• Journal of Korea Foundry Society
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• v.13 no.2
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• pp.187-193
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• 1993

In this study, it was examined the relationship between the microstructure, fatigue properties, mechanical properties and retained austenite volume of Mo-Ni ADI corresponding to various austempering temperatures. When the austempering temperature is increased to $370^{\circ}C$, acicular bainite structure was found to be transformed to feathery bainite structure. But at the austempering temperature of $420^{\circ}C$, the dissolved bainite lath was showned. Up to the austempering temperature of $370^{\circ}C$, the volume of retained austenite was increased. However at the austempering temperature of $420^{\circ}C$ a large amount of retained austenite was decreased. In this study, the retained austenite volume was determined by XRD(X-ray diffraction). It was observed that the optimum fatigue properties can be obtained at the condition of austempering temperature $370^{\circ}C$. Under the such conditions, fatigue limit determined as the value of 290 MPa, tensile strength 877MPa elongation 6%, hardness 285(BHN), impact values(CVN) 9.2J and retained austenite volume 30.3%, respectively.

• Journal of the Korean Society for Heat Treatment
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• v.6 no.2
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• pp.49-58
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• 1993

Isothermal transformation behavior during patenting and variations of microstructure and tensile strength of patented wires were investigated in Si-added high carbon steel. The TTT curves of the steels were made for two different austenitizing temperature. As the salt bath temperature was increased, the observed microstructures were bainite at $450^{\circ}C$, the mixture of bainite and pearlite at $500^{\circ}C$, and to pearlite at $600^{\circ}C$, The tensile strength of patented wire exhibited the highest value when the structure was pearlite. while the bainitic structure showed the lowest.

• Korean Journal of Materials Research
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• v.23 no.8
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• pp.423-429
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• 2013

Recently, steel structures have increasingly been required to have sufficient deformability because they are subjected to progressive or abrupt displacement arising from structure loading itself, earthquake, and ground movement in their service environment. In this study, high-strength low-carbon bainitic steel specimens with enhanced deformability were fabricated by varying thermo-mechanical control process conditions consisting of controlled rolling and accelerated cooling, and then tensile and Charpy V-notch impact tests were conducted to investigate the correlation between microstructure and mechanical properties such as strength, deformability, and low-temperature toughness. Low-temperature transformation phases, i.e. granular bainite (GB), degenerate upper bainite(DUB), lower bainite(LB) and lath martensite(LM), together with fine polygonal ferrite(PF) were well developed, and the microstructural evolution was more critically affected by start and finish cooling temperatures than by finish rolling temperature. The steel specimens start-cooled at higher temperature had the best combination of strength and deformability because of the appropriate mixture of fine PF and low-temperature transformation phases such as GB, DUB, and LB/LM. On the other hand, the steel specimens start-cooled at lower temperature and finish-cooled at higher temperature exhibited a good low-temperature toughness because the interphase boundaries between the low-temperature transformation phases and/or PF act as beneficial barriers to cleavage crack propagation.

• Journal of Korea Foundry Society
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• v.26 no.4
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• pp.180-183
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• 2006

The effects of adding Ni on microstructure and mechanical properties of Nodular Indefinite Chilled Iron (NICI) were studied. Thermal fatigue, hardness, tensile properties, wear resistance, are very important factors for NICI used for hot working roll and wire rod mill. The results show that addition 4% nickel has changed pearlite to bainite. Bainite matrix is superior to pearlite matrix on wear resistance, hardness and strength and will increase performance lifetime of NICI conventional roll material. Based in the bainitic microstructure, hardness and tensile property increase up to 48 HRc and $72\;kg/mm^2$, respectively.

• Journal of the Korean Society for Heat Treatment
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• v.26 no.2
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• pp.72-79
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• 2013

The effects of homogenization, hot-forging, and annealing condition on microstructure and hardness of a modified STD61 hot-work tool steel were investigated. The ingot specimen had a dendritic structure consisting of bainite and martensite. Spherical VC particles of approximately 50 nm and cuboidal (V,Ti)C particles of about 100 nm were observed in the ingot specimen. After homogenization, the dendritic structure was blurred, and the difference in hardness between martensite and bainite became narrow, resulting in the more homogeneous microstructure. Needle-shaped non-equilibrium $(Fe,Cr)_3C$ particles were additionally observed in the homogenized specimen. The hot-forged specimen had bainite single phase with spherical VC, cuboidal (V,Ti)C, and needle-shaped $(Fe,Cr)_3C$ particles. After annealing at $860^{\circ}C$, the microstructures of specimens were ferrite single phase with various carbides such as VC, $(Fe,Cr)_7C_3$, and $(Fe,Cr)_{23}C_6$ because of relatively slow cooling rates. The size of carbides in annealed specimens decreased with increasing cooling rate, resulting in the increase of hardness.

• Korean Journal of Metals and Materials
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• v.48 no.3
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• pp.194-202
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• 2010

Application of a stronger and more durable material for reactor pressure vessels (RPVs) might be an effective way to insure the integrity and increase the efficiency of nuclear power plants. A series of research projects to apply the SA508 Gr.4 steel in ASME code to RPVs are in progress because of its excellent strength and durability compared to commercial RPV steel (SA508 Gr.3 steel). In this study, the microstructural characteristics and mechanical properties of SA508 Gr.3 Mn-Mo-Ni low alloy steel and SA508 Gr.4N Ni-Mo-Cr low alloy steel were investigated. The differences in the stable phases between these two low alloy steels were evaluated by means of a thermodynamic calculation using ThermoCalc. They were then compared to microstructural features and correlated with mechanical properties. Mn-Mo-Ni low alloy steel shows the upper bainite structure that has coarse cementite in the lath boundaries. However, Ni-Mo-Cr low alloy steel shows the mixture of lower bainite and tempered martensite structure that homogeneously precipitates the small carbides such as $M_{23}C_6$ and $M_7C_3$ due to an increase of hardenability and Cr addition. In the mechanical properties, Ni-Mo-Cr low alloy steel has higher strength and toughness than Mn-Mo-Ni low alloy steel. Ni and Cr additions increase the strength by solid solution hardening. In addition, microstructural changes from upper bainite to tempered martensite improve the strength of the low alloy steel by grain refining effect, and the changes in the precipitation behavior by Cr addition improve the ductile-brittle transition behavior along with a toughening effect of Ni addition.

• Journal of Korea Foundry Society
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• v.11 no.4
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• pp.323-330
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• 1991

The relationships between the microstructure changes, retained austenite volume and elevated temperature tensile properties of Mo-Ni-Cu ADI corresponding to various austempering temperatures and time were investigated, After the $250^{\circ}C$ tensile test for the test piece austempered at $270^{\circ}C$ the accicular bainite structure was observed blunted under room temperature microscope. In the case of $370^{\circ}C$ austempering, the feathery bainite lath spacing was observed broadened. But after the $450^{\circ}C$ tensile test, bainitic features could not be observed. As the testing temperature increased, retained austenite volume tested at room temperature decreased. Especially, after the $450^{\circ}C$ tensile test retained austenite volume approached nearly to zero. A little higher tensile properties appeared at $250^{\circ}C$ testing than those at room temperature.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.05a
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• pp.178-181
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• 2004

Heat treatment is one of important manufacturing process that determine the quality of the products. Because of a difference of mechanical property by heat treatment, It is necessary to This papers presents flow stress and yield point through tensile test. The goal of this study is to obtain the data of flow stress and yield point at martensite, bainite, ferrite/pearlite phase structure using SCM420, SCr420. The result of tensile test is satisfied and is expected to develop an available FEM analysis.

• Journal of Korea Foundry Society
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• v.10 no.3
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• pp.235-246
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• 1990

This study has been carried out to investigate into the rolling wear characteristics of austempered ductile iron under unlubricated dry rolling condition by Amsler type test with 9.09% sliding ratio. Wear amount is increased with increasing of ferrite and retained austenite and it has been found that the amount of rolling wear is decreased when the hardness of matrix are higher. The rolling life up to generation of abnormal condition is conspicusly increased and amount of rolling wear is increased asrolling revolutionis increased and wear of austempered ductile ironunder dry rolling condition. Matrix has been obtained to upper bainite, low bainite at heat treatment, obtained to bull`s eye structure at as cast. It has been found that the amount of rolling wear is decreased when the tensile strength of the casting are increased and then the rolling life up to generation of abnormal wear in conspicuously increased and amount of wear was intend to graphite content.

• Corrosion Science and Technology
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• v.23 no.1
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• pp.11-19
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• 2024

A Fe-Ni Bainitic steel as a weathering steel application was developed by combining its excellent mechanical properties and corrosion resistance in maritime environments. Nickel concentration (0.4-3 wt%) and inverted austempering multi-step (IAM) process were primary determinants of the microstructure of the Fe-Ni Bainitic steel. The initial austempering steel was performed at 300 ℃ for 600 seconds to obtain a partly bainitic transformation. The steel was heated again for 1800 s at 450 ℃. The microstructure was comprised of ferrite, a blocky martensite/austenite island, and a homogeneous lath-shape bainite structure with widths ranging from 4.67 to 6.89 ㎛. The maximum strength, 1480 MPa, was obtained with 3 wt% nickel. In this study, corrosion behavior was investigated utilizing potentiodynamic and electrochemical impedance spectroscopy (EIS) tests. A higher nickel content in Fe-Ni Bainitic steel refined the grain size, improved the bainite fraction, lowered the corrosion rate to 0.0257 mmpy, and increased the charge transfer of film resistance to 1369 Ω.