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Hobyung Chae,E‑Wen Huang,Jayant Jain,Dong‑Hyun Lee,Stefanus Harjo,Takuro Kawasaki,Soo Yeol Lee 대한금속·재료학회 2024 METALS AND MATERIALS International Vol.30 No.5
We employed two additive manufacturing (AM) strategies perpendicular to each other in AM stainless steel. In contrast tothe vertically-built strategy, the horizontally-built one imposed higher tensile strength and ductility due to distinct transformation-induced plasticity (TRIP) behavior and a tailored initial texture oriented more favorably for deformation texture. Weapplied the direct-aging-treatment without solid-solution to retain unique AM microstructure and retained austenite (RA),which enhanced yield strength (+ 60.2%), tensile strength (+ 41.2%), and ductility (+ 81.2%). This effective strengtheningby the direct-aging-treatment effectively increased the stability of RA delaying the onset of TRIP. This is because RA grainspossessed relatively homogeneous chemical composition by aging and are effectively suppressed by a harder neighboredprecipitated-martensitic matrix despite the increase of stacking fault probability. Our findings offer a promising paradigmfor tailoring properties of texture-controlled AM steels with the aging strategy and advance a fundamental understanding ofthe intrinsic deformation behavior of AM metals.
Stress Corrosion Cracking of a Copper Pipe in a Heating Water Supply System
Hobyung Chae,Huai Wang,Minki Hong,Woo Cheol Kim,Jung‑Gu Kim,Heesan Kim,Soo Yeol Lee 대한금속·재료학회 2020 METALS AND MATERIALS International Vol.26 No.7
Stress corrosion cracking behavior of a copper pipe and its mechanism were investigated in a heating water supply systemusing both metallurgical and mechanical analyses. The X-ray diffraction and energy dispersive spectroscopy measurementsshowed that uniform cupric oxide and cuprous oxide were formed during service on the inside surface of the pipe, whilemalachite and cupric oxide were the main corrosion products on the outer surface of the pipe. Many corrosion pits wereobserved around the cracking areas on the outer surface by optical microscopy. Finite element modeling highlighted a regionwith large hoop stresses mainly brought about by the bolt load, where it became susceptible to stress corrosion cracking(SCC). The morphology of the fracture surface revealed an intergranular mode towards the outer surface but a mixed interandtrans-granular mode near the inner surface. Based on the obtained results, a phenomenological 4-stage fracture processis proposed to describe the failure process of the copper pipe, in which the SCC initiated from the corrosion pits on the outersurface, propagated along the axial direction and penetrated towards the inner surface until final fracture.
채호병 ( Hobyung Chae ),김우철 ( Woo Cheol Kim ),김희산 ( Heesan Kim ),김정구 ( Jung-gu Kim ),김경민 ( Kyung Min Kim ),이수열 ( Soo Yeol Lee ) 한국부식방식학회 2021 Corrosion Science and Technology Vol.20 No.2
In this work, we have performed a corrosion failure analysis of a leaking tube connected to an upper header of a condensate pre-heater in a heat recovery steam generator. It was revealed that the leakage position in the tube was the location where the materials were easily vulnerable due to tensile residual stresses induced by the material manufacturing process and welding process. In addition to an imbalance in the module induced by temperature difference during operation of the pre-heater, the weight of the modules and thermal fatigue provoked a type of stress of tensile-tensile fatigue on the tube. Thus, the leakage position of the pre-heater was exposed to the tensile stress on the inner surface of the tube facing the gas, which rendered the unstable oxide layer susceptible to corrosion and the formation of pits on the water side. The cracks propagated along with the degraded microstructure in a transgranular cracking mode under fatigue loading and finally resulted in water leakage.