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CMSX-10 단결정 초내열합금에서 일방향 응고 공정변수에 따른 응고조직 변화도
구지호 ( Ji Ho Gu ),정형민 ( Hyung Min Jung ),이정석 ( Jeong Seok Lee ),윤혜영 ( Hye Young Yoon ),서성문 ( Seong Moon Seo ),조창용 ( Chang Yong Jo ),이재현 ( Je Hyun Lee ) 대한금속재료학회(구 대한금속학회) 2013 대한금속·재료학회지 Vol.51 No.3
Directional solidification experiments were carried out on a single crystal Ni base superalloy CMSX-10 over a wide range of growth velocity and temperature gradient conditions. Under the imposed temperature gradients, the primary dendrite arm spacings decreased proportionally with the growth velocity. Microstructural characteristics at the solid/liquid interface were found to depend strongly on the growth velocities and temperature gradients. Based on the experimental observations, a growth velocity versus temperature gradient map was selected for planar to cellular and cellular to dendritic transitions during solidification. As a result, the microstructure selection map for the alloy was established by comparing it to theoretical models. Good agreement was obtained from the results for planarcellular and cellular-dendritic transitions. The columnar-equiaxed transition, however, was calculated using Hunt`s model for castings. The results are discussed in terms of the combined effects of constitutional undercooling and nucleation undercooling ahead of the growing dendrite tips.
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.
박경미 ( Kyung Mi Park ),구지호 ( Ji Ho Gu ),서성문 ( Seong Moon Seo ),정희원 ( Hee Won Jeong ),신종호 ( Jong Ho Shin ),심권보 ( Kwon Bo Sim ),이재현 ( Je Hyun Lee ) 대한금속재료학회(구 대한금속학회) 2013 대한금속·재료학회지 Vol.51 No.3
Directional solidification experiments were carried out on modified 9Cr1Mo steel and 10Cr1MoW steel to investigate their solidification and segregation characteristics. Under a given temperature gradient, both steels were solidified at the same growth rates of 1, 5, 50 and 100 μm/s, respectively. A detailed study of the solidification sequence for both steels revealed that delta ferrite was the leading phase and gamma austenite nucleated from the solid solidified earlier. A quick phase transformation of delta into gamma was made at growth rates faster than 50 μm/s. Therefore, almost all delta dendrites were identified as gamma phases to be transformed. On the other hand, compositional analyses performed on the solid/liquid interface confirmed that alloying elements in both steels were hardly segregated ahead of the solid/liquid interface. This appears to be responsible for a very narrow freezing range which suppresses elemental segregation.
니켈기 단결정 초내열합금의 γ’-Rafting 메커니즘 조사
송전영(Jeon-Young Song),구지호(Ji-Ho Gu),이연수(Yeon-Soo Lee),마영화(Young-Wha Ma) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.06
니켈기 단결정 초내열합금은 고온에서의 크리프, 피로, 열-기계적 피로와 같은 우수한 기계적 특성 과 안정성으로 항공 및 발전용 가스터빈 엔진의 블레이드 및 베인 소재로 사용되고 있다 <sup>1)</sup>. 이러한 니켈기 단결정 초내열합금의 우수한 기계적 특성은 γ 기지상에 cuboidal 형태로 분포한 γ' 석출물에 의한 것이며, 이러한 석출물의 역온도의존성으로 특정온도까지 우수한 기계적 특성을 나타낸다 <sup>2)</sup>. 니켈기 단결정 초내열합금의 강화상인 γ' 석출물은 고온 크리프 환경에서 방향성 조대화(γ'-rafting)와 같은 미세조직의 변화를 수반하며, 이러한 석출물의 변화는 니켈기 단결정 초내열합금의 크리프 특성에 영향을 미치는 것으로 알려져 있다 <sup>3)</sup>. 본 연구는 니켈기 단결정 초내열합금의 크리프 특성과, 크리프 환경에서 수반되는 γ' 석출물의 형상변화를 관찰하여, 크리프 변형거동 및 잔여수명을 예측하는 모델을 개발하는데 사용하고자 한다. 다양한 온도에서의 크리프 시험을 통해 γ' 석출물의 방향성 조대화는 온도 및 응력조건에 영향을 받는 것을 알 수 있었을 뿐만 아니라, 이러한 γ' 석출물의 형상변화가 크리프 특성에 미치는 영향을 확인할 수 있었다. Nickel-based single crystal superalloys have been used for high temperature components such as blade and vane in aerospace and industries gas turbine engine because of their outstanding high temperature mechanical properties such as creep, fatigue and thermo-mechanical fatigue [1]. Their excellent high temperature mechanical properties were derived from unique microstructure consisted of disordered face-centered-cubic(fcc) gamma matrix and L1<sub>2</sub> ordered-Ni<sub>3</sub>(Al, Ti) gamma prime precipitate. Turbine blade and vane were generally exposed to complex thermal and mechanical creep load. These severe environment leads to morphological evolution of gamma prime precipitate which can significantly affect the creep behavior of nickel-based single crystal superalloy. This study focuses on morphological evolution of gamma prime precipitate and their effect on creep property in first generation nickel-based single crystal superalloy.