• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.12 s.255
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• pp.1526-1533
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• 2006

The aim of this research was to confirm the existence of the thermal creep degradation by hydrides of Zr-2.5Nb pressure tube materials. Small punch creep tests were performed to obtain the relationship between a creep displacement and a loading period at $300^{\circ}C$. A creep stress and a creep strain rate were also converted from the previous results. The creep material constants and the creep stress exponents at the different hydride contents were compared. Finally the hydrides of the axial and circumferential section were observed using OM, SEM and TEM. The following conclusions were made: 1) The degradation of the thermal creep by hydrides was existed and it strongly depended on the hydride contents. 2) As the hydride contents were increased, the creep stress exponents (m) were also increased. 3) Even though the hydride was not precipitated in 50 ppm materials at $300^{\circ}C$, the degradation of thermal creep was found. Therefore, it was believed that this phenomenon strongly related to the hydride precipitation at room temperature.

• Journal of the Korean Ceramic Society
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• v.37 no.9
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• pp.839-846
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• 2000

통전활성 소결법을 이용하여 단상의 세라믹스(PSZ) 및 금속(NiCrAlY)과 PSZ/NiCrAlY 복합체를 각 소결온도에서 제조하였으며, 이에 대한 특성평가는 개량형 소형펀치 시험법을 이용하여 수행하였다. 복합체의 밀도 및 경도에 미치는 소결온도의 영향은 NiCrAlY의 부피분율 증가에 따라 감소되었으며, 중간조성에서의 밀도 및 경도는 연속적인 변화를 나타내었다. 파괴인성의 변화에서는 60vol%-NiCrAlY 이상의 조성에서 급격한 증가를 나타내었으며, 짧은 소결시간 및 빠른 승온속도 등 통전활성 소결법의 장점에 의해 단상의 PSZ에서도 10 MPa.m$^{1}$2/ 의 높은 파괴인성 값을 얻을 수 있었다. PSZ/NiCrAlY 복합체에서의 특성은 소결온도와 NiCrAlY의 부피분율에 의해 지배적으로 영향을 받았으며, 단상의 PSZ은 전형적인 취성거동을 나타내었고 80vol%-NiCrAlY 까지의 복합체에서는 부분적인 취성 및 소성거동을 함께 나타내었다. 각 소결체에서 최고의 특성을 나타내고 소결온도는 단상의 PSZ 경우인 140$0^{\circ}C$에서부터 단상의 NiCrAlY 경우인 120$0^{\circ}C$까지 최대 20$0^{\circ}C$의 차이를 나타내었으며, PSZ과 NiCrAlY의 조성비에 따라서 연속적인 변화를 보였다.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2013.05a
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• pp.87-87
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• 2013

복합조직강의 미세조직제어를 통한 자동차용 고강도 강판재의 개발이 주요 연구과제로 있다. 하지만 고강도화에 따라 수소에 의한 지연파괴의 문제점이 있어, 이를 규명하고 해결하기 위한 많은 연구가 함께 이루어지고 있다. 본 연구에서는 연구, 개발되고 있는 복합조직강 중 DP강과 TRIP강의 수소취성에 미치는 미세조직의 영향을 분석하고, 수소주입조건에 따른 수소취성 및 지연파괴 거동에 대하여 조사하고자 하였다. 이를 위해 음극전기분해법을 이용, 주입수소량을 달리하여 주입수소가 복합조직강의 지연파괴에 미치는 영향을 분석하였다. Hydrogen determinator를 통해 시편 내 수소량을 측정하였고, 소형펀치시험에 의한 기계적물성 변화를 조사하였다. 또한 파단부위의 넓이와 깊이를 비교측정하였고, 파단면을 SEM으로 관찰하여 수소지연파괴 거동을 평가하고자 하였다.

• 대한공업교육학회지
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• v.35 no.2
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• pp.204-223
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• 2010

The purpose of this study was to investigate the high temperature creep behavior of anisotropic Zr-2.4%Nb alloy which includes the hydride. To minimize the effect of the anisotropy and to achieve the bi-axial stress condition, SP creep test was performed using the Zr alloys which have the 50 ppm, 100 ppm and 200 ppm hydride. Each SP creep curve was obtained and compared. While the creep degradation of 50 ppm and 100 ppm hydride specimens was clearly found, the degradation of 200 ppm was not cleared. By the comparison of SP creep constant and stree exponent, this fact was confirmed. As the degradation of 50 ppm and 100 ppm hydride was processed, the SP creep constant was decreased and the stress exponet was increased. However, while the SP creep constant of 200 ppm hydride was decreased, the stree exponent was decreased. Finally, it was confirmed that the creep degradation of 200 ppm was not found. In conclusion, the hydride was the major parameter to control the hight temperature creep degradation of Zr alloy.

• Journal of the Korean Society for Nondestructive Testing
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• v.19 no.6
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• pp.411-419
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• 1999

It has been well recognized that a long term service at elevated temperature of $350^{\circ}C{\sim}550^{\circ}C$ induces embrittlement damage due to carbide precipitation and/or P, Sb and Sn segregation at grain boundaries and thereby deteriorates the grain boundary strength of heat resisting components in the energy-related plants. Therefore, it is very important to assess quantitatively the extent of embrittlement damage of heat resisting components to secure the reliable and efficient service condition and to prevent brittle failure in service. However, because fracture tests are limited in size and number of specimen obtained from the structural components, nondestructive test method is required. In this study, the optimum electrochemical parameters are investigated and discussed to evaluate nondestructive embrittlement damage for aged 2.25Cr-1Mo steels by means of electrochemical polarization test method (ECPTM) in proper corrosive environment. In addition, the electrochemical test results are compared with embrittlement degree evaluated by semi-nondestructive SP test.

• Proceedings of the KSME Conference
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• 2001.11a
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• pp.143-148
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• 2001

This research was undertaken to clarify effects of thermal aging on electrochemical and mechanical properties of superaustenitic stainless steel. The steel was artificially aged at $300{\sim}650^{\circ}C$ for $240{\sim}10,000hrs$. and investigated at $-196{\sim}650^{\circ}C$ using small punch(SP) test. Also, the change in electrochemical properties caused by effects of thermal aging was investigated using electrochemical anodic polarization test in a KOH electrolyte. Carbides and ${\eta}-phase(Fe_2Mo)$ precipitated in the grain bounderies seem to deteriorate the mechanical properties by decreasing cohesive strength in the grain bounderies and promote the current density observed in electrochemical polarization curves. The electrochemical and mechanical properties of superaustenitic stainless steel was drastically decreased in the specimen aged at $650^{\circ}C$.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.7
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• pp.1332-1339
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• 2002

This research was undertaken to clarify effects of thermal aging on electrochemical and mechanical properties of superaustenitic stainless steel and to detect the material degradation nondestructively. The steel was artificially aged at $300{\sim}650^{\circ}C$ for $240{\sim}10,000h$ and the mechanical properties were investigated at $-196{\sim}650^{\circ}C$ using small punch(SP) test. Also, the change in electrochemical properties caused by effects of thermal aging on superaustenitic stainless steel was investigated using electrochemical anodic polarization test in a KOH electrolyte. Carbides and ${\eta}-phase(Fe_2Mo)$ precipitated in the grain boundaries seem to deteriorate the mechanical properties by decreasing cohesive strength in the grain boundaries and to promote the current density observed in electrochemical polarization curves, The electrochemical and mechanical properties of superaustenitic stainless steel decreased significantly in the specimen aged at $650^{\circ}C$ corresponding to the sensitization temperature for conventional austenitic stainless steels.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.3
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• pp.631-638
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• 1995

In this study, mechanical characteristics test of Functionally Gradient Materia (FGM) was performed by means of Modified Small Punch (MSP) Test with FGM; NiCrAlY-8YSZ and PSZ-Ni. To determine fracture mechanic factor, it was carried out MSP test that has possibility with small specimen (10*10*0.5 mm$^{t}$ ) and AE test to analyze micro fracture mechanism. As a result, fracture behavior became varied from brittle fracture to ductile as the content of Ni(or NiCrAlY) composition was increased and fracture energy was increased too. AE characteristics demonstrated that AE technique can detect the onset of fracture processes and AE energy was suddenly increased in the vicinity of maximum load. Since Young's modulus, fracture stress and fracture toughness was determined by MSP test, it can be known that the composition of NiCrAly 75%/8YSZ25% has the best mechanical property and furthermore this result is supported with fracture surface observation.

• Journal of Welding and Joining
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• v.12 no.4
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• pp.63-75
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• 1994

The conventional SCC(stress corrosion cracking) test methods have much difficulty in evaluating the SCC behaviors of the localized zone like HAZ, bond line and weld metal because of the specimen size. Accordingly, the purpose of this paper is to develop the new SCC test method of the welded zone by evaluating the SCC susceptibility on parent metal and various microstructures of the welded zone by SP(small punch) test method using miniaturized small specimen and SSRT(slow strain rate test) method(SP-SSRT). Besides, this study is to verify the efficiency of the SP-SSRT results through AE(acoustic emission) test which is a useful technique to monitor the microfracture processes of the material. From the results of SCC susceptibility, SEM observation and AE test, it can be concluded that the SP-SSRT test using miniaturized small specimen(10mm*10mm*0.5mm) will be a good test method to evaluate the SCC susceptibility on the local zone such as the welded zone.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.5
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• pp.554-562
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• 2004

An empirical relationship between parameters from SP curves and tensile properties has been systematically investigated by experimental tests and FEM simulations. A series of SP and tensile tests were performed. SP tests were also simulated by FE analysis with various tensile properties. It was found that the yield loads(Py) and the maximum loads( $P_{MAX}$) in SP curves were linearly related with the yield strength($\sigma$$_{o}$) and the tensile strength($\sigma$$_{UTS}$), respectively. The yield loads defined from the intersection point of two lines tangent to the elastic bending region and plastic bending region showed better relation to the yield strength than those from offset line. The maximum loads in SP curves showing plastic instability region was linearly related with the tensile strengths. The slope of SP curves in simulation results had a close correlation with the hardening coefficient and hardening strength as well.l.l.l.