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Alloy 600의 Primary Water Stress Corrosion Cracking 반응기구에 대한 고찰
정한섭,국일현 한국부식학회 1996 Corrosion Science and Technology Vol.25 No.1
Characteristics of Primary Water Stress Corrosion Cracking of Alloy 600 nuclear steam generator tubing material were reviewed on the basis of literature data. Thermodynamic characteristics and effect of hydrogen overpressure, behavior in water and steam phase, and the effects of important variables, such as microstructure, temperature and stress and strain, were focused. The classical theory of Slip Dissolution and Film Rupture model, and Hydrogen Embrittlement model were found to be inadequate for the thorough explanation of PWSCC. Models proposed recently to overcome the difficulties encountered by the classical models, included Void-Linkage model, Internal Oxidation model, Corrosion Deformation Interaction model, Creep model, and Selective Dissolution-Vacancy-Creep model. Theoretical background and coincidence with experimental evidences of each model were discussed. Existance of grain boundary cavities near advancing crack tip is in controversy among different researches. Dependence of chemistry and structure of Alloy 600 surface film on the corrosion potential, which has to be revealed by further research efforts, may be helpful for understanding the mechanism of PWSCC.
12Cr-1MoVW 강의 열처리에 따른 탄화물의 화학조성 변화
김성호,유우석,국일현,송병준 대한금속재료학회(대한금속학회) 2000 대한금속·재료학회지 Vol.38 No.3
Dissolution and precipitation reactions of the carbides occurred during heat treatment of 12Cr-1MoVW steel were studied. Transmission electron microscopy and energy dispersive X-ray spectroscopy were used to determine types and chemical compositions of precipitates. The dissolution of the M_(23)C_6 carbide took place during normalizing treatment through the diffusion of chromium atoms. Upon tempering, types of M₃C and M_(23)C_6 precipitates were formed depending on the tempering conditions. The chemical composition of M₃C formed at low tempering temperature is similar to the base material, but the enrichment of chromium and depletion of iron in M₃C occur with increasing the tempering time and temperature. On the other hand chromium content decreases and iron content increases in M_(23)C_6 carbide with increasing the tempering temperature. M_(23)C_6 forms at the δ-ferrite/tempered martensite interface regardless of tempering temperatures. The chromium content of M_(23)C_6 formed at low temperature is low, but chromium content increases and iron content decrease in M_(23)C_6 with increasing the tempering time and temperature.
Microstructure and Mechanical Properties of Cr-Mo Steels for Nuclear Industry Applications
Kim, Sung-Ho,Ryu, Woo-Seong,Kuk, Il-Hiun Korean Nuclear Society 1999 Nuclear Engineering and Technology Vol.31 No.6
Microstructure and mechanical properties of five Cr-Mo steels for nuclear industry applications have been investigated. Transmission electron microscopy, energy dispersive spectrometer, differential scanning calorimeter, hardness, tensile, and impact test were used to evaluate the Cr and W effect on the microstructure and mechanical properties. Microstructures of Cr-Mo steels after tempering are classified into three types : bainitic 2.25Cr-lMo steel, martensitic Mod.9Cr-lMo, HT9M, and HT9W steels, and dual phase HT9 steel. The majority of the precipitates were found to be M$_{23}$C$_{6}$ carbides. As minor phases, fine needle-like V(C,N), spherical NbC, fine needle-like Cr-rich Cr$_2$N, and Cr-rich M$_{7}$C$_3$were also found. Addition of 2wt.% W in Cr-Mo steels retarded the formation of subgrain and dissolution of Cr$_2$N precipitates. Hardness and ultimate tensile strength increased with increasing Cr content. Though Cr content of HT9W steel was lower than that of HT9 steel, the hardness of HT9W was higher due to the higher W content. W added HT9W steel had the highest ultimate tensile strength above $600^{\circ}C$. But impact toughness of W added steel (HT9W) and high Cr steel (HT9) was low.w.w.
질소를 첨가한 316L 스테인리스 강의 고온 저주기 피로균열 생성
김대환,류우석,국익현,김우곤 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.5
The effect of nitrogen on the nucleation of a low cycle fatigue (LCF) crack was studied at high temperature using type 316L stainless steel with different nitrogen contents (0.04-0.15%). Strain-controlled LCF tests were conducted in the temperature range of R.T. ∼600℃ and in air. The waveform of LCF was a symmetrical triangular with a total strain amplitude of 1.0% and in a constant strain rate of 2×10^(-3)/s for all tests. The sites for crack nucleation, crack population, and crack depth from the surface of the specimens were measured after LCF testing up to 40% of fatigue life. The preferred sites for crack nucleation were grain boundaries, but some of cracks also nucleated at twin boundaries and slip bands. Crack population on the surface of the specimen decreased but the maximum depth of the crack from the surface of the specimen did not change with the addition of nitrogen at high temperatures. The cycles for the nucleation (100㎛) of LCF crack increased with the addition of nitrogen. So, the increase of LCF life with the addition of nitrogen at high temperatures may be explained by the fact that nitrogen increases the resistance of crack nucleation.
원자력용 HT9 강과 HT9M 강의 미세조직과 기계적 특성 변화
김성호,박순동,류우석,국일현,송병준 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.12
Microstructure and mechanical properties of HT9 and HT9M steels for liquid metal reactor(LMR) core materials have been investigated. The tempering temperature at which cell structure was formed by rearrangement of dislocations was same in two steels. More carbides were precipitated in HT9 steel due to high carbon content and precipitation of carbides was delayed in HT9M steel. Thus the decrease of hardness with tempering temperature in HT9M steel was lower than that of HT9 steel. The yield strength was decreased, the fine prior austenite grain was formed, and the formation of δ-ferrite was restricted in HT9M steel by decreasing the Cr and carbon content and increasing the Nb content. Thus the impact property of HT9M steel was improved.
이용복,장진성,이덕열,이동희,국일현 대한금속재료학회(대한금속학회) 1997 대한금속·재료학회지 Vol.35 No.8
Precipitation behavior of solution heat treated Alloy 690 tubes (60Ni-30Cr-10Fe: UNS N 06690) has been studied. Carbide precipitates along grain boundary were revealed as M_(23)C_6 type carbide having semi-coherent interface with one adjacent grain. Also drastic change of precipitate density was observed on a twin intercepted grain boundary, i.e, abundant precipitates on general grain boundaries, but no precipitate on boundary segments intercepted by twins. This selective precipitation phenomenon was attributed to the different orientation relationships of grain boundary segments, i.e. different misorientation characteristics, and eventually drastically different boundary energy. Different misorientation characteristics of interfaces were investigated through Kikuchi pattern analyses in TEM. The grain boundary with precipitates was random high angle grain boundary and the clean segment of boundary was CSL (Coincidence Site Lattice) special boundary (e.g. ∑=15), judged by Brandon's criterion. Of course coherent twin boundary with ∑=3 were free of precipitate. (Received April 2,1997)
질소를 첨가한 Type 316L 스테인리스 강의 동적변형시효 거동
김대환,홍준화,최시경,류우석,국익현 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.3
Tensile tests were conducted with varying strain rate (1×10^(-2)/sec∼2×10^(-4)/sec) at the test temperature range between room temperature and 750℃ to evaluate the effect of nitrogen on dynamic strain aging (DSA) behaviors in nitrogen added type 316L stainless steels. Yield and tensile strength increased without reduction of elongation with adding nitrogen. The temperature range and the critical strain for DSA increased with nitrogen addition. Types A and B serrations were observed at 600℃ and the strain for transition from type A to type B increased with nitrogen addition. Solute atoms for DSA were vacancies and chromium and the activation energy for DSA increased with adding nitrogen. The reason that nitrogen retarded DSA was explained by the fact that nitrogen reduced the chromium diffusion to dislocations due to the strong interaction between nitrogen and chromium.