Fe-Mn-Al 합금의 응력부식균열 특성에 관한 연구

허성강,맹선재,백남익 대한금속재료학회(대한금속학회) 1985 대한금속·재료학회지 Vol.23 No.11

Stress corrosion cracking (SCC) testing on Fe-Mn-Al alloys has been carried out in the boiling 42% MgCl₂ solution, and the effects of phase difference or of some chemical contents on SCC sensitivity have been studied. Comparing with the austenitic 18-8 stainless steel, Fe-Mn-Al alloy of austenitic structure has shown much higher resistivity to SCC. By increasing the ferrite phase in the dual phase (γ+α) Fe-Mn-Al alloys, SCC sensitivity was increased. Under the studied condition, pitting occurred more easily at ferrite phase than at austenite phase. These pittings acted as the crack initiation sites, resulting in increased SCC sensitivity. Silicon-added austenitic Fe-Mn-Al alloy has shown increased SCC sensitivity than without it. Increased carbon content of the similar dual phase alloys, resulted in the higher SCC sensitivity.

프라즈마 소결법으로 제조된 NiCrAlY/PSZ 계 복합재의 물성에 관한 연구

허성강,권경재 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.10

Physical properties were investigated in the NiCrAlY/PSZ composites with various NiCrAlY contents(0vol%, 20vol%. 40vol%. 60vol%, 80vol%, 100vol%). Samples of the NiCrAlY/PSZ composites were sintered in the temperature range of 1450 to 1200℃ by a Plasma Activated Sintering(PAS) process. SEM micrographs revealed that grain size of the PSZ, as a matrix in NiCrAlY/PSZ composites, decreased with increasing NiCrAlY content. Hardness values also decreased with increasing NiCrAlY contents. Sintering temperature was found to decrease with increasing NiCrAlY contents in hardness and density measurements. X-ray analysis revealed that the volume fraction of the tetragonal phases in PSZ, as a matrix in NiCrAlY/PSZ composites, increased gradually with increasing NiCrAlY contents and reached the maximum in the NiCrAlY content of 40vol%, and then decreased remarkably. Optical microscopy showed that the continuous phase in the NiCrAlY/PSZ composites changed from PSZ to NiCrAlY in the NiCrAlY content of 80vol%. On the other hand, fracture toughness values linearly increased with increasing NiCrAlY contents, reached the maximum in the NiCrAlY content. of 60vol%, and then decreased remarkably in the NiCrAlY content of 80vol%.

플라즈마 용사에 의한 ZrO2 (8wt% Y2O3) 코팅층의 미세조직과 마르텐사이트 상변태에 관한 연구

허성강,나형용,이주동,홍경태 대한금속재료학회(대한금속학회) 1992 대한금속·재료학회지 Vol.30 No.12

The microstructures and effect of the residual stresses on the stress induced martensitic transformation in plasma-sprayed ZrO₂(8wt% Y₂O₃) coatings were discussed in this study. Acoustic counts and hardness decreased with an increase of compressive axial(or radial) residual stress since martensitic transformation was repressed. The critical stress for inducing phase transformation increased and was influenced greatly by the residual stress as the deposition rate decreased. This is because stress intensification decreases as well with decreasing the deposition rate. Transformable equilibrium tetragonal phase precipitated only at bundaries, which are believed to have small cooling rates due to a collision between solid/liquid interfaces, and was transformed readily by applied stress. It was revealed that the critical stress depended not on the effective stress of Von mises criterion but on the residual stress(σ_(xy) σ_y or σ_z). This implies that martensitic transformation occurs predominently at these boundaries.

알미늄 다결정에서 결정립의 결정방위가 피로변형에 미치는 영향

허성강,남수우,맹선재,천성순 대한금속재료학회(대한금속학회) 1977 대한금속·재료학회지 Vol.15 No.5

알미늄多結晶(純度 99.9%)을 두 應力 사이에서 引張疲勞시켜 多結晶內의 各 結晶粒에 形成되는 slip line을 觀察하여 이들과 各 結晶粒의 方位 즉 Schmid factor와의 關係를 調査하였다. 알미늄 多結晶의 結晶粒은 直徑 約 1.5∼3.0㎜의 크기로 성장시켰고 Schmid factor는 Laue backre flection 촬영으로 決定한 各 結晶粒의 方位에서 計算하였다. 試片은 引張應力 189∼47g/㎟ 또는 198∼32g/㎟ 사이에서 70cpm의 낮은 周期로 反復疾勞시켰으며 10², 10³및 10⁴cyc1e 후에 slip line의 생성 및 分布상태를 光學顯微鏡으로 觀察하였다. Schmid factor 큰 結晶粒일 수록 slip band의 形成에 必要한 疲勞 cycle의 수가 적었으며 Schmid factor가 작은 結晶粒(Schmid factor: 0.34 및 0.37)에서는 10³또는 10⁴cycle의 試驗 후에도 slip band가 觀察되지 않았다. The relation between formation and distribution of slip bands in the grains of aluminum polycrystals (purity: 99.9%) and crystal orientation of each grain have been studied after tensile fatigue stressing of the samples. Grains of the samples were grown to the size of 1.5∼3.0㎜ diameter by heat treatment. Crystal orientation and Schmid factor of each grain were determined by means of Laue back reflection. Cyclic tensile stresses were applied to the samples between 189 and 47g/㎟ or 198 and 32g/㎟ at a low frequency of 70 cpm, and the formation and distribution of slip bands were observed by optical microscope after the cyclic stressing of 10², 10³, and 10⁴ cycles. As the value of Schmid factor of a grain was increased, the number of fatigue cycles, in which slip bands are formed, was decreased, Not any slip bands were found in the grains of low Schmid factor values, such as 0.34 and 0.37, even after 10⁴ cycles.

Mod.9Cr1Mo강에서 발생되는 일시적 취성현상

허성강,구지호,신기삼,신종호,Hur, Sung-Kang,Gu, Ji-Ho,Shin, Kee-Sam,He, Yincheng,Shin, Jong-Ho 한국재료학회 2011 한국재료학회지 Vol.21 No.11

It is well known that modified 9Cr-1Mo steel has a low thermal expansion and high thermal conductivity with excellent high temperature properties compared to austenitic stainless steel. For these advantages, the steel is very popular for the boiler tube of thermal power plants. Normalizing is commonly utilized to obtain martensite in this steel, which shows an unusual toughness for martensite. However, some accidents related to this steel have been reported recently, opening the necessity for further study. As a particular behavior of the steel, an abrupt drop of the impact value has been identified upon tempering at 750$^{\circ}C$ for about 1 hour. It is well known that $Fe_3C$ forms during autotempering and turns to $Cr_2C$ at an early stage and then transforms to $Cr_{23}C_6$. In this study, the cause of the abrupt drop of the impact value was investigated with an impact test, microstructural observation, nanodiffraction and phase analyses using instruments such as optical and transmission electron microscopes (TEM) with an extraction carbon replica of the carbides. The analyses revealed that the $M_2C$ that formed when retained for about 1 hour at 750$^{\circ}C$ causes a drastic decrease in the mechanical properties. The sharp drop in mechanical properties, however, disappeared as the $M_2C$ transformed into $M_{23}C_6$ with longer retention.

Mod.9Cr-1Mo강의 항온변태시 기계적 특성변화에 미치는 Cr탄화물의 영향

허성강,이재현,구지호,신기삼,신종호,Hur, Sung-Kang,Lee, Jae-Hyun,Gu, Ji-Ho,Shin, Kee-Sam,He, Yinsheng,Shin, Jong-Ho 한국재료학회 2012 한국재료학회지 Vol.22 No.1

In this study, mechanical tests and microstructural analyses including TEM analyses with EDX of precipitates in modified 9Cr-1Mo steel were carried out to determine the cause of embrittlement observed after heat-treatment, which limits the usage of the alloy for power plants. Mod. 9Cr-1Mo steel specimens at austenite temperature were quenched to the molten salt baths at $760^{\circ}C$ and $700^{\circ}C$, in which the specimens were kept for 10 min ~ 10 hr with subsequent air-cooling. Impact tests showed that the impact value dropped abruptly when the specimens were kept longer than 30 min at $\sim760^{\circ}C$ reaching to minima in about 1 hr, and then increasing at further retention. The tensile strength of the specimens reached the minimum value without much change afterward, whereas the values of elongation showed the same trend as that of the impact value. The isothermally heat-treated steel at $700^{\circ}C$ also showed a minimum impact value in about 1 hr. These results suggest that the isothermal heattreatment at 760 and $700^{\circ}C$ for about 1 hr induces temporal embrittlement in Mod. 9Cr-1Mo steel. The microstructural examination of all the specimens with extraction replica of the carbides revealed that the specimens with temporal embrittlement had $Cr_2C$, indicating that the cause of the embrittlement was the precipitation of the $Cr_2C$. In addition, TEM/EDX results showed that the Fe/Cr ratio was 0.033 to 0.055 for $Cr_2C$, whereas it was 0.48 to 0.75 for $Cr_{23}C_6$, making the distinction of the $Cr_2C$ and $Cr_{23}C_6$ possible even without direct electron diffraction analyses.

파괴손상해석법과 해석례

허성강 대한금속재료학회 1990 대한금속재료학회 학술대회 개요집 Vol.1990 No.1

STS 304 스테인리스강의 플라즈마 페이스트 침붕처리에 관한 연구

李聖道,李贊揆,孫明淑,尹在弘,許聖康 國立 昌原大學校 産業技術硏究所 1997 産技硏論文集 Vol.11 No.-

Various surface properties of boride layer formed by plasma paste boronizing were examined. A boride layer of flat structure was formed on the STS 304 stainless steel substrate by plasma paste boronizing treatment with several different temperature from 1023K to 1223K under gas mixture of Ar/H₂: 2/1(vol.%). The paste was composed of 30wt% boron and 70wt% borax. The properties of boride layer were evaluated by micro vickers hardness tester, SEM, EPMA, and corrosion tester. Metallographic cross section of the borided specimen was consisted of the boride layer, Ni rich layer, Cr rich layer, and matrix structure. It was found to form boride layer in a short time to conventional boronizing methode. In XRD analysis of boride surface, various peaks such as FeB, Ni₃B and CrB were presented. Especially, the peak of CrB was more intensive than other peaks. Activation energy for the formation of boride layer was found to be 123kJ/mol which is lower than the conventional boronizing treatment. The microhardness of these boride layers were in the range of HV 1800∼2000. However, from the results of anodic polarization curve in 1M H₂SO₄solution on the borided specimens, it is found that the corrosion resistance is improved by boronizing treatment. Primary current density is, however, increased with the Boronizing temperature.

급속응고된 Al-Fe-(Mo, Si) 계 합금분말 압출재의 미세조직과 기계적 특성

허성강,나형용,백남익 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.8

A study has been made on the microstructure and mechanical properties of rapidly solidified Al-Fe-(Mo,Si) alloys. Examination of the extruded bar using TEM reveals the microstructure to be inhomogeneous along with the bimodal distribution of precipitates. It is believed that. such inhomogeneous microstructure is caused by the structural difference between zone A and zone B in the starting powder. The tensile testing shows an increase in tensile strength but a decrease in elogation as the content of iron in the alloy increases. It has been found that decrease in elongation can be prevented by the addition of Si to Al-8Fe-2Mo alloy. For the elevated temperature mechanical properties, all extruded specimens with 8% iron show tensile strength of 180-190㎫ at 573K, but their elongation values were decresed with incresing the test temperatures, reaching a minimum at around 423 K.

석유화학공업에서의 Coking 및 침탄현상

허성강 대한금속재료학회 1991 대한금속재료학회 학술대회 개요집 Vol.1991 No.1