Papers by Author: Keun Bong Yoo

Abstract: Effects of welding and long-term service on the microstructural evolution of superheater tubes of TP347H stainless steel used in power plants were investigated by optical microscope (OM), scanning electron microscope (SEM), electron back-scattered diffraction (EBSD), and transmission electron microscope (TEM). Analyses after welding or long-term service, showed fine NbCs in grains, which will precipitaion strengthen the matrix. When TP347 was long-term serviced in power plants, M₂₃C₆ formed preferentially on the grain boundaries and on twin boundaries, which was attributed to the embrittlement and the intergranular corrosion and fracture. The steam side had less recrystallization rate and more oxide compared to the fire side, which is part of the reason for the cracking from steam side to the fire side. And HAZ is more brittle than the matrix, because of α-Fe phase and coarse grains, due to which, cracks tend to initiate in the steam side of HAZ and propagate to the fire side.

Abstract: The gas turbine components operated in hot combustion gas undergo material degradation due to the thermal cycles. The failure mechanism of the hot gas components would be accompanied by material degradation in the properties of high temperature strength and creep rupture time. Many stationary parts in gas turbine are made of Co-based superalloy because of their high temperature performance. In this work, we survey the time and temperature dependent degradation of Co-based superalloy. We prepared the specimens of Co-based superalloy ECY768 that are exposed at 871 and 982 oC in 1000 ~ 5000 hours. We carried out the mechanical test and microstructural observation.

Abstract: Gas turbine components operated by hot combustion gas undergo material degradation due to the thermal cycle by daily startup and shutdown. The failure mechanism of the hot gas components is accompanied by degradation in the properties of high temperature strength and creep rupture time. Many hot gas components in gas turbines are made of Ni-based superalloy because of their high temperature performance. In this work, we survey the time and temperature dependent degradation of Ni-based superalloy. We prepared specimens from Inconel738LC that were then exposed at 871~982°C in 1,000~5,000hours. We carried out stress-rupture tests and microstructural investigation.

Abstract: Many investigations about superalloys and coatings have been done in the laboratory, but evaluating the degradation condition of hot section components during service is still important not only for repair and reuse but also for outage prevention. Time dependent degradation for 1st stage blades of gas turbine was investigated. The degradation analysis for used blades was divided into microstructure changes by position in the blade and mechanical tests. In the microstructure analysis, the rafting and coarsening of γ', MC decomposition and TCP(Topologically closed packed) phase formation occurred and progressed with increasing service time, and especially at the leading and trailing edge of top layer which should be a check points for used blades. Stress-rupture test results of 25,000 and 52,000 EOH(Equivalent Operating Hours) for used blades were also compared with serviced time and position in each blade.

Abstract: High strength nickel-base super alloys have been used in turbine blades for many years because of their superior performance at high temperature. The prediction of fatigue life for superalloys is important for improving the efficiency. In this study, low cycle fatigue tests are performed the variables of total strain range, and room and elevated temperature. The relations between plastic and total strain energy densities and number of cycles to failure are examined in order to predict the low cycle fatigue life of IN738LC super alloy. The fatigue life is evaluated by the Coffin-Manson equation, also the predicted lives by plastic and total strain energy density are compared with experimental results.

Abstract: The rotating components in the hot sections of land-based gas turbine are exposed to severe environment during several ten thousand hours at above 1100 oC operating temperature. The failure mechanism of the hot gas components would be accompanied by material degradation in the properties of high temperature and creep rupture strength. Many hot gas components in gas turbine are made of Ni-based superalloy because of their high temperature performance. In this work, we surveyed the time and temperature dependent degradation of Ni-based superalloy. We prepared the specimens from GTD111 that are exposed at 871 oC and 982 oC in 1,000 ~ 10,000 hours. We carried out the mechanical test and microstructural observation.

Abstract: Many investigations about superalloys and coatings have been done in the laboratory, but evaluating the degradation condition of hot section components during service is still important not only for repair and reuse but also for outage prevention. Time dependent degradation for second stage blades of gas turbine was investigated. The degradation analysis for used blades was divided into microstructure changes by position of the blade and mechanical tests of high temperature tensile test. In the microstructure analysis, the rafting and coarsening of γ', MC decomposition and TCP phase formation occurred and progressed with increasing service time, and especially the leading and trailing edge of top layer should be a check points for used blade. High temperature tensile results of 25,000 and 52,000 hrs used blades were also compared with serviced time and position in each blade.

Abstract: The objective of this study is to estimate the feasibility of advanced nondestructive method(X-ray diffraction, AE(acoustic emission) and BN(barkhausen noise)measurement application for fatigue life evaluation of the high-temperature pipeline steel such as main steam pipe etc. in power plant. In this study, various nondestructive tests using various types of specimen simulated low cycle fatigue damage were performed in order to analyze fatigue properties when fatigue damage conditions become various stages such as 1/4, 1/2 and 3/4 of fatigue life, respectively. As a result of X-ray diffraction tests for specimens simulated fatigue damages, we conformed that the variation of the full width at half maximum intensity decreased in proportion to the increase of fatigue life ratio. And also, AE and BN signal due to fatigue damage has linear relationship with fatigue life ratio algebraically. From this relationship, it was suggested that direct expectation of the life consumption rate was feasible.

Abstract: Welding residual stress has important influence on the performance of engineering components. In this paper, the residual stress and FWHM were measured by X-ray diffraction method to investigate characteristics for P92 steel welds. The aim of the study is to estimate the residual stress and FWHM distribution characteristics. A compressive residual stress distributed as a function of depth has a different pattern in welds and base metal. A large tensile residual stress occurs welds and near the HAZ, but approaches gradually zero as away from the welding center. Residual stress and FWHM undergo rapid relaxation after PWHT. Also, FWHM is a scalar quantity without any directional difference and is proportional to hardness on the whole.

Abstract: The objective of this study is to examine the feasibility of the X-ray diffraction method for the fatigue life assessment of high-temperature steel pipes used for main steam pipelines, re-heater pipelines and headers etc. in power plants. In this study, X-ray diffraction tests were performed on the specimens simulated for low cycle fatigue damage, in order to estimate fatigue properties at the various stages of fatigue life. As a result of X-ray diffraction tests, it was confirmed that the full width at the half maximum (FWHM) decreased with an increase in the fatigue life ratio, and that the FWHM and the residual stress due to fatigue damage were algebraically linearly related to the fatigue life ratio. From this relationship, a direct assessment of the remaining fatigue life was feasible.