• Korean Journal of Materials Research
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• v.7 no.11
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• pp.999-1006
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• 1997

0.14wt.%C-0.82wt.%Mn-0.20wt, %Si-0.10wt.%V-0.03wt.%Nb의 조성을 갖는 직접 소입강에 몰리브데늄을 각각 0.30wt.%, 0.58wt.% 첨가하여 기계적 성질을 조사하고 투과 전자 현미경을 이용하여 각각의 미세 조직을 관찰하여 석출물을 분석하였다. 직접 소입시의 석출물은 M$_{2}$C, MC의 비평형 상태의 석출물이 생성되었고 석출물들은 마르텐사이트 래스내의 침상형탄화물, 구형의 M$_{2}$C탄화물, 직사각형 형태의 MC탄화물이 관찰되었다. 유냉시에는 래스내의 침상형 탄화물만 관찰되었으며 수냉시에는 M$_{2}$C 탄화물이 관찰되었다. 0.58wt.%의 몰리브데늄의 첨가된 강에서는 MoN이 관찰되었으며, 이 강의경우 강도의 손실 없이 인성이 증가하는 경향을 나타내었다.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.18 no.5
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• pp.252-261
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• 2017

This study examined the effect of heat treatment on the microstructure and mechanical properties of J55 line pipe steel. The experiments were carried out at under the following various conditions: austenization temperature($880^{\circ}C$, $910^{\circ}C$, $940^{\circ}C$), cooling methods(water quenching, oil quenching) and tempering temperature(none, $550^{\circ}C$, $650^{\circ}C$). The phase diagram and CCT curve were simulated based on the chemical composition of J55 steel to predict the microstructures. In the results, A1, A3 temperature decreased. As the austenization temperature increased, existing austenite grains grew exponentially which seriously degraded their mechanical properties. Various microstructures, including martensite, bainite, ferrite, and pearlite, developed in accordance with the heat treatments and were closely correlated with hardness, tensile strength and toughness. Martensite was formed after water quenching, but bainite and ferrite appeared after oil quenching. FeC precipitation formed and coarsened during tempering, which improved their toughness.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.4
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• pp.69-76
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• 2018

This study examined the effect of the heat treatment and alloying elements (B, Ti) on the microstructures and mechanical properties of API J55 steel. The experiments were carried out using various austenization temperatures ($880^{\circ}C$, $910^{\circ}C$, $940^{\circ}C$), cooling methods (water quenching, oil quenching) and tempering temperatures (none, $550^{\circ}C$, $650^{\circ}C$) with J55 and J55+B,Ti steels. The phase diagram and CCT curve were simulated based on the chemical compositions of the J55 and J55+B,Ti steels to predict the microstructures. The results showed that the A1 and A3 temperatures decreased and, as a result, the noses of the ferrite and bainite parts of the CCT curve moved to the right. Various microstructures were formed, namely martensite, bainite, ferrite and pearlite, in accordance with the heat treatment, which had an effect on the hardness, tensile strength and toughness. Martensite was formed after water quenching, but bainite and ferrite appeared after oil quenching with the J55 specimens. On the other hand, martensite was formed, regardless of the cooling method (water quenching, oil quenching), with the J55+B,Ti specimens, because of the improvement of the hardenability caused by the addition of boron. Therefore, the J55+B,Ti specimens exhibited much higher mechanical properties than the J55 specimens, even after the tempering treatment, since the addition of Ti caused fine precipitates to be formed, which inhibited grain growth at the recrystallization temperature.

• Tribology and Lubricants
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• v.32 no.5
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• pp.160-165
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• 2016

AISI 4140 steel is widely used in various mechanical components owing to its superior mechanical properties. Surface hardening techniques are often used to further improve the properties, particularly for applications with moving components. The aim of this research is to understand the effect of heat treatment process on surface properties and tribological characteristics of AISI 4140 steel. In this work, we prepare two different AISI 4140 steel specimens- one cooled by air and one by oil- and determine surface properties such as surface topography and roughness using a confocal microscope. We also observe the cross-sections of the specimens using a scanning electron microscope to understand the difference in the material structure. In addition, we assess the hardness with respect to the distance from the surface using a micro-Vickers hardness tester. After characterizing the surfaces of the specimens, we investigate the wear characteristics of the specimens under hydrodynamic lubrication. The results show the presence of grooves on the surface of the oil-cooled specimens. It is likely that such grooves are formed during the cooling process using the oil. However, we observe no other significant differences in the surface properties of the specimens. The wear test results show the occurrence of severe wear on the oil-cooled specimens, which may be due to the groove formed on the surface. The results of this work may be helpful to improve surface properties using surface hardening techniques from a tribological perspective.

• Korean Journal of Materials Research
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• v.4 no.3
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• pp.364-375
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• 1994

The microstructure of medium-carbon non-heat treated steels by air-cooling shows ferritepearlite structure. Compared to the conventional Q/T steels, the strength of these steels containing V, Nb and Ti are similar to Q/T steels. However, their toughness are inferior. In this study, the mechanical properties and microstructures of low-carbon Mo bearing steels produced by direct-quenching process were investigated. Especially, the effect of Mo on the direct-quenched steels was examined. The direct-quenched steels containing Mo were found to have higher strength and toughness. Mo seemed to be effective in producing the fine structure and dense precipitates because of decreasing transformation temperature. The best results were 1211 MPa in UTS and 127.5 J in toughness in the oil quenched 0.15C+O.llV+ 0.035Nb+ 1.81Mo steel. Compared to 0.4% C microalloyed steels, this results showed that UTS increased by 45% and toughness by 2 times.

• Journal of Welding and Joining
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• v.15 no.5
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• pp.56-63
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• 1997

In The study, the bonding of WC-9%Co to SUJ2 steel using Ag-Cu-Zn-Cd insert metal has performed to investigate the bonding properties by heat-treatment. Bonding was brazed for 5-30min at 95$0^{\circ}C$, performed solution treatment for 5 min at 85$0^{\circ}C$ and sustained subsequently oil quenching. To investigate the effect of heat-treatment, tempering was executed at $600^{\circ}C$ for 30 min after oil quenching. Mechnical properties and chemical compositions on the brazed bonding interface were investigated by means of microstructural observation, 4-point bending test and EDS and XRD measurements. The results obtained were as follows. 1) The bonding strength of WC-9%Co/SUJ2 joints by Ag-Cu-Zn-Cd insert metal obtained about 78, 117 and 72MPa after brazing for 5, 20 and 30 min at 95$0^{\circ}C$. And the highest bonding strength obtained about 131MPa after brazing for10 min at 95$0^{\circ}C$ 2) Higher bonding strength of 288MPa was obtained in the joint that brazed for 10 min at 95$0^{\circ}C$, and carried out tempering for 30 min at $600^{\circ}C$ subsequently. 3) Fracture of joint brazed by Ag-Cu-Zn-Cd insert metal for 5, 10, 20 and 30 min created WC-9%Co/SUJ2 interface. The joint that brazed for 10 min at 95$0^{\circ}C$ and then tempered for 30 min at $600^{\circ}C$ was fractured at the site of WC-9%Co.

• Korean Journal of Materials Research
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• v.8 no.11
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• pp.1031-1037
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• 1998

To investigate the effect of cooling rate and annealing temperature on the corrosion of Zircaloy-4 and Zr-2. 5Nb alloys, autoclave corrosion tests were performed at $500^{\circ}C$ for the specimens prepared by various heat treatments. The specimens were heat-treated at $1050^{\circ}C$ for 30 minutes and cooled by ice-brine quenching, water quenching, oil quenching, air cooling, and furnace cooling. To investigate the effect of annealing temperature, the specimens were annealed at $\alpha$, ($\alpha$+$\beta$)-, and $\beta$-temperatures. It was observed from the $500^{\circ}C$ corrosion test that nodular corrosion occurred on the Zircaloy-4 alloy but did not occur on the Zr-2.5Nb alloy. The corrosion resistance of Zircaloy-4 increased with increasing the cooling rate. On the other hand, the corrosion resistance of Zr-2.5Nb decreased with increasing the cooling rate and the annealing temperature. It is suggested that corrosion resistance of Zircaloy-4 would be controlled by the distribution of Fe and Cr element in the matrix and precipitates, while that of Zr-2.5Nb alloy the niobium concentration and $\beta_{-Nb}$ phase.