• 대한금속재료학회지
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• 제47권3호
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• pp.175-181
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• 2009

Mechanical properties and precipitation behavior of backward extruded 6061 Al alloy for pressure vessel were investigated using tensile test, transmission electron microscopy (TEM) and differential scanning calorimeter (DSC). In this study, solution heat treatment (SHT) was performed at $535^{\circ}C$ for 30~90 min and aging treatment was conducted at 177 and $190^{\circ}C$ for 1~7 h. Maximum tensile strength of $36.6kgf/mm^2$ and yield strength of $33.29kgf/mm^2$ were achieved at the aging time of 5 h at $190^{\circ}C$. TEM observation showed that fine needle-like ${\beta}^{{\prime}{\prime}}$ phase which has 35~45 nm of length was uniformly distributed in the aged 6061 Al alloy specimen. From tensile test, TEM and DSC analysis, it is expected that aging time of 2~5 h at $190^{\circ}C$ is suitable for the extruded A6061 used as pressure vessels.

• Geomechanics and Engineering
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• 제33권2호
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• pp.133-140
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• 2023

Soil erosion can cause scouring and failures of underwater structures, therefore, various soil improvement techniques are used to increase the soil erosion resistance. The microbially induced calcium carbonate precipitation (MICP) method is proposed to increase the erosion resistance, however, there are only limited experimental and numerical studies on the use of MICP treatment for improvement of surface erosion resistance. Therefore, this study investigates the improvement in surface erosion resistance of sands by MICP through laboratory experiments and numerical modeling. The surface erosion behaviors of coarse sands with various calcium carbonate contents were first investigated via the erosion function apparatus (EFA). The test results showed that MICP treatment increased the overall erosion resistance, and the contribution of the precipitated calcium carbonate to the erosion resistance and critical shear stress was quantified in relation to the calcium carbonate contents. Further, these surface erosion processes occurring in the EFA test were simulated through the coupled computational fluid dynamics (CFD) and discrete element method (DEM) with the cohesion bonding model to reflect the mineral precipitation effect. The simulation results were compared with the experimental results, and the developed CFD-DEM model with the cohesion bonding model well predicted the critical shear stress of MICP-treated sand. This work demonstrates that the MICP treatment is effective in improving soil erosion resistance, and the coupled CFD-DEM with a bonding model is a useful and promising tool to analyze the soil erosion behavior for MICP-treated sand at a particle scale.

• 한국재료학회지
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• 제33권2호
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• pp.77-86
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• 2023

High-temperature oxidation of a Ni-based superalloy was analyzed with samples taken from gas turbine blades, where the samples were heat-treated and thermally exposed. The effect of Cr/Ti/Al elements in the alloy on high temperature oxidation was investigated using an optical microscope, SEM/EDS, and TEM. A high-Cr/high-Ti oxide layer was formed on the blade surface under the heat-treated state considered to be the initial stage of high-temperature oxidation. In addition, a PFZ (γ' precipitate free zone) accompanied by Cr carbide of Cr₂₃C₆ and high Cr-Co phase as a kind of TCP precipitation was formed under the surface layer. Pits of several ㎛ depth containing high-Al content oxide was observed at the boundary between the oxide layer and PFZ. However, high temperature oxidation formed on the thermally exposed blade surface consisted of the following steps: ① Ti-oxide formation in the center of the oxide layer, ② Cr-oxide formation surrounding the inner oxide layer, and ③ Al-oxide formation in the pits directly under the Cr oxide layer. It is estimated that the Cr content of Ni-based superalloys improves the oxidation resistance of the alloy by forming dense oxide layer, but produced the σ or µ phase of TCP precipitation with the high-Cr component resulting in material brittleness.

• 열처리공학회지
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• 제36권6호
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• pp.402-411
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• 2023

In order to examine the effect of cementite precipitated on the steel surface on the carburizing rate, the carburizing process was carried out at various boost times to measure the mass gain and carbon flux, phase analysis and carbon concentration analysis were performed on the surface of the carburized specimen. In the case of the only boost type, the longer the boost time, the more the mass gain by the diffused carbon follows the parabolic law and tends to increase. In particular, as the boost time increased, the depth of cementite precipitation and the average size of cementite on the steel surface increased. At a boost time of 7 min, the fraction of cementite precipitated on the surface is 7.32 vol.%, and the carburizing rate of carbon into the surface (surface-carbon flux) is about 17.4% compared to the calculated value because the area of the chemical (catalyst) where the carburization reaction takes place is reduced. The measured carbon concentration profile of the carburized specimen tended to be generally lower than the carbon concentration calculated by the model without considering precipitated cementite. On the other hand, in the pulse type, the mass gain by the diffused carbon increased according to the boost time following a linear law. At a boost time of 7 min, the fraction of cementite precipitated on the surface was 3.62 vol.%, and the surface-carbon flux decreased by about 4.1% compared to the calculated value. As a result, a model for predicting the actual carbon flux was presented by applying the carburization resistace coefficient derived from the surface cementite fraction as a variable.

• Corrosion Science and Technology
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• 제22권6호
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• pp.435-446
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• 2023

Corrosion behaviors of laser-welded super duplex stainless steel (SDSS) tubes after exposure to an actual power plant environment for one year and those of fin-tube welded SDSS were evaluated. Results showed that corrosion damage on the back side of the SDSS tube in the direction of hot air was higher than that on the front side regardless of weldment location. However, corrosion damage showed no difference between weldment and base metal due to recovery of phase fraction in the weldment through post weld heat treatment (PWHT). Nevertheless, the SDSS tube showed severe corrosion damage along grain boundary due to surface phase transformation (δ → γ) and Cr₂N precipitation caused by PWHT with a high N₂ atmosphere. Corrosion resistance of the SDSS tube was recovered when degraded surface was removed. Corrosion sensitivity of a fin-tube increased significantly due to pre-existing crevice, unbalanced phase fraction, and σ phase precipitation adjacent to the fusion line. Although corrosion resistance was improved by recovered phase fraction and sufficient dissolution of σ phase during PWHT, corrosion reaction was concentrated at the pre-existing crevice. These results suggest that welding conditions for fin-tube steel should be optimized to improve corrosion resistance by removing pre-existing crevice in the weldment.

• 한국주조공학회지
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• 제17권6호
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• pp.560-568
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• 1997

The dissolution behavior of secondary solidification phases in squeeze cast Al-3.9wt%Cu-1.5wt%Si-1.0wt%Mg has been studied using a combination of optical microscope, image analyzer, scanning electron microscope(SEM), energy dispersive spectrometer(EDS), X-ray diffractometer(XRD) and differential thermal analyzer (DTA). Special emphasis was placed on the investigation of the effects of the nonequilibrium heat treatment on the dissolution of the second solidification phases. Ascast microstructure consisted of primary solidification product of ${\alpha}-Al$ and secondary solidification products of $Al_2Cu$, $Mg_2Si$ and $Al_2CuMg$. Equilibrium and non-equilibrium solution treatments were carried out at the temperatures of $495^{\circ}C$, $502^{\circ}C$ and $515^{\circ}C$ for 3 to 5 hours. The amount of the dissolved secondary phases increased with increasing solution treatment temperature, for example, area fractions of $Al_2Cu$, $Mg_2Si$ and $Al_2CuMg$ were approximately 0%, 1.6% and 4.2% after solution treatment at $495^{\circ}C$ for 5hours, and were approximately 0%, 0.36% and 2% after solution treatment at $515^{\circ}C$ for 5hours. The best combination of tensile properties was obtained when the as-cast alloy was solution treated at $515^{\circ}C$ for 3hours followed by aging at $180^{\circ}C$ for 10 hours. Detailed DTA and TEM study showed that the strengthening behavior during aging was due to enhanced precipitation of the platelet type fine ${\theta}'$ phase.

• 한국도로학회논문집
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• 제19권4호
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• pp.61-68
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• 2017

PURPOSES : A complete signal system is not always the best solution for improving traffic operation efficiency at intersections. An alternative solution is to use a Protected Permitted Left Turn (PPLT) operation method. However, the PPLT method needs to be developed after a detailed study of driving tendencies, most notably the gap acceptance behavior, for successful implementation. In this study, the gap acceptance behavior was investigated under various variables and weather conditions, especially under rain, and the results were compared to the case of normal weather. The results of this study will be helpful in introducing the PPLT method, and are important considering the tendency of attempting unprotected left turns that is extremely common in Korean drivers. METHODS : Data was obtained by analyzing traffic footage at four intersections on a day when the precipitation was greater than 5 mm/h. The collected data was classified into seven variables for statistical analysis. Finally, we used logistic regression analysis to develop a probability distribution model. RESULTS : Gap, traffic volume, and the number of conflicting lanes were factors affecting the gap acceptance behavior of unprotected left turns under rainy conditions. CONCLUSIONS : The probability of attempting unprotected left turns is higher for larger gaps. On the other hand, the probability of attempting unprotected left turns decreases with an increase in the traffic volume. Finally, an increase in the number of conflict lanes leads to a decrease in the probability of attempting unprotected left turns.

• 소성∙가공
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• 제32권4호
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• pp.173-179
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• 2023

In this study, the age-hardening behavior and tensile properties of a cast AZ91-0.3Ca-0.2Y (SEN9) alloy are investigated and compared with those of a commercial AZ91 alloy. Even after homogenization heat treatment, the SEN9 alloy contains numerous undissolved secondary phases, Al₈Mn₄Y, Al₂Y, and Al₂Ca, which results in a higher hardness value than the homogenized AZ91 alloy. Under aging condition at 200 ℃, both the AZ91 and SEN9 alloys exhibit the same peak-aging time of 8 h, but the peak hardness of the latter (86.8 Hv) is higher than that of the former (83.9 Hv). The precipitation behavior of Mg₁₇Al₁₂ phase during aging significantly differs in the two alloys. In the AZ91 alloy, the area fraction of Mg₁₇Al₁₂ discontinuous precipitates (DPs) increases up to ~50% as the aging time increases. In contrast, in the SEN9 alloy, the formation and growth of DPs during aging are substantially suppressed by the Ca- or Y-containing particles, which leads to the formation of only a small amount of DPs with an area fraction of ~4% after peak aging. Moreover, the size and interparticle spacing of Mg₁₇Al₁₂ precipitates of the peak-aged SEN9 alloy are smaller than those of the peak-aged AZ91 alloy. The homogenized AZ91 alloy exhibits a higher tensile strength than the homogenized SEN9 alloy due to the finer grains of the former. However, the peak-aged SEN9 alloy has a higher tensile elongation than the peak-aged AZ91 alloy due to the smaller amount of brittle DPs in the former.

• 한국지하수토양환경학회지:지하수토양환경
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• 제7권4호
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• pp.68-76
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• 2002

동래 납석광산 지역의 산성광석배수(ARD)에 의해 오염된 동래천의 공간적 화학조성 변화와 중금속의 제거 과정을 중화실험을 통하여 조사하였다. 광산에 인접한 동래천은 강산성(pH 3.0~4.2)이며 많은 양의 Al. Fe, $SO_4$및 중금속(Pb, Cu, Zn, Cd)을 포함하고 있다. 동래천의 화학조성은 산성배수의 침출지에서부터 수영강과의 합류지점까지 체계적으로 변화한다. 즉, 동래천으로 유입되는 침출수는 Al과 Fe가 풍부하나 하류로 갈수록 Al이 풍부한 조성으로 변한다. 침출수(pH 2.3)는 오염되지 않은 동래천(pH 6.5)과 소규모의 지류(pH 6.2)와 혼합되면서 pH가 최대 4.2까지 증가한다. 이러한 낮은 pH 범위(<4.2)에서는 거의 모든 Fe가 침전되나 Al은 대부분 용존상태로 남게 되어 하류로 갈수록 A띠 풍부한 조성으로 변화하게 된다. 한편, Al이 풍부한 하류의 지표수는 유량이 큰 수영강(pH 6.9)과 혼합되면서 pH가 증가(5.7)하여 흰색의 Al 침전물을 형성시킨다. 침출수를 대상으로 한 중화실험 결과. pH 3.5 이하에서 Fe침전물이, pH 4~6범위에서 Al 침전물이, 그리고 pH 6.0이상에서 Mn침전물이 형성되었다. pH의 증가에 따른 이러한 Fe, Al 및 Mn의 단계적인 침전은 실제 동래천에서 관찰되는 침전물의 형성과 잘 일치하고 있다. 또한 pH 증가에 따른 Fe및 Ai침전물에 의한 중금속의 흡착순서는 Pb>Cu>Cd$\geq$Zn이었으며, 50%의 흡착을 보이는 pH의 값($pH_{50}$은 Pb 3.2, Cu 4.5, Cd 5.2 및 Zn 5.8이었다. 이와 비교하기 위해서, Al이 풍부한 하류의 지표수를 대상으로 한 중화실험 결과, $pH_{50}$의 값은 Pb 4.5, Cu 5.8, Cd 7.4 및 Zn 7.0으로서 침출수에 비해 높은 값을 보였다. 이와 같은 결과로부터, 광산배수에서의 중금속의 제거는 pH의 변화뿐 만 아니라 하천수의 Fe 및 Al의 상대적인 양에 따라 결정됨을 알 수 있다.

• 한국기후변화학회지
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• 제2권4호
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• pp.297-307
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• 2011

최근 기후변화에 따라 식품안전에 대한 위험성도 증가되고 있다. 따라서 식품안전과 기후가 어떤 관련성을 가지고 있는지 살펴보기 위하여, 1999년부터 2009년까지 수집된 식품안전 사건사고 보도자료와 60개 지점 평균한 월별 기후자료(평균기온, 최고기온, 최저기온, 강수량, 상대습도)를 사용하여 식품안전과 기후요소와의 관련성을 분석하였다. 11년간 국내 각종 언론매체로부터 조사한 729건의 식품안전사건사고 자료를 추출한 후 식품안전 사건사고 자료를 위해요인에 따라 화학적, 생물학적, 물리적 위해요인의 3가지로 구분하여, 월별 자료를 평균한 연도별 자료(Annual data)에 대하여 각각 기후요소와 식품안전 사건 사고 자료에 대한 피어슨곱적률 상관계수(Pearson product-moment correlation coefficient)를 산출하였다. 분석 결과, 식품안전 사건사고와 강수량은 음의 상관 관계(-0.48)를 보였고, 최저기온과는 양의 상관 관계(0.45)를 보였다. 강수량은 생물학적 위해인자와 물리적 위해인자에 대해 가장 높은 상관 관계를 보였으며, 기온은 화학적 위해인자와 가장 높은 상관 관계를 보였다. 특이할 점은 식품안전 사건사고 자료에는 식품을 제조, 가공하고, 소비를 결정하는 인간 행동과 관련된 인문적인 요소가 포함되어 있으므로, 가시적인 위험이 나타나는 경우, 위해를 방지하는 선택을 통해 식품안전 사건사고가 감소할 개연성이 있다는 것이다. 따라서 가시적인 현상인 강수량과는 부적인 상관 관계를 보였고, 기온의 경우에도 극단적인 경우에는 소비자의 행동요소가 포함되므로 최고기온보다는 평균기온과 최저기온에서 더 높은 관련성을 나타냈다. 즉, 식품안전 사건사고 자료는 직접적인 병원체에 대한 연구와 달리 인간의 행동과 관련된 요소가 포함되어 있는 것으로 분석되었다. 이 연구가 향후 기후변화에 따른 식품안전 영향 평가에 있어서 기본 정보를 제공할 수 있기를 기대한다.