• Journal of the Korean Society for Heat Treatment
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• v.27 no.2
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• pp.79-89
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• 2014

The effect of carbon on the microstructure and texture of low carbon steels was investigated in a series of 1.6 Mn-0.3Cr-0.2Mo-0.001B steels with carbon ranging from 0.021 to 0.048%. Intensity of {111} orientation increased with decreasing the carbon content, resulting in the increase in $r_m$ value. The highest $r_m$ value of 1.30 was obtained in 0.021%C steel annealed at $820{\sim}850^{\circ}C$ according to the typical galvannealing heat cycle. Martensite volume fraction was not substantially affected by the annealing temperature. It was found that the fine and uniformly distributed martensite particles which were present in amounts of about 5% volume fraction were desirable for the highest $r_m$ value. The other factor affecting the high $r_m$ value was the preferred epitaxial growth of retained ferrite with {111} orientation into austenite during cooling.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2005.06a
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• pp.1812-1818
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• 2005

Even though SMA actuators have high power to volume ratio, there exist disadvantages such as hysteresis and saturation. So the model identification for SMA actuators is very difficult. For the qualitative model identification, we described the behavior of SMA actuators using a so-called diagonal model, which can readily expect the turning point of an incomplete phase transformation. For the quantitative model identification, we developed the general dynamics of SMA actuators using the modified Liang's model. Using this dynamics we can describe the hysteresis and the saturation very well. It is also very important to notice that the modified Liang's model maintains a continuous martensite fraction at the change of the phase transformation but the original model cannot.

• Archives of Metallurgy and Materials
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• v.67 no.4
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• pp.1491-1495
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• 2022

The present research deals with the effect of pre-strain on the hydrogen embrittlement behavior of intercritically annealed medium-Mn steels. A slow strain-rate tensile test was conducted after hydrogen charging by an electrochemical permeation method. Based on EBSD and XRD analysis results, the microstructure was composed of martensite and retained austenite of which fraction increased with an increase in the intercritical annealing temperature. The tensile test results showed that the steel with a higher fraction of retained austenite had relatively high hydrogen embrittlement resistance because the retained austenite acts as an irreversible hydrogen trap site. As the amount of pre-strain was increased, the hydrogen embrittlement resistance decreased notably due to an increase in the dislocation density and strain-induced martensite transformation.

• Korean Journal of Materials Research
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• v.29 no.4
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• pp.211-220
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• 2019

In this study, three kinds of bainitic steel plates are manufactured by varying the chemical compositions and their microstructures are analyzed. Tensile and Charpy impact tests are performed at room and low temperature to investigate the correlation between microstructure and mechanical properties. In addition, heat affected zone (HAZ) specimens are fabricated by a simulation of welding processes, and the HAZ microstructure is analyzed. The base steel that has the lowest carbon equivalent has the highest volume fraction of acicular ferrite and the lowest volume fraction of secondary phases, so the strength is the lowest and the elongation is the highest. The Mo steel has a higher volume fraction of granular bainite and more secondary phases than the base steel, so the strength is high and the elongation is low. The CrNi steel has the highest volume fraction of the secondary phases, so the strength is the highest and elongation is the lowest. The tensile properties of the steels, namely, strength and elongation, have a linear correlation with the volume fraction of secondary phases. The Mo steel has the lowest Charpy impact energy at $-80^{\circ}C$ because of coarse granular bainite. In the Base-HAZ and Mo-HAZ specimens, the hardness increases as the volume fraction of martensite-austenite constituents increases. In the CrNi-HAZ specimen, however, hardness increases as the volume fraction of martensite and bainitic ferrite increases.

• Journal of Power System Engineering
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• v.11 no.4
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• pp.86-91
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• 2007

This study was investigated to know the effect of subzero treatment in austempered ductile cast iron. Retained austenite transformed to martensite by subzero treatment. With decreasing subzero treatment temperature, more volume fraction of retained austenite transformed to martensite and transformed to martensite above 30% by subzero treatment temperature at $-196^{\circ}C$. With decreasing subzero treatment temperature, the value of strength and hardness increased but the value of elongation and impact value decreased. In case of subzero treatment at $-196^{\circ}C$, hardness value increased about 18% and impact value decreased by above 20%. We could find that subzero treated specimens had a little of effect on the tensile properties but had very much effect on the hardness and value of the impact.

• Journal of Powder Materials
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• v.3 no.3
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• pp.174-180
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• 1996

Microstructure and phase transformation of mechanically alloyed TiNi powders added to aluminium matrix for enhancing the damping properties were studied. Four compositions between 48.5 and 51.5 at% Ti intermetallic compounds were selected to control the fraction of martensite phase. Mechanically alloyed TiNi powders were heat-treated at vacuum of $10^{-6}$ torr for crystallization. Ball milled AI/TiNi composite powders were swaged at room temperature and rolled at 450 $^{\circ}C$. After mechanical alloying for 10 hours, Ti and Ni elements were alloyed completely and amorphous phase was formed. Amorphous phase was crystallized to martensite (Bl9') and austenite(B2) after heat treating for 1 hour at the temperature of 850 $^{\circ}C$, and TiNi$_3$, intermetallic compound was partially formed. Considerable amount of martensite phase was remained after swaging and rolling.

• Journal of Powder Materials
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• v.27 no.5
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• pp.414-419
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• 2020

In the present study, we investigated the austenite stability of a sintered Fe-based nanocrystalline alloy. The volume fraction of austenite was measured based on the X-ray diffraction data of sintered Fe-based nanocrystalline alloys, which were prepared by high-energy ball milling and spark plasma sintering. The sintered alloy samples showed a higher volume fraction of austenite at room temperature as compared to the equilibrium volume fraction of austenite obtained using thermodynamic calculations, which resulted from the nanosized crystalline structure of the sintered alloy. It was proved that the austenite stability of the sintered Fe-based alloy increased with a rise in the amount of austenite stabilizing elements such as Mn, Ni, and C; however, it increased more effectively with a decrease in the actual grain size. Furthermore, we proposed a new equation to predict the martensite starting temperature for sintered Fe-based alloys.

• Korean Journal of Materials Research
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• v.25 no.11
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• pp.607-611
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• 2015

The microstructural evolution of Grade 91 tempered martensite ferritic steels heat treated at $760{\sim}1000^{\circ}C$ for two hours was investigated using scanning electron microscopy(SEM), energy disperse spectroscopy(EDS), electron backscattered diffraction (EBSD), and transmission electron microscopy(TEM); a microhardness tester was also employed, with a focus on the grain and precipitate evolution process as well as on the main hardening element. It was found that an evolution of tempered martensite to ferrite($760{\sim}850^{\circ}C$), and to fresh martensite($900{\sim}1000^{\circ}C$), occurred with the increase of temperature. Simultaneously, the parabolic evolution characteristics of the low angle grain boundary(LAGB) increased with the increase of the heating temperature(highest fraction of LAGB at $925^{\circ}C$), indicating grain recovery upon intercritical heating. The main precipitate, $M_{23}C_6$, was found to be coarsened slightly at $760{\sim}850^{\circ}C$; it then dissolved at $850{\sim}1000^{\circ}C$. Besides this, $M_3C$ cementite was formed at $900{\sim}1000^{\circ}C$. Finally, the experimental results show that the hardness of the steel depended largely on the matrix structure, rather than on the precipitates, with the fresh martensite showing the highest hardness value.

• Journal of the Korean Society for Heat Treatment
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• v.10 no.3
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• pp.209-217
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• 1997

Effect of Co content on the microstructure and damping capacity of Fe-23%Mn-X%Co alloy was studied. The volume fraction of ${\varepsilon}$ martensite of the alloy was increased with increasing Co content. The hardness was increased with lowering cooling temperature and increasing Co content in Fe-23%Mn-X%Co alloy, which is ascribed to the increase in ${\varepsilon}$ martensite content. The damping capacity of Fe-23%Mn-X%Co alloy was linearly increased with increasing the strain amplitude, and was constant regardless of Co content at the same volume fractions of ${\varepsilon}$ martensite when the low strain amplitudes ($1{\sim}3{\times}10^{-4}$) were applied, while the damping capacity with large strain amplitudes ($4{\sim}6{\times}10^{-4}$) became higher with increasing Co content at all valume fractions of ${\varepsilon}$.

• Korean Journal of Materials Research
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• v.25 no.1
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• pp.9-15
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• 2015

The effect of retained and reversed austenite on the damping capacity in high manganese stainless steel with two phases of martensite and austenite was studied. The two phase structure of martensite and retained austenite was obtained by deformation for various degrees of deformation, and a two phase structure of martensite and reverse austenite was obtained by reverse annealing treatment for various temperatures after 70 % cold rolling. With the increase in the degree of deformation, the retained austenite and damping capacity rapidly decreased, with an increase in the reverse annealing temperature, the reversed austenite and damping capacity rapidly increased. With the volume fraction of the retained and reverse austenite, the damping capacity increased rapidly. At same volume of retained and reversed austenite, the damping capacity of the reversed austenite was higher than the retained austenite. Thus, the damping capacity was affected greatly by the reversed austenite.