• Proceedings of the Korean Nuclear Society Conference
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• 1996.05c
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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$^{\circ}C$.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.20 no.10
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• pp.3234-3242
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• 1996

This paper describes experimental results of modified small punch( MSP) test conducted to evaluate the fracure characteristics and mechanical properties of plasma sparayed zirconia ($ZrO_2$ stabilized with 8wt. % $Y_20_3$ : YSZ) NiCrAlY composite. The mixing ratios of YSZ/NiCrAlY were 0/100, 25/75, 50/50, 100/0 v.%. Test temperatures ranged from 293K to 1473K. This study is directed at development of thermal barrrier coating(TBC) system with superior heat resistance and mechanical properties. The microstructure and fracture process of the composite were examined by SEM and AE method. The mechanical properties of 100% YSZ were nearly independent of the temperatures tested in this study. In contrast, the NiCrAlY-containing composites showed a significant decrease of the mechanical properties above 1273K, showing a ductile- brittle transition behavior up to the temperature. Furthermore, it can seen that 25% YSZ/75% NiCrAlY composite gave the highest fracure strength and fracture energy among the mixing ratio tested over the temperature range.

• 한국가스학회:학술대회논문집
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• 2006.11a
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• pp.183-186
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• 2006

• Proceedings of the KSME Conference
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• 2000.04a
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• pp.43-48
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• 2000

In this study, a small punch creep(SP-Creep) test using miniaturized specimen$(10{\times}10{\times}0.5mm)$ has been described for the development of the newly semi-destructive creep test method for high temperature structural components such as headers and tubes of boiler turbine casino and rotor and reactor vessel. The SP-Creep testing technique has been applied to 2.25Cr-1Mo(STBA24) steel used widely as boiler tube material and the creep test temperature are varied at $550^{\circ}C{\sim}600^{\circ}C$. The overall deformations of SP-Creep curves are definitely depended with applied load and creep test temperature and show the creep behaviors of three steps like conventional uniaxial creep curves. The steady state creep rate${\delta}_{ss}$ of SP-Creep curve for miniaturized specimen increases with increasing creep temperature, but the exponential value with creep loading is decreased. The activation energy$(Q_{spc})$ during SP-Creep deformation with various test temperatures shows 605.7kJ/mol that is g.eater than 467.4kJ/mol reported in uniaxial creep test. This may be caused by the difference of stress states during creep deformation In two creep test. But from the experimental results, e.g. SP-Creep curve behaviors, the steady state creep rate${\delta}_{ss}$ with creep temperature, and the exponential value(n) with creep loading, it can be summarized that the SP-Creep test may be a useful test method to evaluate the creep properties of the heat resisting material.

• Journal of the Korean institute of surface engineering
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• v.46 no.3
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• pp.126-132
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• 2013

It was investigated that the effects of hydrogen charging on the properties of 590 MPa Dual Phase(DP) steels at the surface layers. The hydrogen-charging time was changed from 5 to 50 hours and current densities from 100, 150, and 200 $mA/cm^2$, respectively. It was found that the hydrogen content in the specimen was increased with as the charging time and the current density. The microvickers hardness of the subsurface zone was increased from 215.3 HV to 239.5 HV due to the increase in current density and charging time. The comparison of the absorbed energies tested by a small-punch (SP) test showed that the absorbed energy of the specimen was greatly reduced from 436 to 283 $kgf-mm^2$ because of hydrogen embrittlement. It was confirmed that bulb aspects of fracture surface became more brittle with increasing hydrogen content.

• Journal of the Korean institute of surface engineering
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• v.46 no.5
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• pp.229-233
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• 2013

The small punch(SP) tests that can be applied to high strength sheet steel in automobile were carried out to evaluate the behavior of hydrogen embrittlement of DP sheet steels. In order to charge hydrogen at DP sheet steels, DP sheet steels were treated by the electrochemical hydrogen charging method under the charging conditions of current densities of 100, 150 and 200 $mA/cm^2$ for charging times of 5, 10, 25 and 50 hrs. Respectively, After hydrogen charging with experimental conditions, SP tests were performed. From the SP results, the correlations between the variation of bulb diameters and bulb heights with the hydrogen charging conditions were analysed. It was shown that the variation of bulb diameters were not significant with the hydrogen embrittlement due to the amounts of hydrogen charging. On the other hand, the bulb heights were observed to decrease with increasing hydrogen contents. It was thought that these results of the variation of bulb shapes after SP tests would be estimated as the index of evaluation of hydrogen embrittlement.

• Journal of the Korean Society for Nondestructive Testing
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• v.15 no.4
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• pp.549-560
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• 1996

The purpose of this study is to investigate the usefulness of small punch(SP) test using miniaturized specimens as a method for fracture strength evaluation of CANDU pressure tube embrittled by hydrogen. According to the test results, the fracture strength evaluation as a function of hydrogen concentration at $-196^{\circ}C$ was much better than that at room temperature, as the difference of SP fracture energy(Esp) with hydrogen concentration was more significant at $-196^{\circ}C$ than at room temperature for the hydrogen concentration up to 300ppm-H. It was also observed that the peak of average AE energy, the cumulative average AE energy and the cumulative average AE energy per equivalent fracture, strain increased with the increase of hydrogen concentration. From the results of load-displacement behaviors, Esp behaviors, macro- and micro-SEM fractographs and AE test it has been concluded that the SP test method using miniaturized specimen($10mm{\times}10mm{\times}0.5mm$) will be a useful test method to evaluate the fracture strength for CANDU pressure tube embrittled by hydrogen.

• International Journal of Safety
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• v.6 no.2
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• pp.1-7
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• 2007

In this study mechanical strength of A53B carbon steel was analyzed using several types of test specimens directly machined from oil recycling pipe experienced a failure due to hydrogen attack in chemical plants. High temperature hydrogen attack (HTHA) is the damage process of grain boundary facets due to a chemical reaction of carbides with hydrogen, thus forming cavities with high pressure methane gas. Driven by the methane gas pressure, the cavities grow on grain boundaries forming intergranular micro cracks. Microscopic optical examination, tensile test, Charpy impact test, hardness measurement, and small punch (SP) test were performed. Carbon content of the hydrogen attacked specimens was dramatically reduced compared with that of standard specification of A53B. Traces of decarburization and micro-cracks were observed by optical and scanning electron microscopy. Charpy impact energy in hydrogen attacked part of the pipe exhibited very low values due to the decarburization and micro fissure formation by HTHA, on the other hand, data tested from the sound part of the pipe showed high and scattered impact energy. Maximum reaction forces and ductility in SP test were decreased at hydrogen attacked part of the pipe compared with sound part of the pipe. Finite element analyses for SP test were performed to estimate tensile properties for untested part of the pipe in tensile test. And fracture toughness was calculated using an equivalent strain concept with SP test and finite element analysis results.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.2
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• pp.300-312
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• 1993

The material deterioration of service-exposed boiler tube steels in fossil power plant was evaluated by using the electrochemical technique namely, modified electrochemical potentiokinetic reactivation(EPR). It was focused that the passivation of Mo$_{6}$C carbide which governs the mechanical properties of Mo alloyed steels did not occur even in the passivity region of steel in sodium molybdate solution and the reactivation peak current (Ip) observed as the result of non-passivation indicating the precipitation of Mo$_{6}$C carbides. To obtain the optimal test conditions for the field test by using the specially designed electrochemical cell, the effects of scan rate, the surface roughness and the pH of electrolyte on Ip value were also investigated. Furthermore, the change of mechanical properties occurred during the long time exposure at high temperature was evlauated quantitatively by small punch(SP) tests and micro hardness test taking account of the metallurgical changes. It is known that reactivation peak current (Ip) has a good relationship with Larson-Miller Parameter(LMP) which represents the information about material deterioration occurred at high temperature environment. In addition it was possible to estimate the ductile-brittle transition temperature (DBTT) by means of the SP test. The Sp test could be, therefore, suggested as a reliable test method for evaluating the material degradation of boiler tube steels. From the good correaltion between the SP DBTT and Ip values shown in this study, it was knows that the change of mechanical properties could be evaluated non-destructively by measurring only Ip values.ues.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.7
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• pp.1241-1249
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• 2002

The extent of materials deterioration can be evaluated accurately by mechanical test such as impact test or creep test. But it is almost impossible to extract a large test specimen from in-service components. Thus material degradation evaluation by non-destructive method is earnestly required. In this paper, the material degradation for virgin and several aged materials of a Cr-Mo-V steel, which is an candidated as structural material of the turbine casing components for electric power plant, is nondestructively evaluated by reactivation polarization testing method. And, the results obtained from the test are compared with those in small punch(SP) tests recommended as a semi-nondestructive testing method using miniaturized specimen. In contrast to the aged materials up to 1,000hrs which exhibit the degradation behaviors with increased ${\Delta}[DBTT]_{SP}$, the improvement of mechanical property can be observed on the 2,000hrs and 3,000hrs aged materials. This is because of the softening of material due to the carbide precipitation, the increase of ferritic structures and the recovery of dislocation microstructure by long-time heat treatment. The reactivation rates($I_R/I_{Crit},\;Q_R/Q_{Crit}$) calculated by reactivation current densityt ($I_R$) and charge($Q_R$) in the polarization curves exhibit a good correlation with ${\Delta}[DBTT]_{SP}$ behaviors.