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Seonghoon Yoo,Byungrok Moon,Myeonghwan Choi,Xian Guo,Junghoon Lee,Namhyun Kang 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.11
In this study, the effect of the Ti/N ratio on the coarsening behaviour of (Ti, Nb)(C, N) precipitates and prior austenite grainsize (PAGS) in the coarse-grained heat-affected zone (CGHAZ) during the welding process was investigated. Oscillationand tandem electro-gas welding (EGW) were simulated using the finite element method to calculate the thermal history ofthe CGHAZ. The calculated thermal history was applied to the Gleeble specimens to simulate the CGHAZ of tandem EGW. As the Ti/N ratio decreased to ~1.5, and the amount of N increased, finer and more (Ti, Nb)(C, N) precipitates existed inthe as-rolled and CGHAZ specimens (peak temperature ~ 1335 °C), thereby producing a small PAGS. Comparing with theprevious studies regarding to the isothermal kinetics conducted by isothermal heat treatment of Ti(C,N), the athermal kinetics(simulation of tandem EGW) of (Ti,Nb)(C,N) showed the insignificant growth of PAGS. Furthermore, Nb-rich precipitateswere dissolved in the matrix above 1000 °C, and they did not contribute to the PAGS growth of the CGHAZ.
Seonghoon Jeong,Gitae Park,Bongyoon Kim,Joonoh Moon,Seong‑Jun Park,Changhee Lee 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.10
This study investigates the relationship between the mechanical characteristics and microstructural transitions in heat-affectedzone (HAZ) of various austenitic Fe–Mn–Al–C lightweight steels. The analysis was conducted on the simulated HAZ sampleswith various post-weld heat treatment (PWHT) conditions. The base steel was prepared via vacuum induction melting, hotrolling, and solution treatment prior to water quenching. Gleeble simulator was used to manufacture the simulated HAZs, andthe samples were subjected to PWHT with various durations by electric furnace. To understand the effects of thermal historyon the mechanical characteristics, ultimate tensile tests, micro-hardness tests, and cryogenic Charpy impact toughness testswere conducted, and microstructural analyses were conducted by optical microscopy, scanning electron microscopy, X-raydiffraction analysis, and transmission electron microscopy. The experimental results confirmed the influence of Al contenton the austenitic Fe–Mn–Al–C lightweight steels, which promotes κ-carbide precipitation, both in the austenite matrix andgrain boundaries, thereby increasing the strength and hardness while decreasing the ductility and toughness. Each mechanicalproperty showed a linear relationship with the growth kinetics of the κ-carbide particles. However, abnormal coarsening ofthe inter-granular κ-carbide during welding and PWHT caused severe embrittlement, regardless of thermal history, by actingas inter-granular crack propagation channels. The experimental results suggest that Fe–Mn–Al–C lightweight steels shouldbe developed with controlling the alloying elements to overcome HAZ discontinuity and mechanical property degradationoccurring by intra- and inter-granular κ-carbide precipitation.
유한요소해석을 통해 모사된 tandem EGW의 CGHAZ에서 Ti / N 비와 Nb 함량이 초기 오스테나이트 결정립 크기에 미치는 영향
유성훈(Seonghoon Yoo),문병록(Byungrok Moon),최명환(Myeonghwan Choi),강남현(Namhyun Kang) 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.5
조선, 해양플랜트 및 초고층 구조물 등에 적용되는 후판 강재의 경우 용접시간 단축을 통해 획기적인 원가 절감 및 생산성 향상이 가능하다. 이러한 후판 강재의 용접 생산성은 350 kJ/cm 이상의 EGW/tandem EGW 초대입열 용접을 통해 확보할 수 있다. 하지만 초대입열 용접의 과도한 용접 입열로 인해 용착금속과 열영향부 인성이 저하되는 문제점이 있기 때문에 초대입열 용접에도 충격 인성을 확보할 수 있는 강재 개발이 필요하다. 최근 Ti, Nb, V, Mo 등을 첨가하여 탄화물, 질화물 그리고 탄질화물을 형성해 초기 오스테나이트 결정립 성장을 억제시켜 충격 인성을 확보하는 방법이 적용 중이다. 하지만 일정한 주기를 가지고 진동하는 2개의 용접봉이 적용된 tandem EGW의 모델링 및 여러 가지 화학 조성이 기계적 특성에 미치는 영향에 관한 연구가 미비하며 유한요소해석을 통해 tandem EGW의 CGHAZ를 모사해 CGHAZ의 충격인성 확보를 위한 화학조성 최적화에 관련된 연구가 필요하다. 따라서 본 연구는 유한요소해석으로 tandem EGW를 모사했고, 결과로 얻은 CGHAZ에서의 열이력을 글리블 실험을 통해 여러 가지 화학 조성의 강재에 적용해 Ti/N 비와 Nb 함량이 CGHAZ의 초기 오스테나이트 결정립 크기에 미치는 영향에 대해 연구하였다. 글리블 실험 후 얻은 TEM 결과를 통해 석출물의 평균 입도 및 분포와 부피 분율을 측정했다. Ti/N 비가 변화함에 따라 초기 오스테나이트 결정립 크기가 변하였고 hypo-stoichiometric ratio의 비를 가질 때 더 우수한 결정립 미세화 효과를 보였으며 Nb 함량이 증가할수록 Nb - rich 상의 heterogeneous nucleation이 증가해 더 많은 양의 석출물이 관찰되었다.
Jeong, Seonghoon,Kim, Bongyoon,Moon, Joonoh,Park, Seong-Jun,Lee, Changhee Elsevier Sequoia 2018 Materials science & engineering Structural materia Vol.726 No.-
<P><B>Abstract</B></P> <P>This study aims to investigate the effects of κ-carbide precipitation behavior in the heat affected zone (HAZ) in FeMnAlC lightweight steels. Three alloys with different amounts of Al were prepared by vacuum induction melting and hot rolling. After solution treatment, the HAZ samples were simulated by a Gleeble simulator with two heat inputs of 10 and 30 kJ/cm. Microstructural analysis with XRD and TEM were carried out while sub-sized tensile test, hardness test, and V-notched Charpy impact test were performed for investigating the mechanical properties of the base steels and HAZ. The results showed that the mechanical properties and precipitation of κ-carbide within the HAZ were strongly related to the Al content and heat input; the tensile strength and hardness of the HAZ increased as the Al content and heat input increased while elongation decreased. On the other hand, in the Charpy impact test, fracture mode transitions in the HAZ (ranging from ductile fracture to brittle inter-granular fracture) occurred in accordance with the Al content and heat input. The different fracture behavior was explained by TEM results, which showed precipitation behavior of κ-carbide in HAZ. Coherent intra-granular κ-carbide was found to cause a transition from ductile fracture to trans-granular cleavage, and we observed that a severe drop of the impact toughness occurred when partially coherent inter-granular κ-carbide appeared. Therefore, our results illustrate that the HAZ of lightweight steels with the proper Al content can be strengthened with minimal loss of impact toughness due to κ-carbide precipitation during the welding process.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The effect of κ-carbide in the HAZ for FeMnAlC alloys was characterized. </LI> <LI> Relationship between mechanical properties and κ-carbide in HAZ was confirmed. </LI> <LI> Formation of intra- and inter-granular κ-carbide affected impact toughness. </LI> </UL> </P>
Jeong, Seonghoon,Park, Gitae,Kim, Bongyoon,Moon, Joonoh,Park, Seong-Jun,Lee, Changhee Elsevier 2019 Materials science & engineering. properties, micro Vol.742 No.-
<P><B>Abstract</B></P> <P>In this study, we investigated the effect of precipitation during the thermal cycles of age hardening and the welding process in hot rolled austenitic FeMnAlC lightweight steel. After solution treatment, samples were prepared under different age hardening conditions, which was conducted at a temperature of 550 °C in a furnace. Samples representing heat affected zones (HAZs) with three different peak temperature conditions were prepared via a Gleeble simulator. For microstructural analyses, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy (TEM) were carried out. The mechanical characteristics were investigated by tensile, hardness, and cryogenic Charpy impact tests. The results showed that precipitation behavior in the lightweight steels had specific influence in mechanical properties in age hardening and welding process. The age hardening effect was shown with increased tensile strength with loss of ductility. In the solution treated specimen, increased hardness appeared in HAZ compared to the base steel, while HAZ softening occurred in every age-hardened specimen. In the cryogenic Charpy impact test, all of the samples except the solution-treated base steel showed negligible energy absorption with inter-granular fracturing. The trends of the experimented HAZ mechanical properties revealed that the precipitation behavior of κ-carbide was strongly related to the overall mechanical properties. Moreover, we found that the welding process removed the hardening effect of aging as a result of drastic κ-carbide dissolution in the high-temperature HAZ regardless of age hardening conditions. The TEM analysis confirmed that all HAZ simulated specimens showed similar precipitation conditions for all cases. Based on our analysis, we confirmed that the precipitation behavior of κ-carbide was the main factor of different HAZ mechanical properties. We also suggested that discontinuous HAZ mechanical properties should be considered during the welding process because of κ-carbide dissolution in FeMnAlC lightweight steels.</P> <P><B>Highlights</B></P> <P> <UL> <LI> FeMnAlC alloy was analyzed for various age hardening and HAZ conditions. </LI> <LI> Mechanical properties had specific relationship with κ-carbide in all cases. </LI> <LI> Inter-granular κ-carbide rapidly formed in all heat treatment conditions. </LI> </UL> </P>
Jaewon Moon,Seonghoon Yi 대한금속·재료학회 2016 METALS AND MATERIALS International Vol.22 No.5
For thermal neutron shielding applications, high boron amorphous ribbons with the compositions of Fe72-xB25- Mo3Crx (where x = 0, 5, 10, 15 or 20 at%) have been developed. From the morphological and compositional analysis of vein patterns formed on the fracture surfaces of the ribbons, the segregation of boron within shear band regions was identified and discussed with reference to “free volume concentration” and “temperature rise” models that had been proposed independently. High boron amorphous ribbons with optimum amounts of Cr exhibit excellent thermal neutron shielding efficiency in combination with mechanical properties such as tensile strength and bending flexibility. Moreover, corrosion resistance of the amorphous ribbons can be effectively enhanced through proper Cr additions, thereby expanding their structural applications in the nuclear industry.
Bongyoon Kim,Seonghoon Jeong,Seong‑Jun Park,Joonoh Moon,Changhee Lee 대한금속·재료학회 2019 METALS AND MATERIALS International Vol.25 No.4
In the automotive industry, lightweight steel has received much attention because steel comprises a signifi cant portion of avehicle’s total weight. Fe–Mn–Al–C steel is a representative lightweight steel due to its high performance and low density. However, there is insuffi cient research into the welding characteristics of Fe–Mn–Al–C lightweight steels. In this study, hotductility tests were conducted on austenitic Fe–30Mn–9Al–0.9C steel in order to understand the welding characteristics(cracking resistance) of the heat aff ected zone. During the on-heating thermal cycle, ductility was altered by a decrease inmicroband induced plasticity (MBIP) (softening) and an increase in dynamic recrystallization (DRX) (softening) as the temperatureincreased. Specifi cally, in the range of 773–1073 K, ductility was fairly degraded because neither MBIP nor DRXtook place. During the on-cooling thermal cycle, ductility behavior was changed by both softening and hardening factors,including formation of brittle (Fe, Mn) 3 Al intermetallic compounds with grain growth and re-solidifi ed grain boundaries. However, the hardening eff ect of precipitated κ-carbide was insignifi cant and might not play a signifi cant role in the hotductility behavior of the lightweight alloy used in this study.