• 한국재료학회지
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• 제13권7호
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• pp.429-435
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• 2003

In this study the new creep test using miniaturized specimen(10${\times}$10${\times}$0.5 ㎣) was performed to evaluate the creep characteristics for degraded materials of 2.25Cr-1Mo steel. For this creep test, the artificially aged materials for 330 hrs and 1820hrs at $630^{\circ}C$ were used. The test temperatures applied for the creep deformation of miniaturized specimens was X$630^{\circ}C$ and the applied loads were between 45 kg∼80 kg. After creep test, macro- and microscopic observation were conducted by the scanning electron microscope(SEM). The creep curves depended definitely on applied load and microstructure and showed the three stages of creep behavior like uniaxial tensile creep curves. The load exponents of virgin, 330 hrs and 1820 hrs materials based on creep rate showed 14.8, 9.5 and 8.3 at $550^{\circ}C$ respectively, The 1820 hrs material showed the lowest load exponent and this behavior was also observed in the case of load exponent based on creep rupture time. In contrast to virgin material which exhibited fined dimple fractography, a lot of carbides like net structure and voids were observed on the fractography of degraded materials.

• Journal of Welding and Joining
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• 제18권6호
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• pp.62-67
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• 2000

Metallurgical characteristics of bonded region and high temperature mechanical properties of heat resistant alloy, Fe-35Ni-26Cr during liquid phase diffusion bonding were investigated employing AM17 insert metal. The insert metal for bonding, AM17 was newly developed Ni-base metal using interpolation method. Bonding of specimens were carried out at 1,403~1,463K for 600s in vacuum. The microconstituents in the bonded interlayer disappeared in the bonding temperature over 1,423K. The microstructures, alloying elements and hardness distribution in the base metal. The tensile strength and elongation of the joints at elevated temperatures were the same level as one of the base metal in the bonding temperature over 1,423K. The creep rupture strength and rupture lives of joints were almost identical to those of base metal.

• 동력기계공학회지
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• 제9권4호
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• pp.96-101
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• 2005

Magnesium alloys have been widely used for many structural components of automobiles and aircraft because of high specific strength and good cast ability in spite of hexagonal closed-packed crystal structure of pure magnesium. In this study, uniaxial tension tests at high temperature and creep tests are done in order to investigate the characteristics of high temperature and mechanisms for creep deformation of AZ31 Mg alloy. Yield stress and ultimate tensile stress decreased with increasing temperature, but elongation increased from results of uniaxial tension test at high temperature. The apparent activation energy Qc, the applied stress exponent n and rupture life have been determined during creep of AZ31 Mg alloy over the temperature range of 473K to 573K and stress range of 23.42 MPa to 93.59 MPa, respectively, in order to investigate the creep behavior. Constant load creep tests were carried out in the equipment including automatic temperature controller, whose data are sent to computer. At around the temperature of $473K{\sim}493K$ and under the stress level of $62.43{\sim}93.59%MPa$, and again at around the temperature of $553K{\sim}573K$ and under the stress level of $23.42{\sim}39.00MPa$, the creep behavior obeyed a simple power-law relating steady state creep rate to applied stress and the activation energy for the creep deformation was nearly equal, respectively, and a little low to that of the self diffusion of Mg alloy including aluminum. Also rupture surfaces at high temperature have had bigger dimples than those at lower temperature by SEM.

• 한국자동차공학회논문집
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• 제13권3호
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• pp.117-122
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• 2005

Titanium alloy has widely been used as material for glasses frame parts because it has high specific strength. It is also light and harmless to human body. However, we have little design data about the mechanical properties such as the creep behaviors of the alloy. Therefore, in this study, creep tests under four constant stress conditions have been conducted with four different temperature conditions. A series of creep tests had been performed to get the basic design data and life prediction of titanium products and we have gotten the following results. First, the stress exponents decrease as the test temperatures increased. Secondly, the creep activation energy gradually decrease as the stresses became bigger. Thirdly, the constant of Larson-Miller parameter on this alloy was estimated as about 2.5. Finally, the fractographs at the creep rupture showed the ductile fracture due to the intergranullar rupture and some dimples.

• Nuclear Engineering and Technology
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• 제55권7호
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• pp.2454-2465
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• 2023

Film boiling may lead to burnout of the heating element. Even though burnout does not occur, the heating element is subject to deformation because it is not sufficiently strong to withstand external loads. In particular, the ballooning and rupture of a tube under film boiling are important phenomena in the field of nuclear reactor safety. If the tube-type cladding of nuclear fuel ruptures owing to high internal pressure and thermal load, radioactive materials inside the cladding are released to the coolant. Therefore, predicting the ballooning and rupture is important. This study presents numerical simulations to predict the ballooning behavior and rupture time of a horizontal tube at high internal pressure under saturated film boiling. To do so, a multi-step coupled simulation of conjugated film boiling heat transfer and ballooning using creep model is adopted. The numerical methods and models are validated against experimental values. Two different nonuniform heat flux distributions and four different internal pressures are considered. The three-step simulation is enough to obtain a convergent result. However, the single-step simulation also successfully predicts the rupture time. This is because the film boiling heat transfer characteristics are slightly affected by the tube geometry related to creep ballooning.

• 한국공작기계학회논문집
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• 제14권2호
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• pp.42-47
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• 2005

Micro duplex stainless steel is used to denote a fine scale two-phase micro structure consisting austenite and firrite. The development of this structure was done by proper thermo-mechanical processing. The objective of present investigation is to study creep characteristics of this alloy. Since we have little design data about the W behaviors of the alloy. An apparatus has been designed and built fir conducting creep tests under constant load conditions. A series of creep tests on them have been performed to get the basic design data and life prediction of micro duplex stainless steel products and we have gotten the 1311owing results. First the stress exponents decrease as the test temperatures increase. Secondly, the creep activation energy Gradually decreases as the stresses become higher. Thirdly, the constant of Larson-Miller Parameters on this alloy is estimated as about 5. Last, the fiactographs at the creep rupture show both the ductile and brittle fracture modes according to the creep conditions.

• Journal of Welding and Joining
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• 제20권6호
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• pp.30-30
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• 2002

It has been reported that the creep characteristics on weldment which is composed of weld metal(W.M), fusion line(F.L), heat-affected zone(HAZ), and base meta(B.M) could be unpredictably changed in severe service conditions such as high temperature and high pressure. However, the studies done on creep damage in power plant components have been mostly conducted on B.M and not the creep properties of the localized microstructures in weldment have been thoroughly investigated yet. In this paper, it is investigated the creep characteristics for three microstructures like coarse-grain HAZ(CGHAZ), W.M, and B.M in X20CrMoV121 steel weldment by the small punch-creep-(SP-Creep) test using miniaturized specimen(l0×10×0.5mm). The W.M microstructure possesses the higher creep resistance and shows lower creep strain rate than the B.M and CGHAZ. In the lower creep load the highest creep strain rate is exhibited in CGHAZ, whereas in the higher creep load the B.M represents the high creep strain rate. The power law correlation for all microstructures exists between creep rate and creep load at 600℃. The values of creep load index (n) based on creep strain rate for B.M, CGHAZ, and W.M are 7.54, 4.23, and 5.06, respectively and CGHAZ which shows coarse grains owing to high welding heat has the lowest creep loade index. In all creep loads, the creep life for W.M shows the highest value.

• Journal of Welding and Joining
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• 제20권6호
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• pp.754-761
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• 2002

It has been reported that the creep characteristics on weldment which is composed of weld metal(W.M), fusion line(F.L), heat-affected zone(HAZ), and base meta(B.M) could be unpredictably changed in severe service conditions such as high temperature and high pressure. However, the studies done on creep damage in power plant components have been mostly conducted on B.M and not the creep properties of the localized microstructures in weldment have been thoroughly investigated yet. In this paper, it is investigated the creep characteristics for three microstructures like coarse-grain HAZ(CGHAZ), W.M, and B.M in X20CrMoV121 steel weldment by the small punch-creep-(SP-Creep) test using miniaturized specimen($10{\times}10{\times}0.5mm$). The W.M microstructure possesses the higher creep resistance and shows lower creep strain rate than the B.M and CGHAZ. In the lower creep load the highest creep strain rate is exhibited in CGHAZ, whereas in the higher creep load the B.M represents the high creep strain rate. The power law correlation for all microstructures exists between creep rate and creep load at $600^{\circ}C$. The values of creep load index (n) based on creep strain rate for B.M, CGHAZ, and W.M are 7.54, 4.23, and 5.06, respectively and CGHAZ which shows coarse grains owing to high welding heat has the lowest creep loade index. In all creep loads, the creep life for W.M shows the highest value.

• 한국정밀공학회지
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• 제29권10호
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• pp.1137-1143
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• 2012

Creep characteristic is an important failure mechanism when evaluating engineering materials that are soft material as polymers or used as mechanical elements at high temperatures. One of the popular thermo-plastic polymers, Acrylonitrile Butadiene Styrene (ABS) which is used broadly for machine elements material, as it has excellent mechanical properties such as impact resistance, toughness and stiffness compared to other polymers, was studied for creep characteristic at different levels of stress and temperatures. From the experimental results, the creep limit of ABS at room temperature is 80 % of tensile strength which is higher than PE and lower than PC or PMMA. Also the creep limits decreased to linearly as the temperatures increased, up to $80^{\circ}C$ which is the softening temperature of Butadiene ($82^{\circ}C$). Also the secondary stage of creep among the three creep stages for different levels of stress and temperature was non-existent which occurred for many metals by strain hardening effect.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2005년도 추계학술대회 논문집
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• pp.818-823
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• 2005

Creep damage is one of the mosl important characteristics for the stability of high temperature structures such as huge energy converting facilities. Creep failure of Type 316LN stainless steel is highly correlated to generation and growth of the voids. In this paper, in order to investigate the correlation of creep rupture time and ultrasonic parameters (group velocity, angular velocity), creep-damaged Type 316LN specimens and measurements for the ultrasonic parameters were made. However, bi-directional measurements were applied along the load direction and the perpendicular direction to the load line by means of the contact type probe of which the central frequencies are 10MHz, 15MHz and 20MHz. Analyzing the angular velocities of the ultrasonic signals obtained from the load direction, it was confirmed that the angular velocities were declined as the creep time passed when 15MHz and 20MHz probes were used. Also, the group velocities were declined for all three frequencies as the creep time increased. Thus, positive feasibility for the creep damage evaluation by means of the angular and group velocities was confirmed. Moreover, result of analysis for the ultrasonic signal which was obtained from the perpendicular direction upon the angular and group velocities indicated little variation for both of the angular and group velocities. Therefore, the creep damage is likely to represent anisotropic itself.