• Journal of Industrial Technology
• /
• v.30 no.B
• /
• pp.17-23
• /
• 2010

Since the propagation of a short fatigue crack is directly related to the large crack which causes the fracture of bulk specimen, the detailed study on the propagation of the short crack is essential to prevent the fatigue fracture. However, a number of recent studies have demonstrated that the short crack can grow at a low applied stress level which are predicted from the threshold condition of large crack. In present study, the threshold condition for the propagation of short fatigue crack is examined with respect to the micro-structure and cyclic loading history. Specimens employed in this study were decarburized eutectoid steels which have various decarburized ferrite volume fraction. Rotating bending fatigue test was carried out on these specimens with the special emphasis on the "critical non-propagating crack length" It is found that the reduction of the endurance limit of their particular micro-structures can be due to the increase of the length of critical non-propagating crack, and the quantitative relationship between the threshold stress ${\sigma}_{wo}$ and the critical non-propagating crack length $L_c$ can be written as ${\sigma}_{wo}{^m}{\cdot}L_c=C$ where m,C is constant. Further experiments were carried out on cyclic loading history on the length of critical non-propagating crack. It shown that the length of critical non-propagating crack is closely related to cyclic loading history.

• 보존과학연구
• /
• s.13
• /
• pp.37-58
• /
• 1992

Iron artifacts from Busǒ Sansǒng inffered to late Baikjae periodwere studied on the aspects of metallugy. These materials were the largest size ever since excavated. From the analytical results these artifacts were found to be pureiron system without impurities or hypo-eutectoid steel system in below 0.3% in carbon contents. From the content of phosphorus in the range of 0.03∼0.05% as aim purity it was shown that charcoal were used for making these iron artifacts from sponge iron not fusion method. By observing metallugical structure it was found that iron artifacts was manufactured by repetitive folding and hammering forging method and some by heating method for adding carbon with cool water. This method were to improve the quality of the soften steel to harden one. In addition to those above repetitive hammering method eliminated the nonferrous materials such as slag inclusion and remained relatively pure ferrite.

• Journal of the Korean institute of surface engineering
• /
• v.37 no.4
• /
• pp.234-241
• /
• 2004

Cavitation erosion properties of the Cu-9Al-4.5Ni-4.5Fe alloys (Al-bronze) surface-modified by flame quenching process have been investigated. After flame quenching at above $T_{\beta}$, the surfaces of Al-bronze with $\alpha$ + $\textsc{k}$ structure have been changed into the $\alpha$ + $\beta$' martensite phases by the eutectoid reaction of $\alpha$ + $\textsc{k}$\longrightarrow$\beta$ followed by the martensite transformation of $\beta$\longrightarrow$\beta$'. As a result of cavitation test, the measured incubation time and erosion rate of the $\alpha$ + $\beta$' alloy was 1.2 times higher and 1.5 times lower, respectively, compared to those of the conventional $\alpha$ + $\textsc{k}$ alloys, showing a remarkable increase of cavitation resistance with the formation of $\beta$' martensite. This is attributed to a preferential erosion of the $\textsc{k}$ precipitates that show the lowest resistance among the $\alpha$, $\textsc{k}$, $\beta$' phases under cavitation loading.ases under cavitation loading.

• Journal of Magnetics
• /
• v.3 no.1
• /
• pp.9-14
• /
• 1998

A non-coercive cast Nd-Fe-B-type material can be easily converted into a coercive one by employing HDDR process. Applying the conventional HDDR process to the Nd-Fe-B-type material generally leads to a powder-like material. HDDR treated material in a solid form can, however, be realised if the process is properly modified (solid-HDDR). In the present study, the change of rigidity (compressive strength) of the Nd-Fe-B-type material during the solid-HDDR has been investigated using a homogeneous sintered magnet with composition $Nd_{13.8}Dy_{0.7}Fe_{78.25}Si_{0.15}Mn_{0.6}B_{6.5}.$ It has been found that the low strength of the hydrided material was improved by the subsequent disproportionation. The restoration of the strength was explained by the eutectoid-like disproportionation structure containing fine neodymium hydride rod embedded in tough iron matrix. The high strength of disproportionated material was reduced radically in earlier stage of recombination, and this wes explained by the reduction of the disproportionated phase. The reduced strength was, however, recovered by further recombination, and this was explained by the fact that as the recombination continues the recombined grains adhere together. The optimally HDDR processed material has a comparable or even higher strength with respect to the initial sintered material prior to the solid-HDDR. The present study suggested that the rigidity of Nd-Fe-B-type material could be retained even after the solid-HDDR.

• Journal of Korea Foundry Society
• /
• v.9 no.6
• /
• pp.463-473
• /
• 1989

This study is aimed to investigate the effect of cyclic heat treatment which is special heat treatment on the strength and toughness in Multi-phase(Ferrite-Bainite-Martensite) Ductile cast Irons. Spceimens were austenitized at eutectoid transformation temperature range(${\alpha}+{\gamma}$) for 30min and austempered at $300^{\circ}C$ and $400^{\circ}C$ for different holding times, and then quenched in ice water to obtain the multi-phase(Ferrite-Bainite-Martensite) structure from various prior structures, which was obtained by various cyclic heat treatments. As the number of cycle in cyclic heat treatment increased, volume fraction of pearlite increased and the its morphology was refined. As the number of cycle in cyclic heat treatment increased, the multi-phase(Ferrite-Bainite-Martensite) was dispersed in whole matrix as refined island phase. Particularly, martensite among the multi-phase gradually became a spherical shape. Good combination in impact energy and tensile strength was detained in $840^{\circ}C-300^{\circ}C-15min$ condition after 10 cycles in cyclic heat treatment, and its multi-phase volume fraction is Ferrite(50%)-Martensite(l3%)-Bainite(37%).

• Journal of Conservation Science
• /
• v.34 no.1
• /
• pp.39-50
• /
• 2018

This study replicated traditional smelting methods to produce iron blooms from sand iron. The metallurgical properties of the slag and the iron blooms were analyzed. The sand iron materials used in the smelting experiments, which were based on ancient documents, were collected from Gyeong-Ju and Pohang. Analysis by WD-XRF and XRD showed that Gyeong-Ju's sand iron contains a high-titanium, with magnetite, and Pohang's sand iron contains a low-titanium, which magnetite and ilmenite were mixed. Analysis of the slag with XRD, and the micro-structure with metal microscopes and SEM-EDS, confirmed that the major compounds in the slag of the Gyeong-Ju's sand iron were fayalite and $w\ddot{u}stite$, and those in the slag of the Pohang's sand iron were titanomagnetite and fayalite. The differences in the main constituents were confirmed according to the Ti quantity. Finally, we observed the microstructures of the iron blooms. In the case of the iron bloom produced from Gyeong-Ju's sand iron, the outside was found to be dominantly a pearlite of eutectoid steel, while the inside was a hypo-eutectoid steel where ferrite and pearlite were mixed together. While, the major component of the iron bloom produced from Pohang's sand iron was ferrite, which is almost like pure iron. However, there were many impurities inside the iron blooms. Therefore, this experiment confirmed that making ironware required a process that involved removing internal impurities, refining, and welding. It will be an important data to identify the characteristics of iron by-products and the site through traditional iron-making experiments under various conditions.

• Journal of the Korean Society for Heat Treatment
• /
• v.16 no.6
• /
• pp.320-328
• /
• 2003

A specimen of Cu-35%Sn alloy has been subjected to the unidirectional heat treatment in an attempt to examine the evolution of microstructures under varying thermal conditions. The specimen was cast in the form of a cylinder 10 mm in diameter and 200 mm in length, which was then installed in the temperature gradient field established inside a vertical tube furnace. The furnace temperature was adjusted to make the upper part at $750^{\circ}C$ and bottom end part at $300^{\circ}C$ of the specimen. The experiment was terminated by dropping it into water after the 30 minutes holding at given temperature. By the rapid cooling, the high temperature phases, ${\gamma}$ and ${\zeta}$, were retained at ambient temperature with some of ${\gamma}$ phase transformed to ${\varepsilon}$ phase, especially at the grain boundaries of ${\gamma}$ phase. The presence of ${\varepsilon}$ phase was found to determine the nature of phase transformations of the ${\zeta}$ phase undergoes upon cooling. In the close area of the ${\varepsilon}$ phase, ${\varepsilon}$ phase grew separately out of ${\zeta}$, and adds to the preexisting ${\varepsilon}$ whereas in areas away from ${\varepsilon}$, both ${\delta}$ and ${\varepsilon}$ grew simultaneously out of ${\zeta}$, and formed a lamella eutectoid structure. The transformation to ${\delta}$ was found to occur only in slow cooling. The hardness on each phase showed that the retained phases, ${\gamma}$ and ${\zeta}$, could be plastically deformed without brittle fracture while the phases, ${\varepsilon}$ and ${\delta}$, were too brittle to be deformed.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.12 no.3
• /
• pp.505-512
• /
• 1988

Since the propagation of a short fatigue crack is directly related to the large crack which causes the fracture of bulk specimen, the detailed study on the propagation of the short crack is essential to prevent the fatigue fracture. However, a number of recent studies have demonstrated that the short crack can grow at a low applied stress level which are predicted from the threshold condition of large crack. In present study, the threshold condition for the propagation of short fatigue crack is examined with respect to the microstructure and cyclic loading history. Specimens employed in this study were decarburized eutectoid steels which have various decarburized ferrite volume fraction. Rotating bending fatigue test was carried out on these specimens with the special emphasis on the '||'&'||'quot;critical non-propagating crack length.'||'&'||'quot; It is found that the reduction of the endurance limit of their particular microstructures can be due to the increase of the length of critical non-propagating crack, and the quantitative relationship between the threshold stress .DELTA. .sigma. $_{th}$ and the critical non-propagating crack length Lc can be written as .DELTA. .sigma. $_{th}$, Lc=C where m, C is constant. Further experiments were carried out on the effect of pearlitic structure and cyclic loading history on the length of critical non-propagating crack. It is shown that the length of critical non-propagating crack is closely related to both pearlite interlamellar spacing and cyclic loading history.ory. cyclic loading history.

• Korean Journal of Metals and Materials
• /
• v.49 no.2
• /
• pp.93-103
• /
• 2011

Precipitation behavior of high-nitrogen duplex Fe-24Cr-7Mn-4Ni-4Mo-0.43N stainless steel aged at $850^{\circ}C$ was investigated using scanning transmission electron microscopy. Based on the analyses of selected area diffraction patterns, four kinds of precipitates (intermetallic sigma (${\sigma}$) and chi (${\chi}$), $Cr_2N$ and secondary austenite) were identified. At the ferrite/austenite phase boundary, the ${\sigma}$ phase and secondary austenite were formed via ${\alpha}{\rightarrow}{\gamma}+{\sigma}$ eutectoid reaction. The precipitation of $Cr_2N$ occurred at the austenite grain boundary as well as the interior of the ferrite. The intermetallic ${\chi}$ phase also formed within the ferrite and showed a cube-cube orientation relationship with the ferrite. Further aging produced a lamellar structure composed of $Cr_2N$ and austenite along the ferrite/austenite boundary and enhanced the precipitation of the ${\chi}$ phase. The crystallographic features of the precipitates were also examined in terms of the orientation relationship with the austenite or ferrite matrix.

• Journal of Conservation Science
• /
• v.36 no.3
• /
• pp.197-212
• /
• 2020

This study analyzed the microstructure of ring-pommel swords, excavated from Ancient Tombs, Hadae, Ulsan and examined their production technique, using non-destructive testing and a metallurgical method. The results confirmed that the five ring-pommel swords, unearthed in Ancient Tombs, Hadae, Ulsan, as identified by radiographic non-destructive testing, had been solely manufactured using iron, through forging based on the single-piece technique. Furthermore, these results were compared with previous studies, and the manufacturing techniques of single-piece ring-pommel swords were categorized into three types: pure iron - changing the shape, pure iron - changing the shape - carburization, and steel - changing the shape - quenching. The ring-pommels of four swords had around 0.7% of carbon content, which is as much as for eutectoid steel and higher than for other parts of these swords, such as the backs of their blades and handles. The weapon function of a small ring-pommel sword, under 60cm in length, was maximized by quenching focusing on its blade. Conversely, the martensite quenching structure was not observed in four ring-pommel swords shorter than 75cm. In other words, the same types of single-piece ring-pommel swords(late in 2C~early in 4C) were unearthed from Ancient Tombs, Hadae, and the group who has manufactured these swords is presumed to have limited their effectiveness, functionally depending on purposes, through an iron-making process and heat-treatment techniques.