• 한국정밀공학회:학술대회논문집
• /
• 한국정밀공학회 2008년도 춘계학술대회 논문집
• /
• pp.405-406
• /
• 2008

• 보존과학회지
• /
• 제26권4호
• /
• pp.407-416
• /
• 2010

토양 중에 존재하는 철환원능력을 갖춘 세균은 철제유물의 부식생성물을 에너지원으로 이용할 수 있다. 이러한 세균의 활동은 부식생성물의 변화를 초래하여 유물의 부식을 촉진시키는 부식생성물의 판단을 어렵게 할 수 있다. 본 연구의 목적은 철환원세균이 부식생성물에 일으키는 변화를 조사하여, 철제유물의 부식에 관한 이해를 높이고자 한다. 실험은 출토철제유물을 재현하기 위해서 부식촉진인자 중에서 가용성염류(염화물이온, 황산이온)를 이용하여 부식시킨 철편을 준비하였다. 이 부식철편을 세균이 존재하는 배지에서 42일간 배양하였다. 실험 후, 부식철편의 부식생성물은 SEM, EDS, XRD를 이용하여 관찰, 분석을 실시하였다. 관찰결과, 부식철편이 세균의 활동으로 인해 녹색으로 변화하였으며 부식철편에 판상 결정과 마름모꼴 결정이 새롭게 생성된 사실을 알게 되었다.

• 한국소성가공학회:학술대회논문집
• /
• 한국소성가공학회 2004년도 추계학술대회논문집
• /
• pp.211-214
• /
• 2004

적절한 가공조건을 통하여, 중 탄소강에서 SIDT를 적용하여 페라이트-시멘타이트 미세조직이 압연 직후에 얻어졌다. 또한 이와 같은 조건은 압하온도 $690^{\circ}C$, 높이 감소율 $70\%$ (변형량 1.2) 및 변형율 속도 1/s에서 가공하였을 때 얻어졌으며, 가공 직후 $730^{\circ}C$로 승온하여 약 30분간 유지 후 서냉한 조건에서 매우 연한 미세조직이 얻어짐을 알 수 있었다. 이는 기존 20 시간 이상의 구상화 열처리에 비해 매우 짧은 시간이며, 구상화 열처리를 통한 경험한 미세조직과 유사한 특성을 나타낼 것으로 예상된다.

• 한국소성가공학회:학술대회논문집
• /
• 한국소성가공학회 2000년도 추계학술대회 논문집
• /
• pp.33-36
• /
• 2000

The dynamic recrystallization (DRX) of medium carbon steels (SCM 440 and POSMA45) was studied with torsion test in the temperature range of $900-1100^{\circ}C$ and the strain rate range of $5.0x10^{-2}\;-\;5.0x10^0/sec$. To establish the quantitative equations for DRX, the evolution of flow stress curve with strain was analyzed. The critical strain (${\varepsilon}_c$) and strain for maximum softening rate ( ${\varepsilon}^{*}$) could be confirmed by the analysis of work hardening rate ($d{\sigma}/d{\varepsilon}\;=\; \theta$). The volume fraction of dynamic recrystallization ($X_{DRX}$) as a function of processing variables, such as strain rate ( $\dot{\varepsilon}$ ), temperature (T), and strain ( $\varepsilon$ ) were established using the ${\varepsilon}_c$ and ${\varepsilon}^{*}$. For the exact prediction, the ${\varepsilon}_c$, ${\varepsilon}^{*}$ and Avrami' exponent (m') were quantitatively expressed by dimensionless parameter, Z/A respectively. The transformation-effective strain-temperature curve for DRX could be composed. It was found that the calculated results were agreed with the experimental data for the steels at any deformation conditions.

• 열처리공학회지
• /
• 제23권6호
• /
• pp.338-343
• /
• 2010

Mechanical properties and microstructures of medium carbon high manganese steels were investigated in terms of alloying elements such as Mn, C contents, and heat treatment condition. Austenite volume fraction was increased with increasing Mn content, leading to hardness decrease in the range of Mn content of above 10% after quenching and tempering. Such results are also supported by microstructural analysis and X-ray diffraction in that the increase in mangaese content results in the increase in austenite fraction. Studies on tempering condition indicated that not only hardness and tensile strength but also charpy impact values were reduced as tempering temperature were raised in the range of $250^{\circ}C$ to $600^{\circ}C$. It was also observed that fracture mode was changed from dimple to intergranular fracture. Such results are thought to be due to very fine carbide precipitation or impurity segreagation at grain boundaries as tempering temperature goes up. Heat treatment of Fe-5Mn-2Si-1Al-0.4C can be optimized by austenitizing at $850^{\circ}C$, air cooling and tempering at $250^{\circ}C$, resulting in 1950 MPa in Tensile strength, 17% in elongation and 23.3 $J/cm^2$ in charpy impact energy with high work hardening characteristics.

• 한국소성가공학회:학술대회논문집
• /
• 한국소성가공학회 2009년도 추계학술대회 논문집
• /
• pp.362-365
• /
• 2009

Microstructural evolution and the mechanical properties of various carbon steels were investigated with the variation deformation temperature to explore the optimum microstructure with excellent combination of strength and ductility. For this purpose, three carbon steels containing different carbon contents were deformed using Gleeble 3500 at temperatures including austenitic, austenitic/ferritic, austenitic/cementitic, ferritic/cementitic regions. The results showed that in the medium and high carbon steels, cementite particles became finer with decreasing deformation temperature resulting higher hardness but lower ductility. Further effort is needed to find out optimum microstructures with enhanced mechanical properties.

• 열처리공학회지
• /
• 제5권4호
• /
• pp.215-223
• /
• 1992

This study has been performed to investigate into some effects of temperature, gas mixing ratio and time on the optical microstructure, hardness and wear characteristics of medium carbon alloy steel treated by plasma nitriding. The results obtained from the experiment are summarized as follows: (1) Optical micrographs of AISI 4140 steel plasma-nitrided by the double stage technique have revealed that the nitrided layer is composed of the compound layer and the diffusion layer. The variation in temperature at the first stage gives effects, on the formation of compound layer and the growth rate is shown to be relatively fast at $460^{\circ}C$. (2) The thickness of compound layer has been found to increase with increasing nitrogen percentage in the gas mixture and the holding time. It is therefore recommended that a shorter holding time and a lower nitrogen percentage are more effective to produce a tougher compound layer and a diffusion layer only. (3) X-ray diffraction analysis for AISI 4140 steel has shown that the compound layer consist of ${\gamma}^{\prime}-Fe_4N$ and ${\alpha}-Fe$ and that tough compound layer diffustion layer only can be obtained by the double stage plasmanitriding process. (4) There is also a tendency that the total hardened layer depth increases with increasing temperature, time and nitrogen percentage in the first stage during the double stage plasma nitriding. (5) The wear resistance of plasma nitrided specimens has been found thobe considerably increased compared to the untreated specimens and the amount of increment has appeared to increase further with increasing nitriding temperature, holding time and notrogen percentage of gas mixture in the first stage treatment.

• Corrosion Science and Technology
• /
• 제17권2호
• /
• pp.81-89
• /
• 2018

In this study, the effect of Nb alloying element on the corrosion fatigue properties of high strength steel is investigated by conducting fatigue experiments under corrosive condition and hydrogen induced condition, potentiodynamic polarization test, tensile test and surface analyses. Nb element is added to enhance the mechanical property of medium carbon steel. This element forms MX-type phases such as carbides and nitrides which are playing an important role in the grain refinement. The grain refinement is one of the effective way to improve mechanical property because both tensile strength and toughness can be improved at the same time. However, MX-type phase precipitates can be a susceptible site to localized corrosion in corrosive environment due to the potential difference between matrix and precipitate. The obtained results showed that Nb-added steel improved corrosion fatigue property by grain refinement. However, it is degraded for hydrogen-induced fatigue property due to Nb, Ti-inclusions acting as a stronger trap.

• Journal of Mechanical Science and Technology
• /
• 제16권4호
• /
• pp.501-511
• /
• 2002

This paper presents a study of the effect of rolling temperature, roll gap (pass height), initial specimen size and steel grades of specimens on the roll force in round-oval-round pass sequence by applying approximate method and verifications through single stand pilot rod rolling tests. The results show that the predicted roll forces are in good agreement with the experimentally measured ones. The approximate model is independent of the change of roll gap, specimen size and temperature. Thus, the generality of the prediction methodology employed in the approximate model is proven. This study also demonstrates that Shida's constitutive equation employed in the approximate model needs to be corrected somehow to be applicable for the medium and high carbon steels in a lower temperature interval (700∼900$\^{C}$).

• Steel and Composite Structures
• /
• 제23권6호
• /
• pp.623-631
• /
• 2017

Present paper deals with the temperature-dependent buckling analysis of sandwich nanocomposite plates resting on elastic medium subjected to magnetic field. The lamina layers are reinforced with carbon nanotubes (CNTs) as uniform and functionally graded (FG). The elastic medium is considered as orthotropic Pasternak foundation with considering the effects of thermal loading on the spring and shear constants of medium. Mixture rule is utilized for obtaining the effective material properties of each layer. Adopting the Reddy shear deformation plate theory, the governing equations are derived based on energy method and Hamilton's principle. The buckling load of the structure is calculated with the Navier's method for the simply supported sandwich nanocomposite plates. Parametric study is conducted on the combined effects of the volume percent and distribution types of the CNTs, temperature change, elastic medium, magnetic field and geometrical parameters of the plates on the buckling load of the sandwich structure. The results show that FGX distribution of the CNTs leads to higher stiffness and consequently higher buckling load. In addition, considering the magnetic field increases the buckling load of the sandwich nanocomposite plate.