Yu-Sik Kong

Creep Life Prediction for Udimet 720 Material Using the Initial Strain Method (ISM)

Despite of considerable research results of uniaxial tension creep available for superalloys, few studies have been made on high temperature creep using the Initial Strain Method (ISM). In this paper, the real-time prediction of high temperature creep strength and creep life for the nickel-based superalloy Udimet 720 (high-temperature and high-pressure gas turbine engine materials) was performed on round-bar type specimens under pure static load at the temperatures of 538°C, 649°C, and 704°C. The predictive equation derived from the ISM in creep tests showed better reliability than those from LMP(Larson-Miller Parameter) and LMPISM (Larson Miller Parameter-Initial Strain Method) specially for long time creep prediction (103-105h)

Weibull Statistical Analysis of Micro-Vickers Hardness using Monte-Carlo Simulation

Abstract In the present study, the Weibull statistical analysis using the Monte-Carlo simulation has been performed to investigate the micro-Vickers hardness measurement reliability considering the variability. Experimental indentation test were performed with a micro-Vickers hardness tester for as-received and quenching and tempering specimens in SCM440 steels. The distribution of micro-Vickers hardness is found to be 2-parameter Weibull distribution function. The mean values and coefficients of variation (COV) for both data set are compared with results based on Weibull statistical analysis. Finally, Monte-Carlo simulation was performed in order to evaluate the effect of sample size on the micro-Vickers hardness measurement reliability. For the parent distribution with shape parameter 30.0 and scale parameter 200.0 (COV=0.040), the number of sample data required to obtain the true Weibull parameters was founded by 20. For the parent distribution with shape parameter 10.0 and scale parameter 200.0 (COV=0.1240), the number of sample data required to obtain the true Weibull parameters was founded by 30.

Mechanical Properties of Dissimilar Friction Welded Steel Bars in Relation to Post Weld Heat Treatment

Dissimilar friction welding were produced using 15(mm) diameter solid bar in chrome molybedenum steel(KS SCM440) to carbon steel(KS S45C) to investigate their mechanical properties. 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 H.A.Z and microstructure investigations. The specimens were tested as-welded and post-weld heat treated(PWHT). The tensile strength of the friction welded steel bars was increased up to 100% of the S45C base metal under the condition of all heating time. Optimal welding conditions were n

Mechanical Properties of Friction Welded SM 45C-SF 45 Joints for Automobile Reverse Idle Gear Shaft Applications

Friction welding is a common practice to join axially symmetrical parts for automobile industry applications. The shaft for automobile reverse idle gear is generally produced by forging steel, SF 45. This method is not so good because of high cost of material and production. In this study, in order to investigate the possibility of application of SM 45C to SF 45 dissimilar friction welding, the dissimilar friction welded joints were performed using 20 mm diameter solid bar in forging steel(SF 45) to carbon steel(SM 45C). The optimal friction welding parameters were selected to ensure reliable quality welds on the basis of visual examination, tensile test, micro-Virkers hardness surveys of the bond of area and optical microstructure investigations for welded joint parts. Finally, post weld heat treatment(PWHT) of the high-frequency induction hardening was performed for the friction welded specimens under the optimal welding conditions. And then, the mechanical properties were compared for as-welded and PWHT in SM 45C to SF 45.

Application of Acoustic Emission Technique and Friction Welding for Excavator Hose Nipple

Friction welding is a very useful joining process to weld metals which have axially symmetric cross section. In this paper, the feasibility of industry application was determined by analyzing the mechanical properties of weld region for a specimen of tube-to-tube shape for excavator hose nipple with friction welding, and optimized welding variables were suggested. In order to accomplish this object, friction heating pressure and friction heating time were selected as the major process variables and the experiment was performed in three levels of each parameter. An acoustic emission(AE) technique was applied to evaluate the optimal friction welding conditions nondestructively. AE parameters of accumulative count and event were analyzed in terms of generating trend of AE signals across the full range of friction weld. The typical waveform and frequency spectrum of AE signals which is generated by friction weld were discussed. From this study the optimal welding variables could be suggested as rotating speed of 1300 rpm, friction heating pressure of 15 MPa, and friction heating time of 10 sec. AE event was a useful parameter to estimate the tensile strength of tube-to tube specimen with friction weld.

Optimization of Friction Welding for Motor Vehicle Safety Belts: Part 1-Mechanical Properties and Microstructure

*Bukyong Education Center for Vehicle Safety Component Technolo gy, Pukyong National University, Busan, KoreaKEY WORDS: Friction welding 마찰용접, Post weld heat treatment (PWHT) 용접후열처리, As-welded 마찰용접재, Base metal (BM) 모재, Heat affected zone (HAZ) 열영향부, Weld interface (WI) 용접계면, Welding condition 용접조건ABSTRACT: Dissimilar friction welds were produced using 15-mm diameter so lid bars of chrome molybdenum steel (KS SCM440) and carbon steel (KS SM20C) to investigate their mechanical properties. Th e main friction welding parameters were selected to ensure good quality welds on the basis of visual examination, tensile tests, Vickers hardnes s surveys of the bond area and HAZ, and macro-structure investi gations. The specimens were tested as-welded and post-weld heat treated (PWH T). The tensile strength of the friction welded steel bars was increased to 100％of the SM20C base metal under the condition of a heating time of more than four seconds. Optimal welding conditions were n = 2,000 (rpm), HP = 60 (MPa), UP = 100 (MPa), HT = 5 (s),and UT = 5 (s), when the total upset length was 7.8 (mm). The hardness distribution peak of the friction welded joints could be eliminated using PWHT. The two different kinds of materials were strongly mixed to show a well-combined structure of macro-particles, with no molten material, particle growth, or defects.교신저자 안석환: 부산광역시 남구 용당동 산100, 051-629-7565, shahn@pknu.ac.kr

EVALUATION OF THE JOINT-INTERFACE PROPERTIES OF SCM440-S45C STEELS JOINED BY FRICTION WELDING

The purpose of this study is to investigate experimentally the hardness distributions and micro-structural properties of the dissimilar joints using chrome molybdenum steel (SCM440) to carbon steel (S45C) parts. The experiments were carried out using a beforehand designed and constructed experimental friction welding set-up, constructed as a continuous-drive brake type. The pilot dissimilar welding experiments under different friction pressure and friction time were carried out to obtain optimum welding parameters using visual examination and tensile tests. Vickers hardness distributions and microstructures in the interfaces of the dissimilar joints for PWHT were also obtained and examined. The obtained results were compared with those of the previous study.

New Considerations on Variability of Creep Rupture Data and Life Prediction

This paper deals with the variability analysis of short term creep rupture test data based on the previous creep rupture tests and the possibility of the creep life prediction. From creep tests performed by constant uniaxial stresses at 600, 650 and 700 elevated temperature, in order to investigate the variability of short-term creep rupture data, the creep curves were analyzed for normalized creep strain divided by initial strain. There are some variability in the creep rupture data. And, the difference between general creep curves and normalized creep curves were obtained. The effects of the creep rupture time (RT) and steady state creep rate (SSCR) on the Weibull distribution parameters were investigated. There were good relation between normal Weibull parameters and normalized Weibull parameters. Finally, the predicted creep life were compared with the Monkman-Grant model.

A Study on Prediction of Stress Intensity Factor and Fatigue Crack Growth Behavior Using the X-ray Diffraction Technique

This study verified the relationship between fracture mechanics parameters(ΔK, ΔKeff/, Kmax/) and X-ray parameters (r/, B) for SG365 steel at elevated temperature up to 300. The fatigue crack propagation test were carried out and X-ray diffraction technique according to crack length direction was applied to fatigue fractured surface. The residual stress on the fracture surface was found to increase low ΔK region, reach to a maximum value at a certain value of Kmax/ or ΔK and then decrease. Residual stress were independent on stress ratio by arrangement of ΔK and half value breadth were independent by the arrangement of Kmax/. The equation of r/ - ΔK was established by the experimental data. Therefore, tincture mechanics parameters could be estimated by the measurement of X-ray parameters.

Evaluation on the Creep Life Prediction Using Initial Strain Method

The high temperature creep behavior of heat machine systems such as aircraft engines, boilers and turbines in power plants and nuclear reactor components have been considered as an important and needful fact. There are considerable research results available for the design of high temperature tube materials in power plants. However, few studies on the Initial Strain Method (ISM) capable of securing repair, maintenance, cost loss and life loss have been made. In this method, 3 long time prediction Of high temperature creep characteristics can be dramatically induced through a short time experiment. The purpose of present study is to investigate the high temperature creep lift of Udimet 720, SCM 440-STD61 and 1Cr-0.5Mo steel using the ISM. The creep test was performed at 40 to under a pure loading. In the prediction of creep life for each materials, the equation of ISM was superior of Larson-Miller Parameter(LMP). Especially, the long time prediction of creep life was identified to improve the reliability