• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2001.05a
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• pp.131-135
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• 2001

Evaluation of microstructural changes during open die forging of heavy ingots is important for process control. The objective of the control of forging parameters, such as shape of the dies, reduction, temperature and sequence of passes, is to maximize the forging effects md to minimize inhomogeneities of mechanical properties. The hot working die steel is produced by using the multistage open die forging. The structure is altered during forging by subsequent processes of plastic deformation, recrystallization and grain growth. A numerical analysis using an rigid visco-plastic finite element model was performed to predict microstructural evolution of hot working die steel.

• Journal of Welding and Joining
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• v.22 no.2
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• pp.78-84
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• 2004

Effects of manganese and carbon on the HAZ microstructural evolution in 500㎫ grade titanium oxide steels were investigated. Microstructural evolution primarily depends on supercooling. When cooled at 3$^{\circ}C$/s in 0.15%C-1.5%Mn steel, grain boundary and Widmanst tten ferrite formed at 640 and 62$0^{\circ}C$, respectively, followed by competitive formation of acicular ferrite and upper bainite inside of grain at 58$0^{\circ}C$. With an increase of manganese, degree of supercooling increased while critical cooling rate for the formation of gain boundary ferrite decreased. Consequently, the amount of acicular ferrite in HAZ was decreased in 2.0%Mn after initial increase in 1.0 and 1.5%Mn. Therefore, optimum supercooling should be maintained to accelerate acicular ferrite formation in titanium oxide steels. Low carbon steel, 0.11%C-1.5%Mn, showed larger amount of acicular ferrite than higher carbon steel because of effectiveness of diffusionless transformation in low carbon steel.

• Journal of Powder Materials
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• v.6 no.1
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• pp.81-87
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• 1999

Based on the pore filling theory, the microstructure evolution during liquid-phase sintering has been analyzed in terms of interrelationship between average grain size and relative density. For constant liquid volume fraction, the microsturucture trajectories reduced to a single curve in a grain size(x)-density(y) map, regardless of grain growth constant. The slope of curves in the map was inversely proportional to average pore size, while it increased fapidly with liquid volume fraction. Increase in pore volume fraction retarded the densification considerably, but showed marginal effect on the slope. The activation energy of densification was predicted to be the same as that of grain growth as long as the liquid volume fraction is constant for any temperature range studied. The present analyses on microstricture evolution may demonstrate the usefulness of pore filling theory and provide a guideline for process optimization of liquid-phase sintering.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.05a
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• pp.408-410
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• 2008

The evolution of texture and microstructure during recrystallization was tracked after different cold rolling of aluminum sheets. Texture of the sheet center were differentiated by different strain states due to prior deformation. The evolution of recrystallization texture was studied with the amount of shear applied during cold rolling. The final grain size after recrystallization annealing was varied due to the effective strain during deformation.

• Applied Microscopy
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• v.45 no.1
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• pp.9-15
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• 2015

In the present study, microstructural evolution in CuCrFeNi, CuCrFeNiMn, and CuCrFeNiMnAl alloys has been investigated. The as-cast CuCrFeNi alloy consists of a single fcc phase with the lattice parameter of 0.358 nm, while the as-cast CuCrFeNiMn alloy consists of (bcc+fcc1+fcc2) phases with lattice parameters of 0.287 nm, 0.366 nm, and 0.361 nm. The heat treatment of the cast CuCrFeNiMn alloy results in the different type of microstructure depending on the heat treatment temperature. At $900^{\circ}C$ a new thermodynamically stable phase appears instead of the bcc solid solution phase, while at $1,000^{\circ}C$, the heat treated microstructure is almost same as that in the as-cast state. The addition of Al in CuCrFeNiMn alloy changes the constituent phases from (fcc1+fcc2+bcc) to (bcc1+bcc2).

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2001.11a
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• pp.46-46
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• 2001

The microstructure of the 1020 mild steel target in the region ahead of craters, made by colliding against Cu and W-Cu shaped-charge jets. has been investigated in the present work. The region ahead of the crater impacted by the Cu shaped-charge jet reveals grain refinement implying the formation of sub-grains, while that of W-Cu one leads to martensitic transformation indicating that the region was heated up to an austenitic region which was followed by rapid cooling. The pressure of W-Cu shaped-charge jet impacting against the target when calculated is higher than that of Cu one. The microhardness of the region ahead of the crater impacted by the W-Cu shaped-charge jet is also higher than that of the Cu one. The microstructure of W-Cu slug that remains inside of the crater depicts the occurrence of the remarkable elongation of W particles during the liner collapse. The microstructural evolution of the region ahead of the crater is discussed on the basis of the pressure dependency of the ferrite/austenite transformation in the steel.

• Journal of Korea Foundry Society
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• v.34 no.1
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• pp.1-5
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• 2014

The austempering transformation behavior in Fe-0.7wt.%C-2.3wt.%Si-0.3wt.%Mn steel is investigated. Each specimen was austenitized for 60 min at $900^{\circ}C$, and austempered at $380^{\circ}C$ for different time periods varying from 2 min to 256 min. After the austempering heat treatment, the Stage I and II evolutions are performed using optical metallography, X-ray diffraction and image analyses. Variations in the X-ray diffraction patterns and lattice parameters of the ferrite and austenite demonstrate that the residual austenite decomposes into ferrite and carbide during the Stage II evolution; moreover the amount of ferrite increases during the Stage I evolution. While the amount of austenite increases during Stage I, it dicreases during Stage II. Overall, the variations in the volume fractions of the microstructure and carbide formation in stages I and II meet high temperature austempering reaction of the ausferrite microstructure.

• 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.

• Transactions of Materials Processing
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• v.15 no.6 s.87
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• pp.441-444
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• 2006

The deformation behavior of copper during equal channel angular pressing(ECAP) was calculated using a three-dimensional version of a constitutive model based on the dislocation density evolution. Finite element simulations of the variation of the dislocation density and the dislocation cell size with the number of ECAP passes are reported. The calculated stress, strain and cell size are compared with the experimental data for Cu deformed by ECAP in a modified Route C regime. The results of FEM analysis were found to be in good agreement with the experiments. After a rapid initial decrease down to about 200nm in the first ECAP pass, the average cell size was found to change little with further passes. Similarly, the strength increased steeply after the first pass, but tended to saturate with further pressings. The FEM simulations also showed strain non-uniformities and the dependence of the resulting strength on the location within the workpiece.

• The Korean Journal of Ceramics
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• v.5 no.3
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• pp.260-264
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• 1999

Effect of the particle size of $MgTiO_3$ and $CaTiO_3$ on the microstructural evolution during sintering of $0.93MgTiO_3-0.07CaTiO_3$ system was investigated. Microwave dielectric characteristics of the sintered ceramics were also measured. The microstructural evolutions were explained with an emphasis on the entrapping behavior of $CaTiO_3$ grain into the $MgTiO_3$ grain and were correlated with microwave dielectric characteristics. With an increasing particle size ratio between $CaTiO_3$and $MgTiO_3$, the fraction of entraped $CaTiO_3$ grains increased, which grain growth of $MgTiO_3$were concurrently accelerated due to decreasing drag force of its boundary migration. Besides, $CaTiO_3$-grain entrapment into the $MgTiO_3$grain interior led to decreaseing quality factor values.