• Journal of the Korean Society for Precision Engineering
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• v.15 no.7
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• pp.129-138
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• 1998

The objective of this study is to demonstrate the ability of a computer simulation of microstructural evolution in hot forging of C-Mn steels. The development of microstructure is strongly dependent on process variables and metallurgical factors that affect time history of thermodynamical variables such as temperature, strain. and strain rate during deformation. Then finite element method is applied for the prediction of microstructural evolution, and it should be coupled with heat transfer analysis to consider the change of thermodynamical properties during forming process. In this study, Yada's recrystallization model and rigid-thermoviscoplastic finite element method are employed in order to analyze microstructural evolution during hot forging process. To show the validity and effectiveness of the proposed method, experiments are accomplished and the results of experiments are compared with those of simulations.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2012.05a
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• pp.825-826
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• 2012

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2010.05a
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• pp.915-916
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• 2010

• Proceedings of the Materials Research Society of Korea Conference
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• 1998.08a
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• pp.113.4-113
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• 1998

• Journal of the Korean Society for Heat Treatment
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• v.14 no.2
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• pp.110-117
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• 2001

Artificial aging was performed to simulate the microstructural degradation in 2.25CrMo steel arising from long time exposure at $540^{\circ}C$. It was found that the carbides became coarser and spheroidized as aging time increased. An attempt was made to evaluate the microstructural degradation in artificially aging heat treated 2.25CrMo steel by the ultrasonic attenuation and velocity measurements. Ultrasonic velocity was found essentially insensitive to the microstructural changes resulting from aging heat treatment. However, the ultrasonic attenuation was observed to increase with increasing aging time. Also, it was noticed that the change of ultrasonic attenuation with aging time was more sensitive at high frequency regions.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.10b
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• pp.55-58
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• 2003

Many researches have been already done on the issues of high temperature deformation and the microstructural evolution. The information has been very useful for the plasticity industry, especially successful for the extrusion. However, the parts made with forging usually have a complex shape. It is difficult to control the distribution of the variables like strain, strain rate and temperature rise due to the working heat during a hot-forging process. Consequently, the microstructural variation could be occurred depending on the plastic deformation history that the forged part would get during a hot forging. In the present study, the microstructural characteristic of a hot-forged 6061 aluminum alloy has been discussed on the aspect of grain size evolution. A forging of 6061 aluminum alloy has been carried out for a complex shape with a dimensional variation. Also, finite element analysis has been done to understand how the deformation variables such as strain, strain rate give an influence on the microstructure of a hot forged aluminum product.

• Korean Journal of Materials Research
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• v.17 no.8
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• pp.425-432
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• 2007

The microstructural evolution of ferritic 9Cr-1Mo-V-Nb steel, subjected to creep-fatigue at $550^{\circ}C$, was evaluated nondestructively by measuring the ultrasonic velocity. The variation of the ultrasonic velocity with the fatigue life fraction exhibited three regions. In the first region ($N/N_f$<0.2), a significant increase in the velocity was observed, followed by a slight increase between the fatigue life fractions of $0.2N_f$ and $0.8N_f$, and then a decrease in the final region. The change of the ultrasonic velocity during creep-fatigue was interpreted in relation to the microstructural properties. This study proposes an ultrasonic nondestructive evaluation method of quantifying the level of damage and microstructural change during the creep-fatigue of ferritic 9Cr-1Mo-V-Nb steel.

• Bulletin of the Korean Chemical Society
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• v.33 no.1
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• pp.194-198
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• 2012

In this work, we studied the microstructural changes of ITO during the annealing process. ITO nanoparticles were prepared by the sol-gel method using indium tin hydroxide as the precursor. The prepared sample was investigated using TEM, powder XRD, XPS, DRIFT, and 2D correlation analysis. The O 1s XPS spectra suggested that the microstructural changes during the annealing process are closely correlated with the oxygen sites of the ITO nanoparticles. The temperature-dependent in situ DRIFT spectra suggested that In-OH in the terminal sites is firstly decomposed and, then, Sn-O-Sn is produced in the ITO nanoparticles during the thermal annealing process. Based on the 2D correlation analysis, we deduced the following sequence of events: 1483 (due to In-OH bending mode) ${\rightarrow}$ 2268, 2164 (due to In-OH stretching mode) ${\rightarrow}$ 1546 (due to overtones of Sn-O-Sn modes) ${\rightarrow}$ 1412 (due to overtones of Sn-O-Sn modes) $cm^{-1}$.

• Korean Journal of Materials Research
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• v.32 no.2
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• pp.72-79
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• 2022

A cold roll-bonding process using AA1050, AA5052 and AA6061 alloy sheets is performed without lubrication. The roll-bonded specimen is a multi-layer complex aluminum alloy sheet in which the AA1050, AA5052 and AA6061 sheets are alternately stacked. The microstructural evolution with the increase of annealing temperature for the roll-bonded aluminum sheet is investigated in detail. The roll-bonded aluminum sheet shows a typical deformation structure in which the grains are elongated in the rolling direction over all regions. However, microstructural evolution of the annealed specimen is different depending on the type of material, resulting in a heterogeneous microstructure in the thickness direction of the layered aluminum sheet. Complete recrystallization occurs at 250 ℃ in the AA5052 region, which is lower by 100K than that of the AA1050 region. Variation of the misorientation angle distribution and texture development with increase of annealing temperature also differ depending on the type of material. Differences of microstructural evolution between aluminum alloys with increase of annealing temperature can be mainly explained in terms of amounts of impurities and initial grain size.

• Journal of the Korean Society for Heat Treatment
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• v.36 no.4
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• pp.215-222
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• 2023

3Y-TZP (3 mol% yttria-stabilized tetragonal zirconia polycrystals, 3Y-TZP) ceramics are emerging as dental implant materials due to their superior optical and mechanical properties as well as excellent biophysical properties, in spite of low bioactivity. In this study, we investigated to sintered properties and microstructural defects of dental zirconia implants fabricated by ceramic injection molding and post-HIP (Hot isostatic pressing) processing and analyzed the processing parameters related with the obtainment of its high sinterd density. Sintered and microstructural parameters, i.e, apparent density, grain size and phase composition of zirconia implants fabricated by injection molding were dependent on the fixtute size and implant type. Maximum sintered density of 99.2% and minimum grain size of 0.3-0.4 ㎛ were obtained from large-scaled 2-body sample. In 1-body ceramic implant, high sintered density of 99.5% was obtained, but it had a little monoclinic phase and wide grain size distribution.