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Derivation of Q* Parameter for Evaluating Creep Crack Growth Rate of Modified 9Cr-1Mo Steel
I.M.W. Ekaputra,Woo-Gon Kim(김우곤),Jae-Young Park(박재영),Seon-Jin Kim(김선진),Min-Hwan Kim(김민환),Yong-Wan Kim(김용완) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.12
The concept of the Q* parameter was first proposed by Yokobori et al. The Q* parameter is defined as the exponent of the exponential function in the thermal activation process equation. In this study, the creep crack growth (CCG) behavior of the modified 9Cr-1Mo steel is evaluated using the Q* parameter. The CCG tests were carried out under various loads at 550oC and 600oC. The K and C* parameters have been used to characterize the CCGR of modified 9Cr-1Mo steel. The results show that the K parameter exhibits the largest scatter data, and there is no systematic trend in each series of tests, while C* showed a narrower scattering of data of CCGR than the K parameter. However, C* decreased during the early stage of crack growth and subsequently increased, i.e., a dual value due to the nose appearance, and it did not distinguish among the series of data under various loads at 550oC and 600oC clearly in a one linear line. The Q* parameter was able to evaluate the CCG by a simple monotical linear function without a dual value owing to nose existing in the early stage, and it exhibited an increase or decrease regardless of the testing conditions. The Q* was regarded as an independent parameter that is not depending on stress and temperature, whereas the C* was regarded as a dependent parameter depending on the creep stress and temperature.
Analysis of Creep Behavior of Alloy 617 for VHTR Application
Woo-Gon Kim(김우곤),Jae-Young Park(박재영),I.M.W. Ekaputra,Min-Hwan Kim(김민환),Seon-Jin Kim(김선진),Yong Wan Kim(김용완) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.12
Creep rupture data were obtained from a series of creep tests with different applied stresses at 850℃, 900℃ and 950℃ of Alloy 617, which is considered as a prime candidate material for the VHTR application. On the basis of the creep experimental data, the analysis of creep rupture behavior was performed using various creep relations and laws such as Norton’s power law, Monkman-Grant Relationships (MGR), Modified Monkman-Grant Relationships (MMGR), creep damage tolerance factor λ, and Zener-Hollomon Parameter (Z), and then the creep constants used in these equations were determined. The MMGR appeared to be more narrowed in data scattering than the MGR, and it followed well a straight line of m ? 1.0 as m=0.97. In the plot of the Z parameter vs. stress, it obeyed a straight line of the slope of n’=5.87 regardless of the three different temperatures. It would be thus inferred that the same creep mechanism was operative within the present stress and temperature ranges, and creep damage tolerance factor of Alloy 617 was found to be 2.40.
Analysis of the creep crack growth rate for modified 9Cr-1Mo steel using Q* parameter
I. M. W. Ekaputra,김우곤,박재영,김선진,김용완 대한기계학회 2014 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.28 No.10
This paper proposes a Q* parameter to analyze the creep crack growth rate (CCGR) for modified 9Cr-1Mo steel. A C* parameter is notreasonable for some practical applications because it has a dual value in the early stage, although the CCGR has been generally evaluatedby the C* parameter. The Q* parameter is able to evaluate the CCGR by a simple monotical linear function without a dual value due tothe “nose” in the early stage of the CCG curve. The K, C*, and Q* parameters have been used to analyze the CCGR for this steel, and thescattered data among the three parameters were obtained and compared. In the scattered data from the early stage to the acceleration stage,the Q* parameter was found to be superior to the C* parameter, while the K parameter had the largest scatter and there was no systematictrend among each series of tests. It is obvious that the Q* was regarded as an independent parameter, whereas the C* was regarded as adependent parameter that depends on the creep deformation rate, which is a function of applied load, temperature, specimen shape, andactivation energy of deformation, etc.
Woo-Gon Kim,박재영,I. M. W. Ekaputra,홍성덕,김선진,김용완 대한기계학회 2013 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.27 No.8
This paper presents a comparative investigation on the high-temperature tensile and creep properties of Alloy 617 base metal (BM)and weld metal (WM) fabricated by a gas tungsten arc weld process. The WM had higher yield strength and lower ultimate tensile strength than the BM does; however, its elongation was significantly lower than that of the BM. The creep curve of the BM and WM was somewhat different from that of typical heat-resistance steel, and did not show a textbook creep. The WM exhibited a longer creep rupture life, lower creep rate, and lower rupture ductility than the BM. However, as the creep rupture time reached approximately 36,800 h,the creep life of the WM was expected to be almost similar to that of the BM; and after 36,800 h, its creep life was expected to be worse than the BM. Loner creep tests is needed to investigate the long-term creep life of the WM. The creep failure mode of the BM and WM was obviously an intergranular cracking of the cavity formation and growth mechanisms, although it was more evident in the WM. The BM had a more ductile fracture surface than the WM.