• Title/Summary/Keyword: Load Exponent

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High Temperature Creep Characteristics Evaluation for Degraded Heat Resistance Steel of Power Plant by Mini-Specimen (미소시험편에 의한 재질열화된 내열강의 고온 크리프 특성 평가)

  • Lyu, Dae-Young;Baek, Seung-Se;Yu, Hyo-Sun
    • Korean Journal of Materials Research
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    • v.13 no.7
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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.

Development of Evaluation Technique of High Temperature Creep Characteristics by Small Punch-Creep Test Method (I) - Boiler Superheater Tube - (SP-Creep 시험에 의한 고온 크리프 특성 평가 기술 개발(I) - 보일러 과열기 튜브 -)

  • Baek, Seung-Se;Na, Seong-Hun;Na, Ui-Gyun;Yu, Hyo-Seon
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.25 no.12
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    • pp.1995-2001
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    • 2001
  • In this study, a small punch creep(SP-Creep) test using miniaturized specimen(10${\times}$10${\times}$0.5mm) is described to develop the new creep test method for high temperature structural materials. The SP-Creep test is applied to 2.25Cr-lMo(STBA24) steel which is widely used as boiler tube material. The test temperatures applied for the creep deformation of miniaturized specimens are between 550∼600$^{\circ}C$. The SP-Creep curves depend definitely on applied load and creep temperature, and show the three stages of creep behavior like in conventional uniaxial tensile creep curves. The load exponent of miniaturized specimen decreases with increasing test temperature, and its behavior is similar to stress exponent behavior of uniaxial creep test. The creep activation energy obtained from the relationship between SP-Creep rate and test temperature decreases as the applied load increases. A predicting equation or SP-Creep rate for 2.25Cr-lMo steel is suggested. and a good agreement between experimental and calculated data has been found.

Development of Evaluation Technique of High Temperature Creep Characteristics by Small Punch-Creep Test Method (ll) - Boiler Header - (Sp-Creep 시험에 의한 고온 크리프 특성 평가 기술 개발(ll) - 보일러 헤더 -)

  • Baek, Seung-Se;Lee, Dong-Hwan;Ha, Jeong-Su;Yu, Hyo-Seon
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.26 no.1
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    • pp.55-60
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    • 2002
  • For the development of a new creep test technique, the availability of SP-Creep test is discussed for 1Cr-0.5Mo boiler header material. And some results are also compared with those of 2.25Cr- 1Mo steel which widely uses as boiler superheater tube. The results can be summarized as follows. The load exponents(n) obtained by SP-Creep test for 1Cr-0.5Mo steel are decreased with increasing creep temperature and the values are 15.67, 13.89, and 17.13 at 550$^{circ}C$ ,575$^{circ}C$ and 600$^{circ}C$, respectively. The temperature dependence of the load exponent is given by n = 107.19 - 0.1108T. This reason that load exponents show the extensive range of 10∼16 is attributed to the fine carbide such as M$_{23}$C$_{6}$ in lath tempered martensitic structures. At the same creep condition, the secondary creep rate of 1Cr-0.5Mo steel is lower than the 2.25Cr-1Mo steel1 due to the strengthening microstructure composed by normalizing and tempering treatments. Through a SEM observation, it can be summarized that the primary, secondary, and tertiary creep regions of SP-Creep specimen are corresponding to plastic bending, plastic membrane stretching, and plastic instability regions among the deformation behavior of four steps in SP test, respectively.y.

Creep Damage and Hardness Properties for 9Cr Steel by SP-Creep Test Technique (SP-Creep 시험기법에 의한 9Cr강의 크리프 손상과 경도 특성)

  • Baek, Seung-Se;Lyu, Dae-Young;Kim, Jeong-Ki;Kwon, Il-Hyun;Chung, Se-Hee;Yu, Hyo-Sun
    • Proceedings of the KSME Conference
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    • pp.105-110
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    • 2001
  • It has recently been raised main issue how solve the problem of insufficient energy. One of the solution is to increase the thermal efficiency of power generation system. For the purpose of high efficiency, it is necessary to increase the steam temperature and pressure. So, the use of modified $9{\sim}12%Cr$ steel having superior creep rupture strength and oxidation resistance is required to endure such severe environment. The evaluation of creep properties of those heat resistance material is very important to secure the reliability of high temperature and pressure structural components. Since creep properties are determined by microstructural change such as carbide precipitation and coarsening, It is certain that there are some relationship between creep properties and hardness affected by microstructure. In this study, SP-Creep ruptured test for newly developed 9Cr steel being used as boiler valve material was performed, and creep properties of the material were evaluated. Also, hardness test were performed and hardness results were related to the creep properties such as LMP and creep strength to verify the availability of SP-Creep test as creep test method.

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A Study on Evaluation of High Temperature Creep Properties of 9Cr1MoVNb Steel by Small Punch-Creep test (소형펀치-크리프 시험에 의한 9Cr1MoVNb강의 고온 크리프 특성 평가 연구)

  • Yu, Hyo-Sun;Na, Sung-Hoon;Baek, Seung-Se;Kwon, Il-Hyun;Ahn, Byung-Guk;Na, Eui-Gyun
    • Proceedings of the KSME Conference
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    • pp.99-104
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    • 2000
  • This paper describes the high temperature creep characteristics for virgin material of 9Cr1MoVNb steel using small punch creep(SP-Creep) test technique which is developing recently. In addition, the several results of SP-Creep test are compared with that of 2.25Cr- 1Mo steel which is widely used as boiler materials and that of conventional uniaxial creep test. The obtained SP-Creep curves show the creep behaviors of three regimes like that obtained from conventional uniaxial creep test, and SP-Creep properties are definitely depended on applied load and test temperature. The correlation of SP-Creep rate and creep rupture life with applied load has been determined like the correlation between creep rate/rupture life and stress in uniaxial creep test, and also is satisfied with Power law. The creep rupture times of newly 9Cr1MoVNb steel are higher than those of 2.25Cr1Mo steel at the same creep temperature and applied loading condition, and the decrease extent of creep rupture life with loads is very lower compared with 2.25Cr1Mo steel.

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