• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2001.10a
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• pp.293-296
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• 2001

The dynamic softening mechanisms of AISI 316, AISI 304 and AISI 430 stainless steels were studied with torsion test in the temperature range of $900 - 1200^{\circ}C$ and the strain rate range of $5.0x10^{-2}-5.0x10^0/sec$. The austenitic stainless steels, such as AISI 316 and AISI 304 were softened by dynamic recrystallization (DRX) during hot deformation. Also, the evolutions of flow stress and microstructure of AISI 430 ferritic stainless steel show the characteristics of continuous dynamic recrystallization (CDRX). To establish the quantitative equations for DRX of AISI 316 stainless steel, 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 ( $\varepsilon$ ), temperature (T), and strain ( $\varepsilon$ ) were established using the ${\epsilon}_c$ and ${\varepsilon}^{*}$. For the exact prediction the ${\varepsilon}_c,\;{\varepsilon}^{*}$ and Avrami' exponent (m') were quantitatively expressed by dimensionless parameter, Z/A, respectively. It was found that the calculated results were agreed with the experimental data for the steels at my deformation conditions. Also, we can reasonably conclude that the DRX, CDRX and grain refinement of stainless steels can be achieved by large strain deformation at high Z parameter condition.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2003.10a
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• pp.100-105
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• 2003

25Cr-20Ni series strainless steel have an excellent high temperature strength, high oxidation and high corrosion resistance. However, further improvement can be creep strength by work hardening prior creep. In the present study, the effect of prestraining at room temperature on the creep behavior of a Class M(STS310S) and a Class A(STS310J1TB) alloy containing precipitates have been examined. Prestraining was carried out at room temperature and range of prestrain was $0.5{\sim}2.5$ % at STS310J1TB and $2.0{\sim}7.0$ % at STS310S. Creep behavior and creep rate of pre-strained specimens were compared with that of virgin specimens. Room temperature prestraining produced the creep life that is longer than that of a virgin specimen both for STS310J1TB and STS310S when creep test carried out at the temperature lower than recrystallization temperature. The reason for this improvement of creep life was ascribable to the interaction between dislocations and precipitates in addition to the dislocation-dislocation interaction in STS310J1TB and the dislocation-dislocation interaction in STS310S. The beneficial effect of prestraining in STS310J1TB was larger than that of STS310S.

• Journal of the Korean Society for Nondestructive Testing
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• v.24 no.2
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• pp.158-162
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• 2004

Reliability of welded structures in power plant facilities is very important, and their reliability evaluation requires exact materials properties. But, the conventional PQR (Procedure Qualification Record) can hardly reflect the real material properties in the field because the test is only done on specimens with simulated welding. Therefore, a continuous indentation technique is proposed in this study for simple and non-destructive testing of in-field structures. This test measures the indentation load-depth curve during indentation and analyzes the mechanical properties such as the yield strength, tensile strength and work hardening index. This technique has been applied to evaluate the tensile properties of the weldment in the main steam pipe and hot reheater pipe in power plants under construction and in operation.

• Korean Journal of Materials Research
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• v.10 no.5
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• pp.335-342
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• 2000

The microhardness of rhenium sheets was determined as a function of indentation load and temperature. The temperature dependence of the microhardness between room temperatures and $1000 ^{\circ}C$was studied using a hot microhardness tester equipped with a Vickers indenter. The load dependence of the microhardness was investigated using oth a Vickers and a Knoop indenter. The indentation size effect (ISE) was well explained using the normalized Meyers law. The hardness of the annealed rhenium sheet approached that of the as-rolled sheets at large indentation loads because of work-hardening under the indenter during indentation. The hardness at zero load(obtained from extrapolation of the load dependence of the hardness) suggested that the hardness is controlled by two different mech-anisms having different thermal activation. At low temperature the activation energy for the mechanism controlling the hardness was approximately 0.02 eV , Whereas at higher temperatures that was approximately 0.15eV. The tranisi-tion temperature between the two different controlling mechanisms was about $250^{\circ}C$.

• Korean Journal of Materials Research
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• v.26 no.12
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• pp.709-713
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• 2016

Grain morphology, phase stability and mechanical properties in binary Ti-Al alloys containing 43-52 mo1% Al have been investigated. Isothermal forging was used to control the grain sizes of these alloys in the range of 5 to $350{\mu}m$. Grain morphology and volume fraction of ${\alpha}_2$ phase were observed by optical metallography and scanning electron microscopy. Compressive properties were evaluated at room temperature, 1070 K, and 1270 K in an argon atmosphere. Work hardening is significant at room temperature, but it hardly took place at 1070 K and 1270 K because of dynamical recrystallization. The grain morphologies were determined as functions of aluminum content and processing conditions. The transus curve of ${\alpha}$ and ${\alpha}+{\gamma}$ shifted more to the aluminum-rich side than was the case in McCullough's phase diagram. Flow stress at room temperature depends strongly on the volume fraction of the ${\alpha}_2$ phase and the grain size, whereas flow stress at 1070 K is insensitive to the alloy composition or the grain size, and flow stress at 1270 K depends mainly on the grain size. The ${\alpha}_2$ phase in the alloys does not increase the proof stress at high temperatures. These observations indicate that improvement of both the proof stress at high temperature and the room temperature ductility should be achieved to obtain slightly Ti-rich TiAl base alloys.

• Proceedings of the Korean Institute Of Construction Engineering and Management
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• autumn
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• pp.379-382
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• 2003

Construction concrete needs to maintain suitable environment which contains temperature and humidity etc. Then concrete shows own strength. The environment is not regular and contains many variables. Especially the climate element occupies many parts of variables. We have the climate environment which goes down to $-4^{\circ}C$. The factor that obstruct to construction is the failure at a construction progress. But it must be processed to be scheduled. Therefore we have to do the special care for factor of climate that obstruct to construction. We must make assurance doubly sure at the quality of concrete. We need maintenance of temperature and humidity for the hardening until the requirement period after a concrete pured in. We must do the care of curing sufficiently not to take the influence of injurious activity. This causes strength of concrete. Specific method of curing is according to each situation which is environment element. We wish to analyze curing course in construction of concrete at the paper. Also we wish to predict the problem as to consider curing and suggest the improvement plan through the paper.

• Journal of the Korean Society for Precision Engineering
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• v.18 no.4
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• pp.142-150
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• 2001

Automotive manufactures have taken more interests in tailored sheet metals for improving the rigidity, weight reduction, crash durability, and cost savings so that their application to auto-bodies has been increased. However, since the tailored sheet metals do not behave like un-welded sheet metals in press forming operations, the stamping engineers no longer rely only on conventional forming techniques. Futhermore, there is no clear understanding of the characteristics of welded metal which influence the overall press formability of tailored sheet metals. Recently, the computer simulations are prevailing for the evaluation of the formability. Unfortunately, the mechanical property of tailored sheet metal has to be quantitatively defined in the simulation. In this study, the analytical equations are formulated in order to find the mechanical properties of the welded metal in the tailored sheet metal welded by co$_2$laser. Based on force distribution assumption, the constitutive behavior of the welded metal is investigated using uniaxial tensile test results of base metals and tailored sheet metal. Then, the strength coefficient, work-hardening exponent, and plastic strain ratio of laser-welded metal are calculate from those of base metals and tailored sheet metal. In addition, the existence of weld defects in the welded metal is indirectly detected by examining the slop of strength coefficient of the welded metal.

• Transactions of Materials Processing
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• v.27 no.5
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• pp.281-288
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• 2018

This study deals with the microstructure and tensile properties of 600 and 700 MPa-grade high-strength seismic reinforced steel bars. High-strength seismic resistant reinforced steel bars (SD 600S and SD 700S) were fabricated by TempCore process, especially the SD 700S specimen was more rapid cooled than the SD 600S specimen during the TempCore process. Although two specimens had microstructure of tempered martensite in the surface region, the SD 600S specimen had ferrite-degenerated pearlite in the center region, whereas the SD 700S specimen had bainite-ferrite-degenerated pearlite in the center region. Therefore, their hardness was highest in the surface region and revealed a tendency to decrease from the surface region to the center region because tempered martensite has higher hardness than ferrite-degenerated pearlite or bainite. The SD 700S specimen revealed higher hardness in the center region than SD 600S specimen because it contained a larger amount of bainite as well as ferrite-degenerated pearlite. On the other hand, tensile test results indicated the SD 600S and SD 700S specimens revealed continuous yielding behavior because of formation of degenerated pearlite or bainite in the center region. The SD 600S specimen had a little higher tensile-to-yield ratio because the presence of ferrite and degenerated pearlite in the center region and the lower fraction of tempered martensite enhance work hardening.

• Korean Journal of Materials Research
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• v.14 no.10
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• pp.713-718
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• 2004

To study the dynamic strain aging behavior of Zr-0.4Sn-1.5Nb-0.2Fe sample tube for nuclear fuel cladding in the range of pressurized water reactor (PWR) operation temperature, the tensile tests of the tube specimens, which had been finally heat-treated at $470^{\circ}C\;and\;510^{\circ}C$, had been carried out with the strain rate $1.67{\times}10^{-2}/s\;and\;8.33{\times}10^{-5}/s$ at the various temperatures from room temperature to $500^{\circ}C$. It was observed that the elongation of the specimens got shortened as the temperature increased from $200^{\circ}C\;to\;340^{\circ}C$. The specimens that were finally heat-treated at $470^{\circ}C$ showed a plateau more remarkably on the plot of yield strength-temperature than those heat-treated at $510^{\circ}C$. In the range of $310\sim400^{\circ}C$, the strain rate sensitivity of the specimens finally heat-treated at $510^{\circ}C$ was $30.4\%\sim33.7\%$ lower but the work hardening exponent index of the specimens was a little higher than that without dynamic strain aging effect.

• Journal of the Korean Geotechnical Society
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• v.19 no.5
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• pp.199-210
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• 2003

The development of strain localization within shear zones is frequently observed during soil deformation. In fact, the phenomenon appears to be more often the norm rather than the exception. Conceptually, any soil condition that renders negative work increment is prone to localization. In this study, a broad range of soil and loading conditions are investigated to test this criterion, including: dilative soil subjected to drained shear (standard case), contractive soil sheared under undrained conditions, cavitation in dilative soil in undrained shear, inhomogeneous soils, particle alignment in contractive soils made of platy particles, soils that experience particle crushing, and the shear of low-moisture and/or lightly cemented loose soils. Unique specimens and test procedures are designed to separately test each of these soil conditions in the laboratory According to experimental test results, soil specimens with post-peak strain softening behavior are prone to progressive failure, localization of deformations, and shear banding. The state of stress, the soil density, inherent mechanical and geometrical properties of soil particles, low water content, and heterogeneity can contribute to triggering strain localization. Considering all possible cases of localization, the best method to obtain the critical state line in the laboratory is to use contractive homogeneous specimens subjected to drained shear.