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Young Wha Ma,Kyong Woon Lee,Byeong Ook Kong,Hyun Uk Hong,Yeon Soo Lee 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.11
A study for evaluating the weld joint strength reduction factor (WSRF) due to creep was conducted for alloy 740H-P92 dissimilarmetal weld joint to be used advanced ultra-supercritical fossil power plants. To this end, a series of the creep rupturetests were carried out for P92 and alloy 740H parent metals and alloy 740H-P92 dissimilar metal weld joint at elevatedtemperatures ranging from 600 to 700 °C. Additionally, for the creep tested specimens, hardness measurement and microstructureobservation were performed. Weldment creep rupture test used specimens of transverse cross-weld type. In the creeptest result, weldment creep rupture strength was almost same as that of the P92 parent metal when analyzed as a function ofLarson-Miller parameter (LMP). However, when analyzed as a function of creep rupture time for individual temperatures,it showed a tendency to be lower than that of the P92 parent metal after certain times. The difference between two creepstrengths increased with increasing time. Based on the analysis result for creep rupture time at each temperature, the WSRFfor 100,000 h creep life was determined as 0.84 at 600 °C, 0.72 at 650 °C and 0.58 at 700 °C through extrapolation. Additionally,from the results of investigation on hardness and microstructure, it was shown that the creep life of alloy 740H-P92dissimilar metal weld joint was dominantly determined by the characteristics of the heat affected zone (HAZ) on P92 side.
Microstructure Evaluation During Short Term Creep of Cr35Ni45Nb Cast Alloy Reformer Tube
Young Wha Ma,Gimo Yang,Kee Bong Yoon,Thi Giang Le 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.12
In this work, a Cr35Ni45Nb alloy tube for use in a reformer furnace is subjected to high temperature (871 °C and 927 °C)creep regimes, and the microstructural changes (e.g. carbide precipitation and phase transformation) and consequent creepdamage are investigated. The results of creep tests under closely similar applied stresses give different values of the Larson–Miller parameter (LMP) at different temperatures. As a result, the calculated creep rupture time at 871 °C based on theLMP value measured at 927 °C is approximately twice that which was actually measured at 871 °C. The microstructure ofthe as-cast tube is found to consist of an austenitic matrix with networks of MC (NbC) and eutectic (M23C6) carbides locatedboth at grain boundaries and between dendrites. It is noted that the M23C6carbide is not a primary eutectic carbide that canbe observed in the as-cast Cr35Ni45Nb alloys. It can be argued that the primary eutectic M7C3carbides were transformedinto M23C6due to the heat of spiral welding during the tube manufacturing process. After creep, the NbC carbides at bothlocations were mostly transformed to the G-phase (Nb3Ni2Si) and all the precipitates formed inside the austenitic matrixwere composed of the M23C6and G-phase. Creep cavities were initiated around the G-phase and grew cracks along the grainboundaries due to the formation of a Cr-depleted zone and the G-phase.
등통로각압축법으로 성형가공된 알루미늄 2024 합금의 이방성 강도 평가
마영화(Young Wha Ma),윤기봉(Kee Bong Yoon) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.5
To assess anisotropic tensile strength of material produced by ECAP, small punch (SP) testing method was adopted as a substitute for the standard uniaxial tensile test because ECAP processing sample is usual1y limited to approximately 10 ㎜ length in the transverse direction perpendicular to the direction of ECAP extrusion. SP tests were performed with specimens taken out along both the longitudinal and transverse directions of over-aged aluminum 2024 alloy subjected to three ECAP passes at 150℃. Failure surfaces of SP specimens tested showed that failure was caused by shear deformation and the ECAP processed material had strength anisotropy. Based on the aspect of the failure surface, a method of assessing tensile strength was proposed.
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.
횡방향으로 등방성인 크리프 재료에서 고온파괴매개변수 C<SUB>t</SUB>
마영화(Young Wha Ma),윤기봉(Kee Bong Yoon) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.11
High temperature crack growth life assessment procedures need to be extended for anisotropic creeping materials such as directionally solidified Ni-based superalloys because they have been developed for isotropic materials. In this study, the previously proposed estimation equation of C<SUB>t</SUB> for the transversely isotropic secondary-creeping materials was applied for the creep crack growth rate data of a directionally solidified Ni-based superalloy which is one of the transversely isotropic materials. Effectiveness of the proposed equation was shown and its limitation was discussed.