• 열처리공학회지
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• 제11권1호
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• pp.54-61
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• 1998

Since Cr-Mo-V steel has excellent fracture and creep properties at elevated temperature, they are extensively used as steam turbine components such as the turbine rotor. However, the turbine rotor steel used to suffer material degradation during long term service. Therefore, the assessment of the safety and residual life of the turbine rotor is periodically required during service. One of the most convenient techniques for that is the hardness method mainly due to its simplicity and nondestructive characteristics. In this research, six specimens with different aging times of turbine rotor steel were artificially prepared by an isothermal heat treatment at $630^{\circ}C$. The micro Vickers hardnesses of specimens were measured at room temperature. The relationships between the fracture properties and the hardness ratio were investigated. And also an indirect method to evaluate the residual life of degraded turbine rotor was proposed based on the micro Vickers hardness measurement.

• 열처리공학회지
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• 제6권3호
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• pp.152-158
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• 1993

Effects of oxide film which is grown by heat treatment on the hydrogen embrittlement (HE) of 17-4 PH stainless steel were investigated. Specimems were tensile tested after cathodic hydrogen charging and the be haviors of HE were evaluated from the elongation change. It was found that specimems solution treated at $1040^{\circ}C$ for 1/2hour showed best retardation ability to HE for both aging conditions i.e. $480^{\circ}C{\times}1hr$ and $620^{\circ}C{\times}4hrs$. X-ray and Auger study revealed that the major composition of oxide films are $Cr_2O_3$ and $CuCr_2O_4$.

• 열처리공학회지
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• 제24권3호
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• pp.144-148
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• 2011

Change in damping capacity with strain amplitude was studied in Mg-Al-Si alloy in as-cast, solution-treated and aged states, respectively. The as-cast microstructure of the alloy is characterized by eutectic ${\beta}$($Mg_{17}Al_{12}$) phase and Chinese script type $Mg_2Si$ particles. The solution treatment dissolved the ${\beta}$ phase into the matrix, while the aging treatment resulted in the distribution of continuous and discontinuous type ${\beta}$ precipitates. The solution-treated microstructure showed better damping capacity than as-cast and aged microstructures both in strain-dependent and strain-independent damping regions. The decrease in second-phase particles which weakens the strong pinning points on dislocations and distribution of solute atoms in the matrix, would be responsible for the enhanced damping capacity after solution treatment.

• 열처리공학회지
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• 제8권4호
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• pp.245-254
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• 1995

The purpose of this study was to examine the effects of low melting point phase(LMPP) on mechanical properties in the Al-Cu-Li-X(In, Be) alloys. This study was performed by the differential scanning calorimetry(DSC), the transmission electron microscope(TEM), hardness test, tensile test and notch tensile test. The shape of LMPP in the specimens homogenized at $570^{\circ}C$ was film type due to remelting at grain boundary during homogenization. Low melting point phases had no effects on mechanical properties in the aging treated materials, because the density of LMPPs was low. Mechanical properties of the aging treated materials were affected by the density of matrix precipitation phases and grain sizes. For the In or In, Be added Al-Cu-Li alloys, the optimum solution treatment temperature was $550^{\circ}C$. The strength of Al-Cu-Li-In-Be $T_6$ treated alloy was higher than that of 2090-$T_8$ alloy.

• Journal of Welding and Joining
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• 제16권3호
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• pp.121-128
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• 1998

The degradation of mechanical properties in the high nitrogen steel HN3 developed for nuclear fusion reactor has been evaluated quantitatively using the small punch(SP) test, X-ray diffraction (XRD) analysis has also been conducted to identify carbides or nitrides precipitated on grain boundaries of the heat treated samples. Mechanical properties of the steel HN3 significantly decreased with increasing heat treatment time and temperature or with decreasing testing temperature. Combination of XRD and metallurgical observation, revealed that the material degradation in the thermally aged steel was caused by precipitation of carbides on the grain boundaries. While the weld metal showed the lowest mechanical properties among various microstructures in GTA weldments. By combining SP test and XRD analysis, cryogenic fracture behaviors and aging degradation for high nitrogen steel could be successfully evaluated in nondestructive manner.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2006년도 추계학술대회 논문집 Vol.19
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• pp.235-236
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• 2006

The contact resistance and grain boundary potential barrier of ceramic $BaTiO_3$ PTCR were investigated. The electroless plated Ni, evaporated Al, and Ag paste were chosen as electrode materials of PTCR device for comparison analysis before and after heat treatment. The contact resistance of electrode were measured by electrometer (dc), digital multimeter (dc), and LCR meter (ac). In the case of Al electroded samples, the heat treatment and protective oxide layer had high resistance and effect on the stability of PTCR effect against contact resistance degradation, but the Ag-paste had comparably high contact resistance before heat treatment and decreased after heat treatment with safe. On the other hand, the samples with electroless plated Ni electrode had good properties of contact resistance against aging.

• 열처리공학회지
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• 제4권3호
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• pp.13-20
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• 1991

The present work was aimed to examine the variation of precipitations and mechanical properties by thermomechanical treatments (TMT) in Al-2.19 wt%Li and Al-2.0 wt%Li-0.11 wt%Zr alloys. This study was performed by TEM, SEM observation, DSC, electrical resistance measurement, hardness and tensile strength measurement. First peak of resistivity aged at $90^{\circ}C$ was caused by precipitation of ${\delta}^{\prime}$-precursor phase, and second peak was caused by precipitation of ${\delta}^{\prime}$ phase. According to this result, the precipitation process of Al-2.19 wt%Li alloy was as follow : $SSSS{\rightarrow}{\delta}^{\prime}$-precursor phase ${\rightarrow}{\delta}^{\prime}$ (Coherent ${\rightarrow}$ Semi-coherent) ${\rightarrow}{\delta}$. In a Al-2.0 wt%Li-0.11 wt%Zr ternary alloy, the first peak of resistivity was appeared at initial aging heat-treatment. It is result from exsistant of ${\delta}^{\prime}$-precursor phase. The effect acceleration in a binary alloy was not appeared and the over-aging ternary alloy was accelerated with increase of the reduction rate. It is caused by combination effect of ${\delta}^{\prime}$ and composite phase.

• 열처리공학회지
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• 제10권4호
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• pp.255-265
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• 1997

Precipitation and strengthening mechanisms in squeeze cast Mg-8.5wt%Li-4.5wt%Al have been investigated by differential scanning calorimetry(DSC), scanning electron microscopy(SEM), in-situ and ex-situ X-ray diffraction analysis and hardness measurement. Special emphasis was placed on the investigation of the precipitation behavior by the DSC technique. Microstructural and calorimetric analysis showed that ${\theta}$ and ${\delta}$ precipitates in the b.c.c. ${\beta}$ phase matrix, forming two exothermic peaks at the temperature ranges of $130^{\circ}C{\sim}180^{\circ}C$ and $236^{\circ}C{\sim}280^{\circ}C$. ${\theta}$ and ${\delta}$ dissolve into the matrix forming an endothermic peak at the temperature range of $280^{\circ}C{\sim}352^{\circ}C$. The as-cast microstructure consists of ${\alpha}$, ${\beta}$ and ${\delta}$. Peak strength was obtained after aging for 1 hour at $50^{\circ}C$. The aging time required for the peak strength decreased as the aging temperature increases. The hardness decrease during overaging was due to the coarsening of ${\theta}$ precipitates. Microhardness measurement showed that variation of the hardness of ${\beta}$ matrix was more pronounced than that of the ${\alpha}$ phase, indicating that the ${\beta}$ phase is more responsible for the strengthening of the Mg-8.5wt%Li-4.5wt%Al alloy.

• 대한금속재료학회지
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• 제49권2호
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• pp.121-127
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• 2011

The mechanical properties of the various heat treatment conditions on Ti-15Mo-3Nb-3Al-0.2Si alloy plates were examined. XRD patterns from the surface of Ti-15Mo-3Nb-3Al-0.2Si were analyzed as a solution-treated Ti alloy has the single-phase ${\beta}$ structure whereas the aged Ti alloys have the ${\beta}$ matrix embedded with ${\alpha}$ needles. High strength (~1500 MPa) with decent ductility (7%) was obtained by the Ti alloy double aged at $300^{\circ}C$ and $520^{\circ}C$ for 8 hours each. The double-aged alloy exhibits the finer structure than the single-aged alloy at $300^{\circ}C$ for 8 hours because of the higher nucleation rate of ${\alpha}$ needles at an initial low aging temperature ($320^{\circ}C$). TEM observation revealed that the fine nanostructure with ${\alpha}$ needles in the ${\beta}$ matrix ensured the excellent mechanical properties in the double aged Ti-15Mo-3Nb-3Al-0.2Si alloy. In the solution treated alloy, the yield drop, stress-serrations and the ductility minimum typically associated with dynamic strain aging can be attributed to the dynamic interaction between dislocations and oxygen atoms. The yield drop and the stress serration were not observed in aged samples because the geometrically introduced dislocations due to phase precipitates suppressed the dynamic strain aging.

• 한국원자력학회:학술대회논문집
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• 한국원자력학회 1996년도 춘계학술발표회논문집(3)
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• pp.79-84
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• 1996

Small punch test was performed on CF8 duplex stainless steel aged at 370 and 400$^{\circ}C$ up to 5,000 h to evaluate the degree of the thermal aging embrittlement. At room temperature, the SP load-displacement curve was in a similar shape to those of ferritic steels and had a good reproducibility in spite of two-phase structure. The aging heat treatment resulted in a slight increase of the yield strength. As test temperature was lowered, the SP load showed a sudden drop followed by serrations before the SP specimen was fractured, resulting from the cracking of ferrite phase. The extent of thermal embrittlement was assessed in terms of the SP energy. Aging treatment at higher temperature led to a larger shift in the transition temperature and the corresponding change in the fracture mode. The main cause of the degradation was the embrittlement of ferrite phase. Additionally the phase boundary separation profoundly contributed to the degradation of the specimen aged at 400$^{\circ}C$.