극저온용 Fe-36%Ni강 자동 용접부의 White spot 생성 원인 분석 = Analysis of White Spots Formation on Autogeneous TIG Welds of Cryogenic Fe-36%Ni Steels

다국어 초록 (Multilingual Abstract)

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.

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