Authors: Nam Yong Kim, Jeoung Han Kim, Yu Sik Kong, Jong Won Yoon, Jong Taek Yeom, Dong Geun Lee, Nho Kwang Park
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.
511
Authors: Jong Taek Yeom, Eun Jeoung Jung, Jeoung Han Kim, Dong Geun Lee, Nho Kwang Park, Seung Sik Choi, Chong Soo Lee
Abstract: The process design of profile ring rolling for a large-scale turbine diaphragm was made
using the calculation method and three dimensional finite element method (FEM). The design criteria
are to achieve uniform distributions of strain and temperature, and defect-free profiled ring products.
Based on the compression test results of the low alloy steel, deformation processing map was
generated using the combination of the dynamic materials model (DMM) and stable or unstable
forming criteria. The processing map was used to determine the optimum ring rolling temperature and
feed rate of the mandrel. FEM analysis was simulated to predict the formation of rolling defects and
deformed shape in the profile ring rolled diaphragm product. Finally, optimum process design to
obtain a sound large-scale turbine diaphragm without forming defects was suggested and its
validation was made by the comparison between the experimental data and FE analysis results.
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Authors: Dong Geun Lee, Yang Gon Kim, Byoung Chul Hwang, Sung Hak Lee, Nack Kim
Abstract: Dynamic deformation and fracture behavior of Zr-based bulk metallic glass (BMG) and
BMG composite containing dendritic β phases was investigated in this study. Dynamic compressive
test results indicated that both maximum compressive stress and total strain of the BMG and BMG
composite decreased with increasing test temperature because shear bands could propagate rapidly
as the adiabatic heating effect was added at high temperatures. Above the glass transition
temperature, total strain decreased more abruptly due to crystallization of amorphous phases.
Maximum compressive stress and total strain of the BMG composite were higher than those of the
BMG because β phases played a role in forming multiple shear bands. The BMG composite having
more excellent dynamic properties than the BMG can be more reliably applied to the structures or
parts requiring dynamic properties.
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Authors: Jong Taek Yeom, Chong Soo Lee, Jeoung Han Kim, Dong Geun Lee, Nho Kwang Park
Abstract: A methodology for evaluating and predicting component lives in creep-fatigue interaction
region was investigated for Waspaloy. A unified viscoplasticity constitutive equation including
multi-back stresses was used to describe cyclic material behaviors. Also, a continuum damage model
coupling with the creep-fatigue damage rules was established based on the analysis of creep and low
cycle fatigue behavior. Multi-axial fatigue and creep equivalent stress concepts were employed to
predict three dimensional component lives. Notched cyclic tests under various stress conditions in the
creep-fatigue interaction region were carried out to validate the life prediction methodology with
FEM simulation based on the continuum damage model. The comparison of experimental data and
prediction results indicates that the continuum damage model is a powerful approach for the
prediction of component lives.
235
Authors: Dong Geun Lee, Yont Tai Lee, Jong Taek Yeon, Jeoung Han Kim, Nho Kwang Park, Sung Hak Lee
Abstract: Dynamic compressive tests were conducted on Zr-based amorphous alloys and
amorphous matrix composite containing dendritic β phases. Dynamic compressive tests were
conducted using a compressive Kolsky bar and then the test data were analyzed in relation to
microstructure and fracture mode. Under dynamic loading, the maximum shear stress and ductility
of the amorphous alloy and composite were considerably lower than those under quasi-static
loading because of the decreased resistance to fracture. Deformation under dynamic loading
lowered strain and compressive strength because of reduced fracture resistance, and the alloy
containing dendritic β phases showed better compressive strength and ductility than the monolithic
alloy.
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