• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.1
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• pp.131-140
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• 1993

The activation energy for high temperature creep is associated with stresses, temperatures, straians And the creep strain appears to be a function of a temperature, compensated time, namely $te^{-}$.DELTA.H/RT/, and the stress. In fact this functional relation appears to be isomorphic to material structure by x-ray analyses. Applying this functional relation, the dependance of activation energy for A17075 creep was investigated. The activation energy for creep is insensitive to stress, temperature, structure, and strain. And phenomenological model agrees with experimental creep data.

• Korean Journal of Materials Research
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• v.2 no.5
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• pp.343-352
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• 1992

The high temperature deformation behavior of microalloyed hot forging steels has been examined as a function of the temperature, the strain rate, and the alloying element by using high temperature compression test. The high temperature deformation mechanism, which was obtained by analyzing the flow stress-strain curve and microstructure, could be considered to dynamic recrystallization. The peak stress of Nb-V-Mo steel was more increased and the dynamic recrystallization of Nb-V-Mo steel was faster than those of Nb-V steel. The peak stress of 1.2Mn-0.09Nb steel was more increased and the dynamic recrystallization of 1.2Mn-0.09Nb was delayed a little bit than those of 1.0Mn-0.05Nb. The peak stress of C-Nb-V steel was more increased and the dynamic recrystallization of C-Nb-V steel was delayed than those of C-steel. The constitutive equation of high temperature deformation had a power law type. The grain size of dynamic recrystallization was refined as the Zener-Hollomon parameter was increased. The relation of the dynamic recrystallization grain size and Zener-Hollomon parameter could be quantified to power law.

• Advances in materials Research
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• v.3 no.4
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• pp.217-225
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• 2014

In this study, a forging steel alloyed with both Nb and V was used as experimental material and the hot deformation behavior has been studied for this steel by conducting the compressive deformation test at temperature of $900-1150^{\circ}C$ and strain rate of $0.01-0.01s^{-1}$ in a MMS-300 thermo-mechanical simulator. The microstructure evolution, particularly the dynamically recrystallized microstructure, of the experimental steel at elevated temperatures, strain rates and strain levels, was characterized by optical microstructural observation and the constitutive equation in association with the activation energy and Zener-Hollomon parameter. The curves of strain hardening rate versus stress were used to determine the critical strain and peak strain, and their relation was connected with Zener-Hollomon parameter. Under the conditions of processing temperature $900^{\circ}C$ and strain rate $0.01s^{-1}$, the dynamic recrystallization took place and the austenite grain size was refined from $164.5{\mu}m$ to $28.9{\mu}m$.

• Transactions of Materials Processing
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• v.7 no.2
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• pp.168-176
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• 1998

Hot workability of the AA5083 alloy ws investigated by torsion test at temperature ranges of $350{\sim}520^{\circ}C$ and strain rates of 0.5, 1.0, and 3.0/sec. The flow stress and hot ductility of the AA5083 alloy as a function of deformation variables such as temperature and train rate were studied. The microstructural evolution of the AA5083 alloy was studied in relation to Zener-Hollomon parameter (Z=exp( /RT) Also the hot restoration mechanism of the AA5083 alloy was small when Z val-ues were higher than $1.73{\times}1016/sec(370^{\circ}C,\;0.5/sec)$ In addition the difference microstructures during hot deformation. It was found that the increase of flow curves and deformed microstructures during hot deformation. It was found that the increase of flow stress of the AA5083 alloy was small when Z val-ues were higher than $1.73{\times}1016/sec(370^{\circ}C.\;0.5/sec)$. However under the low Z values less than $1.73{\times}1016/sec(370^{\circ}C,\;0.5/sec)$ the flow stress increase with increasing the Z values. The large dispersoid particles in the matrix grain decreased the flow strain of the AA5083 alloy because it caused the stress concentration during hot deformation.

• Transactions of Materials Processing
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• v.7 no.2
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• pp.158-167
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• 1998

Hot restoration mechanism flow stress and stain of the Al2024 composites reinforced with 1,8,15,36, and $44{\mu}m\;SiC_p$(10 vol. %) were studied by hot torsion tests. The hot restoration mechanism of all the composites was found to be dynamic recrystallization(DRX) at $320^{\circ}C$ while that of the composites reinforced with 1 and $8{\mu}m\;SiC_p$ was found to be dynamic recovery(DRX) at $480^{\circ}C$. It was found that the Al2024 composite with $15{\mu}m\;SiC_p$ showed the highest flow stress(${\sim}$223 MPa) at $320^{\circ}C$ under a strain rate of 1.0/sec. Also the highest flow strain of the composites was obtained at $430^{\circ}C$. The com-posites reinforced with 1 and $8{\mu}m\;SiC_p$ showed lower flow stress and higher flow strain at $480^{\circ}C$ than those of the composites reinforced with 15, 36, and $44\;{\mu}m\;SiC_p$ These result were discussed in relation to the transition of the hot restoration mechanism. $DRX{\leftrightarrow}DRV$. The dependence of flow stress on strain rate and temperature was attempted to fit with the hyperbolic sine equation ($\dot{\varepsilon}=A[sinh({\alpha}{\cdot}{\sigma}_p]^n$ exp(-Q/RT)and Zener-Hollomon parameter($Z=\;\dot{\varepsilon}\;exp(Q/RT))$.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.32 no.1
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• pp.21-28
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• 2008

Stress-strain curves based on Ramberg-Osgood and Hollomon relations are strongly dependent upon the regressed range of strain. This work investigates mathematical expressions of true stress-strain curves of metallic materials. We first observe the variation of yield strength, strain hardening exponent and stress-strain curve with regressed range of stain. Based on sectional regression and expression using one or two parameters, we propose an optimal strain range for which yield strength and nonlinear material behavior are quite appropriate.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.16 no.1
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• pp.49-55
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• 2020

Creep rupture data for Alloy 690 steam generator tubes in a pressurized water reactor are essentially needed to demonstrate a severe accident scenario on thermally-induced tube failures caused by hot gases in a damaged reactor core. The rupture data were obtained using the tube specimens under different applied-stress levels at 650℃, 700℃, 750℃, 800℃, and 850℃. Important creep constants were proposed using various creep laws in terms of Norton power law, Monkman-Grant (M-G) relation, damage tolerance factor (λ), and Zener-Hollomon parameter (Z). In addition, a creep activation energy (Q) value for Alloy 690 tube was reasonably determined using experimental data. Creep behaviors such as creep strength, creep rates, rupture elongation showed the results of temperature dependence well. Modified M-G plot improved a correlation of the creep rate and rupture life. Damage tolerance factor for Alloy 690 tubes was found to be λ =2.20 in an average value. Creep activation energy for Alloy 690 tube was optimized for Q=350 (kJ/mol). A plot of Z parameter obeyed a good linearity, and the same creep mechanism was inferred to be operative in the present test conditions.