Papers by Author: Yu Sik Kong

Abstract: The long-term corrosion resistances for the carbon steels have been investigated under high temperature pressurized water atmosphere, in the conjunction with the analysis of nondestructive properties by the ultrasonic wave. The corrosion test for carbon steels was carried out at the temperature of 200 °C under a water pressure of 10 MPa. The corrosion test cycles for carbon steels were changed up to 65 weeks. The mechanical properties of carbon steel suffered from the corrosion cycle were investigated by a tensile test, attaching an acoustic emission sensor on the test sample. The tensile strength of carbon steels greatly decreased beyond the corrosion cycle of 35 weeks, accompanying the increase of weight loss by the creation of corrosion damages. The attenuation coefficient of carbon steels by the ultrasonic wave increased with the increase of corrosion cycles.

Abstract: The effect of post weld heat treatment on mechanical properties of friction welded Alloy 718 and SNCRW was investigated. Friction welding tests were carried out at a constant rotation speed and pressure. Optimum friction condition was found to be the friction pressure of 25kg/cm2, friction time of 40sec, upset pressure of 80 kg/cm2, and dwell time of 5sec. After friction welding tests, post weld heat treatments were performed in the temperature range of 500-900°C for 8hrs in order to investigate the microstructure and mechanical properties of weld joint. Specimens with the post weld heat treatment at 720°C for 8hrs show optimal mechanical properties. Residual stress of post weld heat treated specimens was measured to weld joint in the same temperature range. After friction welding tests on samples with a diameter of 80mm, tensile properties of post-weld-heat-treated and non-heat-treated samples were compared.

Abstract: The purpose of this study is to investigate the high temperature creep life of Waspaloy using the Initial Strain Parameter Technique (ISPT). The creep tests were performed at the elevated temperatures from 550oC to 700 oC. Constant stress creep tests were carried out in the experiment. The initial strain was measured for one minute after loading. The creep life of Waspaloy was calculated using the creep life prediction equation of ISPT. The confidence level between the experimental rupture time and the calculated rupture time using the ISPT is within 95%. So, the results show that the creep life prediction by the ISPT was a good agreement with LMP method.

Abstract: In this paper, an analysis of fatigue crack growth (FCG) behavior from a statistical point of view has been carried out. Fatigue crack growth tests were conducted on sixteen pre-cracked compact tension (CT) specimens of the pressure vessel (SPV50) steel in controlled identical load and environmental conditions. The assessment of the statistical distribution of fatigue crack growth experimental data obtained from SPV50 steel was studied and also the correlation of the parameter C and m in the Paris-Erdogan law was discussed. The probability distribution function of FCG life and FCG rate seems to follow the 3-parameter Weibull.

Abstract: In this study, the creep rupture tests of STS304 stainless steels were investigated at three different elevated temperatures of 600
, 650
and 700
under the constant creep stresses. Creep rupture characteristics such as creep stress, creep rupture time, steady state creep rate and so on were evaluated. The behaviors of creep rate curve and initial strain are compared at three different elevated temperatures. The stress exponent (n) at 600
, 650
and 700
based on steady state creep rate showed 22.5, 20.6 and 11.4 respectively. By increasing the temperature, the stress exponent is decreased. At the temperature of 700
, the lowest stress exponents are shown and this behavior is also observed in the case of stress exponent based on rupture time. The creep life prediction by LMP method is presented and the equation of this result is as follows: T(logtr+20)=-0.005152-14.56+24126.

Abstract: This paper deals with the statistical properties of short time creep rupture characteristic values (for example, creep rupture time, steady state creep rate, total creep rate, initial strain, etc.) in STS304 stainless steels. From short time creep rupture tests performed by constant stresses at three different elevated temperatures 600, 650 and 700
, the scatter and probability distributions were investigated for rupture time, total creep rate, steady state creep rate, initial strain, and others. The effect of temperature on the statistical scatter of rupture time was the smallest at 700
. The effect of stress on the statistical scatter of rupture time was smaller with increasing stresses. The probability distributions of short time creep rupture data were well followed 2-parameter Weibull.

Abstract: An experimental work of dissimilar friction welding was conducted using 15 mm diameter solid bar in copper alloy (Cu-1Cr-0.5Zr) to stainless steel (STS316L) for being used as fusion reactor component materials, not only to optimize the friction welding parameters, but also to investigate the elevated temperature tensile strength and creep rupture properties for the friction welded joints under the optimal welding conditions. The main friction welding parameters were selected to endure good quality welds on the basis of visual examination, tensile tests, Vickers hardness survey of the bond area and HAZ. For friction weld joining of copper alloy to stainless steel bars, the total upset increases lineally as increasing heating time. Optimal welding conditions were selected as follows: Rotational speed 2000rpm, friction pressure 80MPa, upsetting pressure 140MPa, heating time 2 second, upsetting time 5 second and total upset 13mm. The weld interface of dissimilar friction welded steel bars was mixed strongly. And also the creep properties and creep life prediction by Larson-Miller parameter method were presented at the elevated temperatures of 300, 400 and 500oC.

Abstract: Constant
K fatigue crack growth tests were performed by applying an intermediate multiple overloads for S45C steel. The purpose of this study is to investigate effects of specimen thickness at various baseline stress intensity factor range levels (
Kb), the application position of the overload (a/W) and the application frequency of the overload (OLHz) on fatigue crack growth retardation behavior. The principal results are summarized as follows. The number of retardation cycles for the constant baseline stress intensity factor level (
Kb ) decreases with increasing specimen thickness. The normalized number of retardation cycles ( Nd / Nc ) decreases with increasing specimen thickness. But, at
Kb = 45 MPa(m)1/2, the cycle increases with increasing specimen thickness.

Abstract: An experimental study of dissimilar friction welding was conducted using 15mm diameter solid bar in chrome molybdenum steel (SCM440) to carbon steel (S45C) not only to optimize the friction welding conditions, but also to investigate the fatigue performance. The main friction welding parameters were selected to endure good quality welds on the basis of visual examination, tensile tests, Vickers hardness surveys of the bond of area and HAZ, and microstructure investigations. In this study, the specimens were tested as welded. For fatigue strength, the notched specimens for the optimal conditions were rotary bending fatigue tested. The results were compared with S-N curves for the base metals.

Abstract: The evaluation of specimen thickness effect of fatigue crack growth life by the simulation of probabilistic fatigue crack growth is presented. In this paper, the material resistance to fatigue crack growth is treated as a spatial stochastic process, which varies randomly on the crack surface. Using the previous experimental data, the non-Gaussian (eventually Weibull, in this report) random fields simulation method is applied. This method is useful to estimate the probability distribution of fatigue crack growth life and the variability due to specimen thickness by simulating material resistance to fatigue crack growth along a crack path.