• Journal of Korea Foundry Society
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• v.24 no.5
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• pp.290-294
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• 2004

The effect of carbon equivalent on the fading behavior of hypoeutectic ductile cast iron was investigated. The carbon content was slightly increased right after graphite spheroidization treatment and remained almost constant with holding at $1,490^{\circ}C$ after 4 minute. The residual magnesium content was decreased slowly with holding. The empirical equation expressing the relationship between this and holding time was proposed ; $C=C_o-P{\times}t$. The proportionality constant, P, was inversely proportional to carbon content.

• Journal of Korea Foundry Society
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• v.6 no.4
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• pp.315-320
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• 1986

• Korean Journal of Metals and Materials
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• v.48 no.11
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• pp.974-980
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• 2010

Various types of steel, namely, 0.35C, 0.2C-Cr, and 0.2C-Cr-Mo steels, were quenched and tempered by high-frequency induction heat treatment. The type, size, and spheroidization of the carbides varied depending on the tempering temperatures ($450{\sim}720^{\circ}C$). During the tempering process, the carbide was precipitated in the martensite matrix. The 0.35C, 0.2C-Cr, and 0.2C-Cr-Mo steels contained carbides that were smaller than 120 nm. The carbide was spheroidized as the tempering temperature increased. Owing to the fine microstructure and spheroidization of the carbides, all three steels had a high tensile strength as well as yield ratio and reduction of area. In the case of the 0.2C-Cr steel, the use of Cr as an alloying element facilitated the precipitation of alloyed carbides with an extremely small particle and resulted in an increase in the spheroidization rate of the carbides. As a result, a large reduction of area was achieved (>70%). The 0.2C-Cr-Mo steel had the highest tensile strength because of the high hardenability that can be attributed to the presence of alloying elements (Cr and Mo). Quenching and tempering steels by induction heat treatment resulted in a high strength of over 1 GPa and a large reduction of area (>70%) because of the rapid heating and cooling rates.

• Journal of the Korean Society for Heat Treatment
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• v.26 no.3
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• pp.126-131
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• 2013

In this study, effects of silicon addition on the spheroidizing annealing of hyper-eutectoid steel was investigated. Heat treatment at various temperatures in the ${\gamma}+{\theta}$ region was also conducted in order to systematically control the kinetics of undissolved cementite. It was found that small amount of Si addition could increase both $A_1$ and $A_{cm}$ transformation temperature by both the JMat Pro evaluation and dilatometric measurement. It was also revealed by the microstructural observation that the volume fraction of retained cementite during heat treatment increased with decreasing temperature as well as increasing Si content. Based on the results obtained, it could be suggested that spheroidization at relatively higher temperature above $950^{\circ}C$ could be achieved by small addition of Si.

• Journal of the Korean Chemical Society
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• v.7 no.1
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• pp.51-57
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• 1963

It is well known that the graphite flakes become spherulite, when a suitable amount of nodulizing element, such as cerium or magnesium, is added to the cast iron. The change of graphite from flake to nodular shape improves not only the tensile strength but the ductility as well. However, the mechanism of spheroidization of graphite in cast iron has not yet been clearly understood, and various theories proposed by a number of investigators were such that it may be due to the special nucleation effect, prevention of flake formation by the adsorption of magnesium vapour on the graphite surface or file surface free energy difference between plain graphite and magnesium-adsorbed graphite. Regardless of the speculations of spheroidizing mechanism of the graphite in the cast iron, the final phenomenon comes to the conclusion that it may be due to the lack of wettability between graphite and iron matrix. In order to collaborate this fact through an experimental method, the authors have constructed a vacuum arc furnace for the wettability measurement as its first step. Our study and experiments were then directed to the comparison of the wettability between iron and graphite on the two cases (namely, the one where magnesium was preliminarily coated on the graphite surface and the other not coated), by means of contact angle measurements. The result was such that a significant difference of the contact angles has been shown between the above two cases. indicating the spheroidization of graphite which might have resulted from the lack of wettability between magnesium-adsorbed graphite and iron matrix.

• Journal of Korea Foundry Society
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• v.9 no.6
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• pp.474-482
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• 1989

The aim of this research is to investigate the effects of gas bubbles on the formation of spheroidal graphite in cast iron, Fe-Si-8%Mg alloy, mischmetal hydride($MmH_2$) and $CaCO_3$, which discharge various amounts of Mg, $H_2$ and $Co_2$, gases, were added to Fe-3.9% C-2.0%Si melt and the melt was innoculated with 0.3wt% of 75%Fe-Si. The spheroidal graphites and/or compacted vermicular graphites were produced with more than 0.625cc/g of Mg gas or more than 0.3125cc/g of $H_2$ gas while $CO_2$ gas did not contribute to graphite spheroidization. Nodule counts increased with the amount of Fe-Si-Mg added ; but they decreased with the amount of $MmH_2$ added because the number of effective gas bubbles decrease with the increase in Mm residual. The bull's eye structure was revealed with 0.625cc/g, 1.25cc/g of Mg and 0.3125cc/g of $H_2$ ; the ledeburite structure was revealed with more than 0.625cc/g of $H_2$.

• Journal of Powder Materials
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• v.18 no.3
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• pp.226-237
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• 2011

W-ZrC and W-HfC composite powders were fabricated by the Plasma Alloying & Spheroidization (PAS) method and the powders were sprayed into hybrid coating layers by using Low Vacuum Plasma Spray (LVPS) process, respectively. Microstructure, mechanical properties, and ablation characteristics of the fabricated coating layers were investigated. The LVPS process led to successful production of W-Carbide hybrid coatings, approximately 400 ${\mu}M$ or above in thickness. As the substrate preheating temperature increased from $870^{\circ}C$ to $917^{\circ}C$, the hardness of the W-ZrC coating layer increased due to decreased porosity. Vickers hardness showed higher value (about 108.4 HV) in W-ZrC hybrid coating material compared to that of W-HfC while adhesive strength was found to be similar in both coating layers. The plasma torch test revealed good ablation resistance of the W-Carbide hybrid coating layers. The relatively high performance W-ZrC coating layer at the elevated temperature is thought to be attributed to both the strengthening effect of ZrC particle remained in the layer and the formation of ZrO2 phase with high temperature stability.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.10a
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• pp.139-141
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• 2009

This study analyzed the effect of the microstructure and alloying element on hydrogen-induced delayed fracture properties for the Energy Saving Wire (ESW) developed recently. Specimens were produced with a diameter 6.5mm post to the deformation (0, 10, 20 and 30%), followed by injecting the hydrogen. The experimental results by using GAS chromatography showed that the more hydrogen was emitted for high-carbon steel (0.45%C steel and 0.35%C steel) than low-carbon steel(0.2%C-Cr steel and 0.2%C-Cr-Mo steel). And, 0.45%C steel, 0.35%C steel and 0.2%C-Cr-Mo steel exhibited the crack for 30% deformed specimen. The hydrogen emitted was analyzed with the amount, the spheroidization, and the size of the carbides.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.05a
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• pp.307-310
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• 2003

Non-heat treated steels are attractive in the steel-wire industry since the spheroidization and quenching-tempering treatment are not involved during the processing. In this study, three different steels such as dual phase steel, low-Si steel, and ultra low carbon bainitic steel were used to investigate their deformation resistance and forming limit. Deformation resistance was estimated by calculating the deformation energy and the forming limit was evaluated by measuring the critical strain revealing crack initiation at the notch tip of the specimens. The results showed that deformation resistance was the lowest in the low-Si steel, and the forming limit strain was the highest in the ultra low carbon bainitic steel.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2001.10a
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• pp.324-329
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• 2001

The destructive method is reliable and widely used for the estimation of material degradation but, it have time-consuming and a great difficulty in preparing specimens from in-service industrial facilities. Therefore, the estimation of degraded structural materials by nondestructive evaluation is strongly desired. In this paper, the use of guided wave was suggested for the evaluation of thermally damaged 2.25 Cr-lMo steel as an alternative way to compensate for limitations of fracture tests. The observation of microstructure variations of the material including carbide precipitation increase and spheroidization near grain boundary was conducted and the correlation with the guided wave features such as energy loss ratio and group velocity changes was investigated. Through this study, the feasibility of ultrasonic guided wave evaluation for thermally damaged materials was explored.