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25Cr-13Ni 스테인리스강의 고온 크리프-피로거동에 관한 연구
송전영 ( Jeon Young Song ),안용식 ( Yong Sik Ahn ) 한국열처리공학회 2015 熱處理工學會誌 Vol.28 No.2
The low cycle fatigue (LCF) and creep-fatigue (hold time tension fatigue, HTTF) tests were performed on the modified 25Cr-13Ni cast stainless steel, which was selected as a candidate material for exhaust manifold in automotive engine. The exhaust manifold is subjected to an environment in which heating and cooling cycle occur due to the running pattern of automotive engine. Several types of fatigue behaviour such as thermal fatigue, thermal mechanical fatigue and creep-fatigue are belong to the main failure mechanisms. High temperature tensile test was firstly carried out to compare the sample with the traditional cast steel for the component. The low cycle fatigue and HTTF tests were carried out under the strain controlled condition with the total strain amplitude from ± 0.6% to ± 0.7% at 800oC. The hysteresis loops of HTTF tests showed significant stress relaxation during tension hold time. With the increase of tension hold time, the fatigue life was remarkably deceased which caused from the formation of intercrystalline crack by the creep failure mechanism. (Received February 5, 2015; Revised February 11, 2015; Accepted February 16, 2015)
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.
송전영(Jeon-Young Song),마영화(Young-Wha Ma),구지호(Ji-Ho Gu),이정섭(Jung-Sub Lee),이연수(Yeon-Soo Lee) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.11
Tensile and Low cycle fatigue behavior of wrought nickel base superalloy was investigated.This alloy was subjected to different solution heat treatment process which results in different microstructure such as grain size, delta phase distribution. Relatively alloy subjected to high temperature solid solution heat treatment (designation: A) results in large grain size and no delta phase. However, alloy subjected to low temperature solid solution heat treatment (designation: B) results in small grain size and precipitation of delta phase. Microstructure evolution and fracture mechanism of two type heat treated alloys determined by scanning electron microscopy (SEM). Usually, grain size distribution is very important factor on low cycle fatigue behavior. After two type heat treatment, alloys show different grain size distribution, which brings about different low cycle fatigue behavior. And more detailed test results will be examined and discussed.
마영화(Young-Wha Ma),이지원(Ji-Won Lee),구지호(Ji-Ho Gu),송전영(Jeon-Young Song),이연수(Yeon-Soo Lee) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.11
An experimental study was conducted on the evaluation of fatigue crack growth behavior under various temperatures ranging from room temperature to 950 °C for a polycrystalline nickel-based superalloy, which was produced by investment casting. From the results, the fatigue crack growth rate showed a tendency to increase with increasing temperature up to 800 ℃. However, the crack growth rate at 950 °C was lower than that at 800 °C. The crack growth behavior was significantly fluctuated, and the crack front showed an irregular growth amount, and it grew as a zigzag-shaped slanted crack. This is considered to be because cracks grew along the slip plane with different directions for each grain.