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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.
Evaluation of Q* Parameter for Characterizing Creep Crack Growth Rate of Type 316LN Stainless Steel
I.M.W. Ekaputra,Woo-Gon Kim,Jae-Young Park,Seon-Jin Kim,Min-Hwan Kim,Yong-Wan Kim 대한기계학회 2014 대한기계학회 춘추학술대회 Vol.2014 No.11
This study evaluated the CCGR (creep crack growth rate) of type 316LN stainless steel using the Q<SUP>*</SUP> parameter. In a previous investigation, the Q<SUP>*</SUP> parameter was applied for modified 9Cr-1Mo steel under various applied loads and temperatures. It is obvious that the Q<SUP>*</SUP> parameter can be depicted as an independent parameter that was able to distinguish the curves under various applied loads and temperatures. Furthermore, the Q<SUP>*</SUP> parameter for type 316LN SS exhibited a similar characteristic as modified 9Cr-1Mo steel. This showed a simple monotical linear function without a dual value due to nose in the early stage of the CCG curve. The relationship between type 316LN SS and modified 9Cr-1Mo steel can also be performed clearly in the graph by the parallel movement of the curves. The type 316LN SS showed a higher Q<SUP>*</SUP> value than the modified 9Cr-1Mo steel, but it has the same slope value. Meanwhile, the CCGR characteristics were not expressed when using the C<SUP>*</SUP> parameter, which was located at the same band. The Q<SUP>*</SUP> parameter was found to be a powerful parameter that not only is able to characterize the CCGR under various applied loads and temperatures, but also under various type of steels. In addition after the CCG tests of type 316LN SS, a fracture micrograph exhibited an intergranular fracture mode.
Evaluation of Work Hardening Parameters in Modelling the Plastic Flow Behavior for Alloy 617
I.M.W. Ekaputra,Woo-Gon Kim(김우곤),Jae-Young Park(박재영),Seon-Jin Kim(김선진),Seon-Eung Kim(김응선) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11
This paper attemps to investigate the work hardening parameters used in modelling the plastic flow of true stress and true plastic strain behavior for alloy 617. The tensile tests were conducted at the strain rate of 5.55×10<SUP>-4</SUP> s<SUP>-1</SUP> under wide range of temperatures (RT ~ 950 ℃). Several equations or flow relationships such as Hollomon, Ludwik, Swift, Ludwigson, and Voce were proposed to investigate the plastic flow behavior. The result showed that the work hardening parameters of Ludwigson can describe the plastic flow behavior adequately, since these parameters yielded the smallest error quantification in fitting line under all temperature ranges as low, intermediate, and high. At the intermediate temperatures, an anomalous work hardening behavior appeared like a plateaus. It was considered as a dynamic strain aging (DSA) phenomenon, where the serrated plastic-flow phenomenon occured due to the interaction between solute atoms and mobile dislocation. In addition, the anomalous behavior was also observed from the graph of yield strength and/or ultimate tensile strength under various temperature ranges.
초고온가스로 압력용기용 Gr. 91 강의 장시간 크리프 수명 예측 방법 개선
박재영,김우곤,김선진,김민환,Park, Jae-Young,Kim, Woo-Gon,EKAPUTRA, I.M.W.,Kim, Seon-Jin,Kim, Min-Hwan 한국압력기기공학회 2014 한국압력기기공학회 논문집 Vol.10 No.1
Gr. 91 steel is used for the major structural components of Generation-IV reactor systems, such as a very high temperature reactor(VHTR) and sodium-cooled fast reactor(SFR). Since these structures are designed for up to 60 years at elevated temperatures, the prediction of long-term creep life is important for a design application of Gr. 91 steel. In this study, a number of creep rupture data were collected through world-wide literature surveys, and using these data, the long-term creep life was predicted in terms of three methods: the single-C method in Larson-Miller(L-M) parameter, multi-C constant method in the L-M parameter, and a modified method("sinh" equation) in the L-M parameter. The results of the creep-life prediction were compared using the standard deviation of error value, respectively. Modified method proposed by the "sinh" equation revealed better agreement in creep life prediction than the single-C L-M method.