• Journal of Korea Foundry Society
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• v.29 no.3
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• pp.120-127
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• 2009

The characterestics of high Si-high Mo ductile cast iron, high Si-high Mo C.V. cast iron and Ni-resist cast iron were compared and evaluated. The nodule count of the last one was lower and the nodularity was higher than those for the first one, respectively. The first two had ferritic matrices with small amounts of molybdenum carbides. The first one had the highest tensile strength and the last one the lowest elongation. This had the highest high temperature strength and that of the second one was greatly increased from the room temperature strength. The volumes of the first two were decreased during cooling and that of the last one changed little. The thermal expansion coefficient of the last one was the highest and the first one the lowest. During high temperature oxidation, even though the volume of the first one was increased, the weight was decreased and the volume and weight of the second one were increased. The change of the increased weight of the last one was more than that of thickness.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.274-274
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• 2010

한국은 국제핵융합실험로 (ITER) 사업에 참여하고 있으며, 삼중수소 증식을 시험하기 위한 시험 모듈(TBM, Test Blanket Module)로서 HCML (Helium Cooled Molten Lithium) TBM을 설계, 개발하고 있다. 헬륨 및 액체 리튬을 냉각재와 증식재로 사용하는 개념으로, 구조재로서 Ferritic Martensitic (FM) 강이 사용될 예정이다. 특히, HCML TBM의 일차벽은 중성자 및 플라즈마로부터 입사되는 입자들을 차폐하기 위한 Be 차폐체와 FM강으로 구성되어 있으며, 일차벽 제작법 개발을 위해서는 Be과 FM강 간의 접합과 FM강 간의 접합 방법이 개발되어야 한다. FM강 간의 접합은 기존의 연구를 통해 접합 조건이 이미 도출되었고, 고열부하 시험을 통해 검증 완료한 상태이다. 그러나, Be과 FM강 간의 접합은 현재 개발단계에 있다. 본 논문에서는 고려 중인 구조재와 Be 차폐체 사이의 접합법 개발을 위해, 고온등방가압(HIP, Hot Isostatic Pressing) 조건을 도출하고, 운전조건과 유사 혹은 가혹한 조건에서 고열부하를 인가하여, 그 건전성을 평가하는 일련의 과정을 기술하였다. 본 연구에서는 Be과 FM강 간의 접합법 개발 및 검증을 위해 제작된 $80{\times}80{\times}1$ Be/FM강 mock-up을 국내에서 구축된 고열부하 시험 장비인 KoHLT를 활용하여 수행한 고열부하 시험에 대한 것이다. 본 mock-up은 $80{\times}80{\times}10mm(t)$의 Be tile 3개를 동일 크기에 두께가 각각 25mm와 50 mm인 FM강과 스테인레스강에 접합된 것으로, 고열부하 장비에 설치하여 고열부하 시험을 수행하였다. 냉각수의 온도 및 속도는 25 C, 0.15 kg/sec로 유지되었고, 열부하는 $0.5\;MW/m^2$로 유지하였다. 시험 조건에 대한 예비해석을 통해, 가열시의 온도 및 stress, strain 분포를 얻었고, 이를 통해, cycle to failure 값을 도출하였다. 1000 사이클의 가열 실험을 마친후 초음파를 활용한 접합 계면의 결함확인 및 파괴검사를 통한 접합 건전성을 확인하였다. 3가지 접합법 모두 일부 접합면이 이탈되었으며, 향후 보다 건전한 접합방법 개발이 진행되어야 할 것으로 보인다.

• Journal of Welding and Joining
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• v.1 no.2
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• pp.45-52
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• 1983

Many pressure vessels for the hot H$\sub$2//H$\sub$2/S service are made of 2+1/4Cr-1Mo steel with austenitic stainless steel overlay to combat agressive corrosion due to hydrogen sulfide. Hydrogen dissolves in to materials during operation, and sometimes gives rise to unfore-seeable damages. Appropriate precautions must, therefore, be taken to avoid the hydrogen induced damages in the design, fabrication and operation stage of such reactor vessels. Recently, hydrogeninduced cracking (or Disbonding) was found at the interface between base metal and stainless weld overlay of a desulfurizing reactor. Since the stainless steel overlay weld metal is subjected to thermal and internal-pressure loads in reactor operation, it is desirable for the overlay weld metal to have high strength and ductility from the stand point of structural safety. In section III of ASME Boiler and Pressure Vessel Code, Post-Weld Heat Treatment(PWHT) of more than one hour per inch at over 1100.deg. F(593.deg. C) is required for the weld joints of low alloy pressure vessel steels. This heat treatment to relieve stresses in the welded joint during construction of the pressure vessel is considered to cause sensitization of the overlay weld metal. The present study was carried out to make clear the diffusion of carbon migration by PWHT in dissimilar metal welded joint. The main conclusion reached from this study are as follows: 1) The theoretical analysis for diffusion of carbon in stainless steel overlay weld metal does not agree with Fick's 2nd law but the general law of molecular diffusion phenomenon by thermodynamic chemical potential. 2) In the stainless steel overlay welded joint, the PWHT at 720.deg. C for 10 hours causes a diffusion of carbon atoms from ferritic steel into austenitic steel according to the theoretical analysis for carbon migration and its experiment. 3) In case of PWHT at 720.deg. C for 10 hours, the micro-hardness of stainless steel weld metal in bonded zone increase very highly in the carburized layer with remarkable hardening than that of weld metal.

• Steel and Composite Structures
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• v.33 no.1
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• pp.37-66
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• 2019

In cold-formed steel structures, such as trusses, wall frames and portal frames, the use of back-to-back built-up cold-formed stainless-steel lipped channels as compression members are becoming increasingly popular. The advantages of using stainless-steel as structural members are corrosion resistance and durability, compared with carbon steel. The AISI/ASCE Standard, SEI/ASCE-8-02 and AS/NZS do not include the design of stainless-steel built-up channels and very few experimental tests or finite element analyses have been reported in the literature for such back-to back cold-formed stainless-steel channels. Current guidance by the American Iron and Steel Institute (AISI) and the Australian and New Zealand (gAS/NZS) standards for built-up carbon steel sections only describe a modified slenderness approach, to consider the spacing of the intermediate fasteners. Thus, this paper presents a numerical investigation on the behavior of back-to-back cold-formed stainless-steel built-up lipped channels. Three different grades of stainless steel i.e., duplex EN1.4462, ferritic EN1.4003 and austenitic EN1.4404 have been considered. Effect of screw spacing on the axial strength of such built-up channels was investigated. As expected, most of the short and intermediate columns failed by either local-global or local-distortional buckling interactions, whereas the long columns, failed by global buckling. All three grades of stainless-steel stub columns failed by local buckling. A comprehensive parametric study was then carried out covering a wide range of slenderness and different cross-sectional geometries to assess the performance of the current design guidelines by AISI and AS/NZS. In total, 647 finite element models were analyzed. From the results of the parametric study, it was found that the AISI & AS/NZS are conservative by around 10 to 20% for cold-formed stainless-steel built-up lipped channels failed through overall buckling, irrespective of the stainless-steel grades. However, the AISI and AS/NZS can be un-conservative by around 6% for all three grades of stainless-steel built-up channels, which failed by local buckling.

• Nuclear Engineering and Technology
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• v.34 no.1
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• pp.42-59
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• 2002

In order to transmute long-lived radioactive nuclides such as transuranics(TRU), Tc-99, and I- l29 in LWR spent fuel, a preliminary conceptual design study has been performed for the accelerator driven subcritical reactor system, called HYPER(Hybrid Power Extraction Reactor) The core has a hybrid neutron energy spectrum: fast and thermal neutrons for the transmutation of TRU and fission products, respectively. TRU is loaded into the HYPER core as a TRU-Zr metal form because a metal type fuel has very good compatibility with the pyre- chemical process which retains the self-protection of transuranics at all times. On the other hand, Tc-99 and I-129 are loaded as pure technetium metal and sodium iodide, respectively. Pb-Bi is chosen as a primary coolant because Pb-Bi can be a good spallation target and produce a very hard neutron energy spectrum. As a result, the HYPER system does not have any independent spallation target system. 9Cr-2WVTa is used as a window material because an advanced ferritic/martensitic steel is known to have a good performance under a highly corrosive and radiation environment. The support ratios of the HYPER system are about 4∼5 for TRU, Tc-99, and I-129. Therefore, a radiologically clean nuclear power, i.e. zero net production of TRU, Tc-99 and I-129 can be achieved by combining 4 ∼5 LWRs with one HYPER system. In addition, the HYPER system, having good proliferation resistance and high nuclear waste transmutation capability, is believed to provide a breakthrough to the spent fuel problems the nuclear industry is faced with.

• Korean Journal of Metals and Materials
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• v.48 no.1
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• pp.39-48
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• 2010

In order to investigate the effects of tempering temperature and heat-treatment path on the microstructural and mechanical properties of ASTM Gr.92 steels, four samples with different tempering temperatures and heat-treatment paths wer prepared. THeree experimental steels showed tempered martensitic microstructures, but the sample tempered at $810^{\circ}C$ was presumed to retain partially untempered martensitic microstructures due to a lower ${\alpha}$+${\gamma}$ phase regime. $M_{23}C_6$, V(C,N), and Nb(C,N) precipitates were observed in all samples. In addition $Cr_2N$ was observed to be precipitated finely and uniformly by isothermal heat-treatment. The lath width and precipitate size in the isothermal heat-treated samples were much smaller than those of the tempered-only specimens. Because of a fine and uniform precipitate, a reduction of lath width would enhance precipitation hardeing, and it was shown that mechanical propertiesincluding the hardness and tensile properties of the steels were improved by isothermal heat-treatment.

• Korean Journal of Metals and Materials
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• v.49 no.7
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• pp.549-556
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• 2011

This study aimed to evaluate the performance of diffusion barriers in order to prevent fuel-cladding chemical interaction (FCCI) between the metallic fuels and the cladding materials, a potential hazard for nuclear fuel in sodium-cooled fast reactors. In order to prevent FCCI, Zr or V metal is deposited on the ferritic-martensitic stainless steel surface by physical vapor deposition with a thickness up to $5{\mu}m$. The diffusion couple tests using uranium alloy (U-10Zr) and a rare earth metal such as Ce-La alloy and Nd were performed at temperatures between 660~800$^{\circ}C$. Microstructural analysis using SEM was carried out over the coupled specimen. The results show that significant interdiffusion and an associated eutectic reaction ocurred in the specimen without a diffusion barrier. However, with the exception of the local dissolution of the Zr layer in the Ce-La alloy, the specimens deposited with Zr and V exhibited superior eutectic resistance to the uranium alloy and rare earth metal.

• Wind and Structures
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• v.32 no.1
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• pp.55-69
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• 2021

Ultrapure ferritic stainless steel provides a new generation of long-span metal roof systems with continuous welding technology, which exhibits many unknown behaviors during wind excitation. This study focuses on the wind-resistant capacity of a new continuous welding stainless steel roof (CWSSR) system. Full-scale testing on the welding joints and the CWSSR system is performed under uniaxial tension and static ultimate wind uplift loadings, respectively. A finite element model is developed with mesh refinement optimization and is further validated with the testing results, which provides a reliable way of investigating the parameter effect on the wind-induced structural responses, namely, the width and thickness of the roof sheeting and welding height. Research results show that the CWSSR system has predominant wind-resistant performance and can bear an ultimate wind uplift loading of 10.4 kPa without observable failures. The welding joints achieve equivalent mechanical behaviors as those of base material is produced with the current of 65 A. Independent structural responses can be found for the roof sheeting of the CWSSR system, and the maximum displacement appears at the middle of the roof sheeting, while the maximum stress appears at the connection supports between the roof sheeting with a significant stress concentration effect. The responses of the CWSSR system are greatly influenced by the width and thickness of the roof sheeting but are less influenced by the welding height.

• Nuclear Engineering and Technology
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• v.54 no.7
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• pp.2376-2385
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• 2022

Two groups of oxide dispersion-strengthened reduced-activation ferritic/martensitic steels (A and B) were prepared by adding Y, Ti, and Zr into steels through vacuum induction melting to investigate the inclusions, microstructures, mechanical properties of the alloys. Results showed that particles with Y, Ti, and Zr easily formed. Massive, Zr-rich inclusions were found in B steel. Density of micron inclusions in A steel was 1.42 × 10¹⁴ m^-3, and density of nanoparticles was 3.61 × 10¹⁶ m^-3. More and finer MX carbides were found in steel tempered at 650 ℃, and yield strengths (YS) of A and B steel were 714±2 and 664±3.5 MPa. Thermomechanical processing (TMP) retained many dislocations, which improved the mechanical properties. YSs of A and B treated by TMP were 725±3 and 683±4 MPa. The existence of massive Zr-rich inclusions in B steels interrupted the continuity of the matrix and produced microcracks (fracture), which caused a reduction in mechanical properties. The presence of fine prior austenite grain size and inclusions was attributed to the low DBTTs of the A steels; DBTTs of A650 and A700 alloy were -79 and -65 ℃. Tempering temperature reduction and TMP are simple, readily useable methods that can lead to a superior balance of strength and impact toughness in industry applications.

• Corrosion Science and Technology
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• v.20 no.6
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• pp.373-383
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• 2021

The effect of Cr addition to high Mn steel on flow-accelerated corrosion (FAC) behavior in a neutral aqueous environment was evaluated. For comparison, two types of conventional ferritic steels (API X70 steel and 9% Ni steel) were used. A range of experiments (electrochemical polarization and impedance tests, weight loss measurement, and metallographic observation of corrosion scale) were conducted. This study showed that high Mn steel with 3% Cr exhibited the highest resistance to FAC presumably due to the formation of a bi-layer scale structure composed of an inner Cr enriched Fe oxide and an outer Mn substituted partially with Fe oxide on the surface. Although the high Mn steels had the lowest corrosion resistance at the initial corrosion stage due to rapid dissolution kinetics of Mn elements on their surface, the kinetics of inner scale (i.e. Cr enriched Fe oxide) formation on Cr-bearing high Mn steel was faster in dynamic flowing condition compared to stagnant condition. On the other hand, the corrosion scales formed on API X70 and 9% Ni steels did not provide sufficient anti-corrosion function during the prolonged exposure to dynamic flowing conditions.