscopic voids were scattered. Nickel was segregated initially at the scale/matrix interface, and later at the lower part of the Fe₂O₃ scale. At 600℃, Fe-2wt.%Ni alloys oxidized parabolically initially, and linearly after 15 h. At 650-700℃, they oxidized linearly from the initial period. Although Fe-2wt.%Ni alloys oxidized slower than pure iron, their oxidation rates were relatively fast.
Plasma Paste Boronizing법에 의한 Ni-Cr-Mo강의 붕화물층 생성거동과 내 토사마모특성에 관한 특성
조재현,박학균,손근수,윤재홍,김현수,김창규,Cho J. H,Park H. K,Son K. S,Yoon J. H,Kim H. S,Kim C. G 한국재료학회 2004 한국재료학회지 Vol.14 No.1
The surface property and formation behavior of a boride layer formed on Ni-Cr-Mo steel in a plasma paste boronizing treatment were investigated. The plasma paste boronizing treatment was carried out at 973~1273 K for 1-7 hrs under the gas ratio of Ar:H$_2$ (2:1). The thickness of the boride layer increased with increasing temperature and time in the boronizing treatment. The cross-section of the boride layer was a tooth structure and the hardness was Hv 2000~2500. XRD analysis revealed that the compound was identified as FeB, $Fe_2$B, and mixed phase of FeB/$Fe_2$B in the boride layer formed at 973~1073 K, 1173K, and 1273K, respectively. The Ni-Cr-Mo alloy boronized at 1173-1273 K showed the best excellent wear resistance against the sand. As a results of corrosion test in 1 M $H_2$$SO_4$ solution, $Fe_2$B formed on the matrix alloy exhibited higher corrosion resistance than FeB.
Fe-Co기 고인성 고강도강의 반복 열처리 거동- Part 1. 조직제어
윤보희 ( Bo Hee Yoon ),박경태 ( Kyoung Tae Park ),이태혁 ( Tae Hyuk Lee ),김재훈 ( Jae Hoon Kim ),김홍규 ( Hong Kyu Kim ),이성 ( Seong Lew ),이종현 ( Jong Hyeon Lee ) 한국주조공학회 2012 한국주조공학회지 Vol.32 No.1
In this study, microstructural evaluation was carried out on secondary hardening type ultrahigh strength steel, Fe-Co-Ni composition. This paper as a first part of whole research presented the microstructural behavior by cyclic heat treatment. The cyclic heat treatment method includes normalizing, stress relieving, solution treatment and aging. Especially, solution treatments performed triple times to get maximized solution hardening. Phase transformation and microstructure were observed by using optical microscope (OM), Electron back-scattered diffraction (EBSD) and X-ray stress analyzer. During the triple solution treatment, size of grain boundary was dramatically decreased by generating a packet from the martensite transformation of residual austenite in the inner part of grain, whereas the hardness increase was not significant.
오스테나이트-마르텐사이트 2상 조직의 Fe계 용가재를 통한 9% Ni 강 용접 특성
최광수,이승훈,정효연 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.11
액화 가스 (LNG) 탱크 제작에 사용되는 9% Ni강의 용접은 극저온 환경에서의 충격 흡수에너지를 유지기 위해 기존에는 FCC 기지의 Ni계 Inconel, Hasteloy 소재 용가재를 사용하고 있다. 하지만, 이러한 Ni기 합금은 고가의 원소가 포함되어 탱크 건조 비용의 경제성이 낮은 단점이 있다. 본 연구에서는 LNG 탱크의 건조 비용의 경제성이 높이고자 모재와 동종 Fe계 용접 소재 및 공정 연구를 개발을 수행하였다. 용가재는 상태도 계산 열역학 소프트웨어를 통하여 오스테나이트 안정화 원소들을 포함하는 Fe계 조성을 설계하였다. 용가재는 오스테나이트-마르텐사이트 2상 조직을 나타내었다. 용가재는 솔리드 타입으로 ø1.5 mm 크기로 제작하였고 TIG 용접을 통하여 용접 샘플을 제작하였다. 용접한 시험편은 비드의 냉각 속도에 따른 오스테나이트-마르텐사이트 조직의 상분율, 미세조직을 분석 그리고 용접 희석 및 HAZ부에 대해서 EDS 성분 분석하였다. 또한, 인장시험과 충격 실험을 통하여 설계된 Fe계 용가재의 용접성을 검토하였다.