• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 1995.06a
• /
• pp.100-123
• /
• 1995

The relationship between plastic deformation and crystal grain change in warm forging processes of SM100 carbon steel is studied. If the carbon steel is deformed in warm forging temperature (about recrystallization range), the crystal grain and cementite of the internal part are changed, so material properties are changed. Some experimental values, such as the elliptic degree of cementite, the grain size of cementitie and ferrite grain size, are investigated. When the plastic deformation proceeds, the elliptic degree of cementite becomes large, the grain size of cementite particle is small, and the size of ferrite grain appears fine by recrystallization. The elliptic degree of cementite has a considerable effect on formability. The distribution of effective strain in the forging is calculated by the rigid visco-plastic FEM analysis. The effective strain distribution obtained from the FEM simulation is compared with the experimental result. At effective strain 0.3 dynamic recovery and dynamic recrystallization begin, over 2.5 the organization of material has better quality that is suitable for the following cold forming.

• 연구논문집
• /
• s.22
• /
• pp.127-139
• /
• 1992

The mechnial properties such as fracture strength, ductility and fatigue strength of Cu shape memory alloy are lower than those of Ti-Ni SMA, because of their high elastic anisotropy and large grain size. And in order to improve the mechanical property of Cu SMA, some techniques such as casting method by addition of refining element, powder metallurgy and rapid solidification process have been studied on the refinement of the grain size of Cu SMA. This study was carried out to refine the grain size of CuAlNi SMA by applying the melt spinning method. According to this study, the conclusions are as follows; - grain size of the melt-spun ribbon was about $1\mum$ - there was not change in grain size, although increasing of hot pressing temperature -grain size of the hot-extruded specimen was about $30-40\mum$, it is more refiner than that of castings

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.11 no.6
• /
• pp.867-876
• /
• 1987

Corrosion fatigue fracture with change in the grain size of M.E.F. dual phase steel is investigated in 3.5% NaCI aqueous solution at pH 2, 4, 6, 9, and 11. Generally speaking, decrease in corrosion fatigue life is strongly dependent on decrease in pH and slightly on the grain size. For the B material with the big grain size, the fatigue life is small due to its large reduction ratio of corrosion fatigue life. The influence of grain size on the reduction ratio of corrosion fatigue life is large at pH 11-6. Whi9le at pH 4-2 the reduction ratio of corrosion fatigue life only depends on the corrosion effect. The larger grain size and the lass pH result in the greater influence on corrosion fatigue crack propagation rate. As pH decreases, the plateau portion in the crack propagation rate curves of the B material are distinct. Crack propagation rate curves become slow down at high .DELTA.K range because crack closure effect by minute corrosion products inside crack causes the oxidation corrosion action less effective for a certain period of time. In A material with small grain size, fatigue life is increased in proportion with increase of martensite intergranular which brings forth restraining the crack propagation decreases crack propagation rate. Corrosion pit which is created in the surface of specimen is found at pH 6,4 and 2 which is noticeable and the unevenness of the surface of the specimen becomes severe as pH decreases. The unevenness of corrosion fatigue fracture surface is severe as the effect of pH increases i.e. as pH decreases. In proportion with increase in the grain size and decrease in pH, the aspect of brittle fracture becomes evident.

• KOREAN JOURNAL OF CROP SCIENCE
• /
• v.47 no.1
• /
• pp.36-41
• /
• 2002

Two rice varieties, 'Oochikara' with large grain and 'Hwayeongbyeo' and their progenies (F$_1$, F$_2$, B$_1$ and B$_2$) were tested to understand gene action of morphological traits of rice grain and their relationships. The evaluated traits were 1,000-grain weight, grain length, width, thickness, length-width ratio and chalkiness of brown rice. Correlation between grain weight and chalkiness was highly significant in the all progenies, and grain length were not associated with width and thickness in an F$_2$ population. Scaling test and jonit scaling test revealed that inheritance of grain traits were fitted to additive-dominance model without epistasis. Additive effects for the traits were much greater than the dominance effects.

• Journal of the Society of Naval Architects of Korea
• /
• v.34 no.1
• /
• pp.102-110
• /
• 1997

In a shipyard, computer calculation is not frequently used for the grain stability calculation because of large difference between calculation values and real values. Therefore, the necessary calculation process for grain stability is done manually. GUI(Graphical User Interface) is adopted for the convenience of users and interactive data I/O. The hold shape (girder, hold, etc.)needed for calculation are visualized using GLBAX which is a 3 dimensional graphic library. The interface with the ship basic calculation package is also implemented. The aim of this paper is to develop a reliable interactive grain stability calculation program which reduces computational time, and is to computerize the grain stability calculation procedure.

• Journal of Advanced Marine Engineering and Technology
• /
• v.29 no.8
• /
• pp.938-945
• /
• 2005

The microstructure evolution in hot forging process is composed of dynamic recrystallization during deformation as well as grain growth during dwell time. Therefore, the control of forging parameters such as strain, strain rate. temperature and holding time is important because the microstructure change in hot working affects the mechanical properties. Modeling equations are developed to represent the flow curve. grain size. recrystallized volume fraction and grain growth phenomena by various tests. The developed modeling equations were combined with thermo-viscoplastic finite element modeling to predict the microstructure change evolution during hot forging process. The large exhaust valve spindle (head diameter of 512mm) was simulated by closed die forging with hydraulic press and cooled in air after forging. The preform was heated to each 1080 and 1150$^{\circ}C$. Numerical calculation was performed by DEFORM-2D. a commercial finite element code. Heat transfer can be coupled with the deformation analysis in a non-isothermal deformation analysis. In order to obtain the fine and homogeneous microstructure and good mechanical properties in forging. the FEM would become a useful tool in the simulation of the microstructure development. In forging, appropriate temperature, strain and strain rate and rapid cooling are required to obtain the fine grain microstructure The optimal forging temperature and effective strain range of Nimonic 80A for large exhaust valve spindle are about 1080$\∼$l120$^{\circ}C$ and 150$\∼$200$\%$.

• Proceedings of the KSME Conference
• /
• 2007.05a
• /
• pp.326-331
• /
• 2007

Large-scale molecular dynamics simulations are performed to verify the deformation characteristics of grain boundaries in nanolithography process. The copper substrate made of 200,000 atoms is constructed by two grains in different crystal orientations using dynamic relaxation method. The grain boundary is located in the middle of the substrate with $45\sim135$ degree angles. The plowing tip is made of diamond-like-carbon atoms in a variety of shapes. In the simulations, the generation, propagation, and accumulation of dislocations are observed inside the substrate. From the numerical results, we address the dynamic behavior of the grain boundaries as well as the frictional characteristics in terms of the morphology of initial grain boundaries.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2003.10b
• /
• pp.123-126
• /
• 2003

A high strength aluminum alloy bolt (A7050, T7 temper treatment) has been developed by the authors. The bolt has a small grain size in the whole area of the bolt because of the large equivalent strain followed by thermo-mechanical treatment. As the bolt made of A 7050 has a risk of stress corrosion cracking, each grain should be strengthened the grain inside than the grain boundary in order to improve the stress corrosion cracking resistance. It has been confirmed that the nano-indentation hardness at each grain inside increased with the increasing equivalent strain by thermo-mechanical treatment processing.

• Journal of the Korean Ceramic Society
• /
• v.32 no.6
• /
• pp.653-658
• /
• 1995

Directional effect of applied pressure during sintering on the microstructure and fracture toughness of the heat-treated silicon nitride ceramics has been investigated. The specimens with a composition of 92Si3N4-8Y2O3(in wt%) were sintered at 172$0^{\circ}C$ by a hot press (HP ) and a hot isostatic press (HIP) and heat-treated for grain growth at 1800~20$0^{\circ}C$. The fracture toughness of the HP samples increased with the grain size while the fracture toughness of the HIP treated samples remained the same even though the grain growth occurred. This discrepancy was explained by a bimodal grain size distribution and large aspect ratio of the HPed samples and a monomodal grain size distributjion and samll aspect ratio of the HIP treated samples.

• Electrical & Electronic Materials
• /
• v.8 no.1
• /
• pp.40-47
• /
• 1995

Electrical properties of ZnO ceramics based on Bi oxide was investigated in relation to sintering temperature. In the temperature range >$1150^{\circ}C$ to >$1350^{\circ}C$ the grain size increased from 9.mu.m to 20.mu.m when the sintering temperature was raised. The leakage current in the low voltage range increased as the potential barrier decreases, which is caused by increasing the grain size at high temperature. The dielectric characteristics of the ZnO ceramics was also affected by sintering temperature. Large dielectric constant was attributed, to the grainboundary layer of polycrystalline ZnO ceramics and decreasing grainboundary width. The variation of breakdown voltage with sintering temperature was attributed to the change of the donor concentration in the ZnO grain and grain size. The results showed that breakdown voltage increased decreasing grain size and donor concentration. Nonohmic coefficient was associated with the lower breakdown voltage per grainboundary layer due to the grain growth and higher donor concentration.