• Korean Journal of Materials Research
• /
• v.7 no.11
• /
• pp.942-950
• /
• 1997

NiAI기 산화물 분산강화(Oxide Dispersion Strengthende:ODS)합금을 기계적 합금화 (Mechanical Alloying: MA)방법으로 제조하였으며, 열간압축방법으로 성형하였다. 연이어 단순항온처리에 의한 정상결정립성장과 특성조건에서의 thermomechanical treatment 에 의한 이차재결정화를 유도하였다. 결정립 조대화된 ODSD MA NiAI의 creep 성질 및 이에 조대화된 미세조직은 creep 성질이 저하된 반면, 이차재결정화된 MA NiAI의 creep성질은 크게 향상되었다. 이 creep 성질의 향상은 이차재결정화의 특성인 급격한 결정립의 조대화, 분산상의 성장억제 및 grain aspect ratio의 증가에 기인한 것으로 사료되었다. 이차재결정화된 ODS MA NiAI의 creep또는 glide controlled dislocation creep임을 제시하지만, 전체 creep속도가 결정립 크기 및 grain aspect ratio의 영향을 크게 받은 것을 볼 때, 결정립계 미끄럼기구가 주 creep 기구와 조합되어 MA NiAI의 전체 creep기구에 영향을 준 것으로 추정할 수 있었다.

• Korean Journal of Materials Research
• /
• v.9 no.7
• /
• pp.740-747
• /
• 1999

Grain refinement is the only strengthening mechanism that improves both strength and toughness. Controlled rolling and accelerated cooling techniques have been known to be effective method to improve the mechanical properties by controlling the recrystallization and/or grain coarsening during processing. Repeat phase transformation $(\gamma/\alpha)$ by repeat heat treating is another way of grain refinement. In this study, a combined effect of controlled rolling and repeat heat treating was investigated. To study the effects of Mo addition and process parameters, Mo alloyed low carbon steels were prepared and thermomechanical controlled processes were simulated in the Gleeble system. The Mo addition resulted in an increasement of the grain coarsening temperature and suppress austenite recrystallization. The optimum condition for the refinement of austenite was obtained when the controlled rolling was performed twice with the same heat treatment condition, and reduction ratio of second pass was higher than that of first pass.

• Proceedings of the KWS Conference
• /
• 2010.05a
• /
• pp.1-10
• /
• 2010

강재의 구조화에 필수적인 용접 공정 후의 저합금강 용접 열영향부 미세조직 및 재질 예측을 위해 가열 중 상변태 거동에 미치는 초기 결정립 크기의 영향, 석출물-free 오스테나이트 결정립 성장 예측 모델, 임계 석출물 크기의 영향을 고려한 용접 열영향부 석출물 조대화 예측 모델, 석출물의 Kinetics을 고려한 결정립 성장 모델, 초기 오스테나이트 결정립크기 및 냉각 속도의 영향을 고려한 용접 열영향부 상변태 모델, 용접 열영향부 경도 예측 모델 등에 대해 연구를 수행하였다. 이를 통해 작성된 최종 모델은 실 용접부와의 비교를 통해 신뢰성을 확인하였으며, 저합금강 용접 열영향부의 미세조직과 경도값을 잘 예측하는 것으로 판단된다. 따라서 본 연구를 통해 작성된 모델을 통하여 용접 열영향부에서의 용접부 품질을 확인하기 위한 시간적, 경제적 비용을 절감할 수 있을 것으로 기대된다.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.18 no.11
• /
• pp.2837-2845
• /
• 1994

Several methods have been developed to predict on the remaining life of the old power plants. However, Larson-Miller parameter, one of existing creep life prediction methods, has not reflected the effect of material degradatioin and grain size. So this study has been carried out to research the effects of material degradation and austenite grain coarsening on the life prediction of 3.5Ni-Cr-Mo-V steel. An experimental result shows that carbide coarsening has no significant effects on the creep rupture life and the Larson-Miller parameter, but grain coarsening has an important influence on the creep ruptrure life and the Larson-Miller parameter. Therefore Larson-Miller constant, K should be determined to consider on the chemical composition and the grain size of materials.

• 한국전기화학회:학술대회논문집
• /
• 2005.07a
• /
• pp.341-346
• /
• 2005

펄스 도금 조건이 Co 도금층의 미세 구조 및 알칼리 $NaBH_4$ 용액의 수소발생특성에 미치는 영향을 조사하였다. 펄스 주기 및 최대전류밀도가 증가함에 따라 polyhedral 형상의 Co 결정립이 triangular형상으로 변화하였으며, 점차 결정립이 조대화 되어, 촉매 표면적이 감소하였다. 결국 알칼리 $NaBH_4$ 용액 내에서 가수분해반응에 참여하는 촉매 site가 감소하여 수소발생속도가 낮아졌다. 펄스도금시간이 증가함에 따라 Co 결정립의 크기가 점차 증가하여 촉매 표면적이 감소하였고, 가수분해반응에 참여하지 못하는 CO의 양이 증가하여 수소발생속도가 크게 감소하였다. 최대전류밀도 $0.1\;A/cm^2$, 펄스 주기 2 mS에서 10 s 동안 펄스 도금 시, $25^{\circ}C\;1\;wt.\%\;NaOH\;+\;10\;wt.\%\;NaBH_4$ 용액에서 $2140\;ml/min{\cdot}g-catalyst$의 높은 수소발생속도를 가지는 것으로 나타났다.

• Journal of Korean Ophthalmic Optics Society
• /
• v.12 no.4
• /
• pp.15-18
• /
• 2007

Titanum and its alloys have been used as the important materials of eyewear frame because of its light weight, mechanical strength and corrosion resistance. This study investigated the characteristics of P-Ti and ${\beta}$-Ti alloys in accordance with joining conditions by electric resistance brazing. Hardness of P-Ti and ${\beta}$-Ti has been reduced because of an increase of grain size by the growth of brazing time. Hardness reduction and reduction area by grain growth were largely appeared in P-Ti than ${\beta}$-Ti.

• Journal of Korean Ophthalmic Optics Society
• /
• v.12 no.3
• /
• pp.55-58
• /
• 2007

Titanum and its alloys have been used as the important materials of eyewear frame due to its light weight, mechanical strength and corrosion resistance. This study investigates hardness and microstructures of titanum alloy groove wires in according with heating time by high frequency induction heating. Because of increase of grain size by the growth of heating time, hardness of ${\beta}-Ti$ has reduced. Hardness of Ti-325 reduced until 2 sec and rapidly increased at 3 sec by high frequency induction heating. It is observed that hardness of Ti-325 reduces by the increase of the grain size until 2 sec and suddenly increased by the development of the detailed ${\alpha}+{\beta}$ lamella at 3 sec.

• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.11 no.2
• /
• pp.60-66
• /
• 2001

($Ba_x$$Sr_{1-x}$)$TiO_3$ (x=0.9, 0.7, 0.5) thin films were prepared on ITO-coated glass by sol-gel method. Perovskite phase formation temperature of BST thin films seemed to be higher than $600^{\circ}C$. Peaks of perovskite phase shift to high diffraction angles as the Sr/(Ba+Sr) ratio was increased, due to the smaller ionic size of $Sr^{2+}$ than $Ba^{2+}$ . As a heating temperature was increased, the grain became coarser. And as Sr/(Ba + Sr) ratio was increased, the grain became finer. Dielectric constants of the BST(50/50) thin film are higher and dielectric losses of that are lower than those of the others. Dielectric constant and dielectric loss of the BST(50/50) thin film were 652 and 0.042 at 1kHz, respectively.

• Korean Journal of Materials Research
• /
• v.5 no.5
• /
• pp.518-524
• /
• 1995

BST 박막을 Pt/Si$O_{2}$/Si 기판위에 co-sputtering 방법으로 형성할 때 열처리 조건이 BST 박막의 결정성과 전기적 특성에 미치는 영향에 관하여 연구하였다. BST 박막의 특성은 급속 열처리나 관상로 열처리과 같은 후열처리 온도보다 증착시의 기판온도에 따라서 민감하게 변화하였고, 기판 온도를 55$0^{\circ}C$로 하여 증착할대 Perovskite상이 가장 안정적으로 성장하여 유전율 1100, 유전손실계수 0.02로 우수한 유전특성을 나타내는 막을 형성할 수 있었다. BST 박막은 기판온도를 증가하면 정합에너지와 표면에너지를 최소로하는 (111) 방향으로 우선방위를 나타내었고 결정립의 조대화 현상으로 누설전류가 증가하였다.

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