• Korean Journal of Materials Research
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• v.9 no.7
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• pp.740-747
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• 1999

Grain refinement is the only strengthening mechanism that improves both strength and toughness. Controlled rolling and accelerated cooling techniques have been known to be effective method to improve the mechanical properties by controlling the recrystallization and/or grain coarsening during processing. Repeat phase transformation $(\gamma/\alpha)$ by repeat heat treating is another way of grain refinement. In this study, a combined effect of controlled rolling and repeat heat treating was investigated. To study the effects of Mo addition and process parameters, Mo alloyed low carbon steels were prepared and thermomechanical controlled processes were simulated in the Gleeble system. The Mo addition resulted in an increasement of the grain coarsening temperature and suppress austenite recrystallization. The optimum condition for the refinement of austenite was obtained when the controlled rolling was performed twice with the same heat treatment condition, and reduction ratio of second pass was higher than that of first pass.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.21 no.6
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• pp.444-452
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• 2009

Textural parameters were calculated from the surface sediments collected from Jinu-do beach in Nov. 2005 and May 2006. In Nov. 2005 and May, 2006, the grain size distribution of surface sediments shows that the mode of $3.0\phi$(i.e., 0.125 mm) dominated the west beach of Jinu-do, but in the east beach the mode of $3.0\phi$(i.e., 0.125 mm) decreased and the mode of $2.5\phi$(i.e., 0.177 mm) was apparently distinct, resulting in the more coarsening trend. Mean grain size of surface sediments also indicates little difference in the west beach between two investigations, but the increasing difference between mean grain sizes in the east beach, showing more coarsening pattern. Such seasonal pattern corresponds to the positive skewness in the east beach in May, 2006. As a result, after the winter in 2005, the surface sediments in the west beach of Jinu-do experienced a little variation, whereas the apparent coarsening of surface sediments occurred in the east beach by removal of $3.0\phi$(i.e., 0.125 mm) fine-grained sand particles. The observed seasonal change may be attributed to the different hydrographic condition and sediment delivery/removal on the surface sediments between the west beach and the east beach of Jinu-do through the increased precipitation and more freshwater discharge from the Nakdong River during the summer and the intensified wave and tide during the winter in the Nakdong River estuary.

• Journal of the Korean Society for Heat Treatment
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• v.37 no.4
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• pp.163-171
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• 2024

SCR420H steel is a low-carbon chromium alloy steel designed for carburizing heat treatment. Recently, research is being conducted on high-temperature carburization heat treatment to reduce costs and CO₂ emissions by shortening the carburization time to meet the international carbon neutral policy. However, this high-temperature carburization heat treatment coarsens the steel grains and causes a decrease in mechanical properties. In this study, a large amount of Ti was added to increase the grain refinement effect in the high-temperature carburizing process. We investigated the microstructure and precipitates of SCR420H steel without Ti (Al steel) and with Ti (AlTi steel). Thermodynamic calculations showed that the AlN and (Ti,Nb)(C,N) precipitated in Al steel, while (Ti,Nb)(C,N) and Ti₄C₂S₂ precipitated in AlTi steel. Addition of Ti increases the fraction of bainite after reheating process. Transmission electron microscopy analysis shows that small amounts of AlN and (Ti,Nb)(C,N) precipitates are formed in the Al steel. The addition of Ti increases the density of (Ti,Nb)(C,N) precipitates and induces the formation of Ti₄C₂S₂ precipitates, increasing the grain coarsening temperature (GCT) under all heat treatment conditions. Higher reheating temperatures also resulted in higher GCT values due to increased precipitation.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09a
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• pp.557-558
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• 2006

In all conventional sintered PM products, the pores present are of two types, primary and secondary. Primary pores forming during compaction and latter during sintering, due to penetration of formed liquid through the matrix grain boundary. Effect of carbon addition on diffusion of Cu in SH737-2Cu system was investigated. After compaction and transient liquid phase sintering at $1120^{\circ}C$ and $1180^{\circ}C$, samples were characterized for densification, showing rise in sintering density and reduction in swelling on carbon addition. Quantitative microstructural characterization (shape factor and pore size) revealed bimodal distribution for 0% carbon, more rounded pores for 0.9% carbon and higher sintering temperature, and pore coarsening at higher sintering temperature.

• Journal of Powder Materials
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• v.5 no.2
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• pp.98-110
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• 1998

$Ti_{52}Al_{48}$ and $(Ti_{52}Al_{48})_{100-x}B_x(x=0.5, 2, 5)$ alloys have been Produced by mechanical milling in an attritor mill using prealloyed powders. Microstructure of binary $Ti_{52}Al_{48}$ powders consists of grains of hexagonal phase whose structure is very close to $Ti_2Al$. $(Ti_{52}Al_{48})_{95}B_5$ powders contains TiB2 in addition to matrix grains of hexagonal phase. The grain sizes in the as-milled powders of both alloys are nanocrystalline. The mechanically alloyed powders were consolidated by vacuum hot pressing (VHP) at 100$0^{\circ}C$ for 2 hours, resulting in a material which is fully dense. Microstructure of consolidated binary alloy consists of $\gamma$-TiAl phase with dispersions of $Ti_2AlN$ and $A1_2O_3$ phases located along the grain boundaries. Binary alloy shows a significant coarsening in grain and dispersoid sizes. On the other hand, microstructure of B containing alloy consists of $\gamma$-TiAl grains with fine dispersions of $TiB_2$ within the grains and shows the minimal coarsening during annealing. The vacuum hot pressed billets were subjected to various heat treatments, and the mechanical properties were measured by compression testing at room temperature. Mechanically alloyed materials show much better combinations of strength and fracture strain compared with the ingot-cast TiAl, indicating the effectiveness of mechanical alloying in improving the mechanical properties.

• Journal of the Korean Society for Heat Treatment
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• v.22 no.2
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• pp.75-79
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• 2009

Precipitate formations and grain size variations in various Mg-Sn-Zn alloys have been investigated and their effects on the tensile properties and sheet metal formability were evaluated. MgSn and MgZn precipitates were observed in the alloy sheets, however any clear difference in morphology or size for the precipitates could not be found even though MgSn precipitates tend to be larger than MgZn. The highest formability in terms of conical cup value was found in the Mg-4 wt%Sn-2 wt%Zn where the high tensile elongation and the reduced grain coarsening at elevated temperatures were observed.

• Proceedings of the KWS Conference
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• 2002.10a
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• pp.499-504
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• 2002

The microstructural change associated with the hardness profile in friction stir welded, age-hardenable 6005 Al alloy had been evaluated. Frictional heat and plastic flow during friction stir welding created the fine recrystallized grain (Stir Zone, SZ), the elongated and recovered grain (Thermo-Mechanical Affected Zone, TMAZ) in the weld zone. Heat affected zone (HAZ), which could be only identified by hardness test because there is no difference in the grain structure compared with that of the base metal, was formed beside the weld zone. A softened region had been formed near the weld zone during friction stir welding process. The softened region was characterized by the dissolution and coarsening of the strengthening precipitate during the friction stir welding. The sound joints of 6005 Al alloys were successfully formed under a wide range of the friction stir welding conditions.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09b
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• pp.725-726
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• 2006

Ultra-fine grained and dispersion-strengthened titanium materials (Ti-Si, Ti-C, Ti-Si-C) have been produced by high energy ball milling and spark plasma sintering (SPS). Silicon or/and carbon were milled together with the titanium powder to form nanometer-sized and homogeneously distributed titanium silicides or/and carbides as dispersoids, that should prevent grain coarsening during the SPS compaction and contribute to strengthening of the material. The microstructures and the mechanical properties showed that strength, hardness and wear resistance of the sintered materials have been significantly improved by the mechanisms of grain refinement and dispersion strengthening. The use of an organic fluid as carrier of the dispersoid forming elements caused a significant increase in ductility.

• Journal of the Korean Society for Heat Treatment
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• v.19 no.4
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• pp.208-218
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• 2006

The effects of the grain size on the magnetic properties in extra-low carbon steel after strain annealing were investigated. Two kinds of sample were prepared. One is the annealed sheet, which was annealed at $670^{\circ}C$ and $850^{\circ}C$ for various time periods after cold rolling. The other is the strain annealed sheet, which was temper rolled by 0.4% and subsequently strain annealed at the temperature ranging between $670^{\circ}C$ and $850^{\circ}C$ for various time periods. The grains after strain annealing became more coarse than those after primary annealing. The grains were coarsened due to the strain induced grain boundary migration (SIGM). It was found that the permeability tended to be increased and coercivity tended to be decreased with the increase of grain size. The optimum magnetic properties was achieved after strain annealing at $850^{\circ}C$ for 30 minites. Under this condition, the coercivity was measured to be 0.6 and the permeability was measured up to be 13000.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1999.08a
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• pp.163-171
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• 1999

The abnormal grain coarsening in wire rolling induces detrimental defects, such as jagged size tolorance, severe bending after heat treatments and drawing troubles, in the following secondary processes. Neishi et al observed that there is a band type region where grain coarsenting occurs in the plastic strain vs. deformation temperature plot. Based on the finding, we have investigate whether grain sizes and ferrite volume fractions are correlation to deformation strain with three kinds of wire rod diameters as for the different average deformation conditions. The samples were chosen from the No.2 Wire Rod Mill of POSCO where 3-roll type of finishing mill stand are used. It was found in the present work that the grain size and ferrite volume fraction of the rolled and cooled microstructure were changed with rolling reduction and rolling temperature. Abnormally grown grains at various observed points were also found. To have homogeneously fine grains of microstructure from the No. 2 Wire Rod Mill, it will be easier to control finish rolling temperature at around 750$^{\circ}C$ rather than to find another rolling schedule.