Papers by Author: Si Yon Bae

Abstract: There exists strong environmental and economic pressure to increase the thermal efficiency of fossil fuel power stations and this has led to a steady increase in operating temperature and pressure resulting in the world wide construction plans for ultra super-critical power plants. Consequently, in order to improve the thermal efficiency of power plant, there has been a strong drive to develop more advanced heat resistant steels with excellent creep, high temperature fatigue and thermal fatigue resistant properties as well as superior oxidation and corrosion resistant properties. In this study, the test material was P122 alloy which was developed for ultra super-critical power plant. To measure the fatigue crack growth rate in low #K range, fatigue tests were performed on the P122 alloy welds by #K decreasing method at three different microstructure (Base metal, HAZ, Weld metal) regions. Microstructure observation and micro-hardness tests performed for all three regions to find the relationship among the crack growth rate, microstructure and hardness. Fatigue tests were performed with compact tension specimens at 600°C, 650°C and 700°C at the loading frequency of 20Hz.

Abstract: A nondestructive inspection is required to check for defects inside of the actually used components and structures and to confirm their generation and growth rate. Using the backward radiated ultrasonic inspection system, we performed nondestructive inspection of cracks and micro cavities resulting from creep. From the ultrasound test results, the generation and growth of cracks and micro-cavities were confirmed, and the fracture life of the components and structures could be predicted. We confirmed degraded region by crack size and cavity based on the amplitude of the backward radiated ultrasound. The size of degraded region at 600oC was about 10mm, while that at 650oC was about 15mm. The size of crack and the cavity area fraction confirmed by ultrasound were very close to the actual size and cavity area fraction, indicating the validity of the predicted creep crack growth rate and creep.

Abstract: The structures connected by welding are fractured often in Heat Affected Zone (HAZ) when cracks grow under high-temperature and high-pressure conditions. In this study, the creep crack growth tests of P92 and P122 steels were performed and the creep fracture characteristics were evaluated. Degradation rate by cavities was investigated and the influence of degradation rate on crack growth rate was evaluated. Crack growth rate of new material was compared with that of the degraded material to confirm the influence of the degradation rate by cavities on the crack growth rate. From these results, the crack growth rates in welded joints were determined. The crack growth rate of the step test specimen was much faster than that of new material under the same test conditions of step temperature, while it was slower than that of new material under the same step loading conditions. It implies that crack growth rate of the welded specimen is faster when degradation rate by cavities is high even under the same temperature and stress conditions. HAZ and weld metal have faster cavity nucleation and growth rates than base metal. Therefore, the HAZ and weld metal specimens showed shorter crack initiation time and the initial crack growth rate (da/dt)c of the HAZ and weld metal was faster than the base metal. If (da/dt)c is fast, fracture life will be reduced.

Abstract: New Ti-base alloys were designed to replace conventional expensive elements with low cost elements of iron and silicon. The prepared Ti-4wt%Fe-(0~4)wt%Si alloys were creep tested at 600oC in air, tensile tested at room temperature and 400oC in air, and exposed to hot air for oxidation tests between 700 and 1000oC. When compared to the Ti-6Al-4V alloy, Ti-4wt%Fe-(0.5~2)wt%Si alloys displayed poorer creep resistance, but exhibited good tensile properties and superior oxidation resistance.

Abstract: Metal matrix composites (MMC) that consisted of Al (-Mg) matrix reinforced with initially added BN particles were fabricated using the pressureless infiltration technique. Initially added BN particles were partly consumed to make AlN, as new reinforcing particles. The other reaction product was MgAlB2 dispersoids. The creep behavior was investigated between 225- 275oC in air. Despite of the presence of the reinforcing particles, the creep resistance was found to be unsatisfactory due to the weak Al (-Mg) matrix.

Abstract: The initiation and growth of internal defects usually cause the failure of long term operated structural components at high temperature. Therefore, by investigating the internal defects of material, the integrity of the system can be maintained and the unexpected failure can be prevented. The main purpose of this study is to investigate the possibility of the application of ultrasound to the measurement and estimation of the creep damage. In this study, the phased array ultrasound and backward ultrasound were employed to detect the internal defects caused by creep. Using the newly developed P92 steel, both creep and NDE ultrasound tests were performed. The results of defect inspection were checked by microstructure examination. Reliable information on the size and position of crack were obtained and successful estimation of the amount of micro-voids and their distribution proved to be obtainable by the ultrasound inspection technique. The width of the creep degraded zone was confirmed by the width of ultrasonic signal, and the severity of degradation was confirmed by ultrasonic amplitude.