Papers by Author: Hyung Ick Kim

Abstract: The Ni-based superalloy Inconel 718 (IN-718) was developed in the 1950s and is currently used for several critical gas turbine components due to its good balance of mechanical properties. Because of its stable mechanical properties at high temperature, the alloy is used for turbine bolts. The mechanical properties of in-service facilities are required to maintain safety operation in power plants. Ultrasonic tests, tensile tests and hardness tests were performed to evaluate the degree of the material degradation of Inconel 718. The mechanical properties decreased as degraded, but the longitudinal velocity of the ultrasonic signal increased. Also, the microstructure of the degraded Inconel 718 was to study the relationship between the result from ultrasonic test and the results from destructive methods.

Abstract: The advancement in superalloys permits the hot gas path components to operate for thousands of hours under severe centrifugal, thermal and vibratory stresses. The blade of a gas turbine must withstand the most severe condition combined of temperature, stress, and environment. After a long operation, the damaged blades of a gas turbine used are welded for build-up and repaired. We analyzed and compared the mechanical properties of GMAW(Gas Metal Arc Welding), a manual welding method, a laser cladding method, and an automatic welding method under research and development.

Abstract: In this study, the plastic η -factors of the SA508Cl.1a narrow-gap welding part, which is used for the primary piping system in a nuclear power plant were obtained by using finite element analysis and the modified fracture toughness testing method was suggested for the narrow-gap welding part. Also, we have performed the fracture toughness test for the SA508Cl.1a narrow-gap welding part by applying the new testing method and then we compared the results with those from the ASTM fracture toughness test.

Abstract: The mechanical properties of in-service facilities under harsh environment a decrease as materials of the facilities degrade. This decrease of mechanical properties can affect the safety operation of the facilities. Therefore, the extent of degradation due to prolonged service exposure must be estimated. Nondestructive evaluation method is a good technique for monitoring the change of mechanical properties of in-service facilities. The most widely used nondestructive methods are the ultrasonic method and the indentation test, which is advantageous with respect of applicability to in-service facilities. The modified theoretical Vary's equation, considering nonlinear response due to material degradation, was proposed for obtaining the correlations between ultrasonic parameters and fracture toughness. Experimental results showed that ultrasonic attenuation, velocity, and nonlinear parameters have significant correlation with fracture toughness and yield strength. The nondestructive evaluation system can be used to obtain the yield strength and ultrasonic parameters simultaneously, and this information can be used to predict the fracture toughness. The predicted results produced good correlations with the experimental results, indicating that the nondestructive evaluation system can be effective in evaluating material properties and degradation, and the life time of facilities.

Abstract: Used pipes in various mechanisms and structures are produced from raw material by extruding and drawing. The properties such as yield strength, tensile strength, and elongation of a pipe produced by these methods are different from the properties of their raw material. But designers use the properties of the raw material because the actual properties of the pipes are difficult to obtain from testing. Also, the pipe is used after it has been bent in a complex manner and cut to fit it to mechanisms and structures. The bending process, especially, induces deformation of the pipe’s section and residual stress, which are involved in the plastic deformation of the bended pipes. This residual stress affects the pipe’s properties, including its fatigue life. Therefore, it is very important to understand the residual stress of a material. But, the distribution of residual stress of a U-shaped pipe, which is examined in this study, is very complicated and cannot be measured exactly.

Abstract: The Ni-based superalloy GTD-111DS has been widely used as the material of the first stage blade of gas turbine. But there are little data available on the microstructure and mechanical property. The strength and ductility of GTD-111DS decrease more rapidly due to the acceleration of its microstructure degradation under high temperature. The results of study showed that the shape of γ' did not change with increased aging time but the amount and volume fraction of the deposition of secondary γ' increased and secondary γ' grew among primary γ'. Also there was difference of yield strength and tensile strength in room temperature according to heat treatment and collecting region.

Abstract: A burst rupture test has been mainly used for evaluating the burst properties of internal pressurized tubes. Although the burst creep test provides accurate results, its complicated and material-wasting process makes it difficult to perform this test. In the burst test, it is known that the hoop stress is a main reason of burst, so it can be expected that the burst rupture properties are strongly related with the hoop creep rupture properties. A ring test is occasionally used for obtaining the hoop directional properties of tube-shaped structures. In this study, the creep rupture properties of Zirlo tubing are investigated at temperature ranging from 365 to 570
using the ring specimens. An analysis of the fractography was performed and the estimation of service life with Larson-Miller parameter was conducted. Finally, In view of the Dorn equation of power-law, the creep mechanism was determined and discussed.

Abstract: The safety and reliability of industrial machineries and structures used in various, and severe conditions has become an increasing concern. This study proposes ductile-brittle transition temperature (DBTT) evaluation technique by the sub-sized specimen test to prevent cleavage fracture. Four classes of the thermally aged 1Cr-1Mo-0.25V specimens were prepared. The fracture toughness tests were performed on sub-sized specimens both at room and low temperatures. The results of the fracture toughness tests were analyzed to obtain the DBT behaviours of the specimen materials

Abstract: In this study, we performed the static test of nuclear piping materials by the unloading compliance method and the normalization data reduction technique and obtained two fracture resistance curves (J-R curves). The two curves were similar, which proves that the normalization data reduction technique can be adopted in the static test. Then we performed the dynamic fracture toughness test for welding part of nuclear piping. The J-R curves were obtained from the dynamic test by the normalization data reduction technique and were compared to those of the static test results.