Effects of long-time service at high temperature on the material strength and J-R curve of Grade 91 steel

Lee, H.Y.,Lim, D.W.,Jeong, J.Y. Pergamon Press ; Elsevier Science Ltd 2017 Engineering fracture mechanics Vol.178 No.-

<P>The effects of long-time service at high temperature on the material strength, ductility, and J-Resistance behavior of Mod.9Cr-1Mo (ASME Grade 91) steel were investigated based on tensile and J-R tests with virgin and service-exposed Gr.91 steel specimens. The service exposed Gr.91 steel specimens were sampled from a tee junction in a reheat steam piping system of an ultra-supercritical (USC) plant in Korea with an accumulated operation time of 73,716 h. Comparisons of the test data were made not only between the test results of the virgin materials and service-exposed materials but also between the test results and the material properties in the elevated temperature design (ETD) codes. These test data of yield strength (YS) and tensile strength (TS) for the virgin and service-exposed Gr.91 steel were compared with those of the ETD code, which showed that the current ETD code overestimates the YS and TS properties for long-time service in a non-conservative manner for Gr.91 steel. J-R test data currently not available in ETD codes were produced over a range of temperatures for virgin and service-exposed Gr.91 steel. The effects of longtime service in J-R curve were investigated. (C) 2017 Elsevier Ltd. All rights reserved.</P>

Grade 91 강의 응력이완 곡선 모델링

김우곤(Woo-Gon Kim),박재영(Jae-Young Park),박진솔(Jin-Sol Park),장진성(Jinsung Jang) 대한기계학회 2016 대한기계학회 춘추학술대회 Vol.2016 No.12

This study is to model high-temperature stress relaxation curves for Grade 91 steel, which is used for major structural material of a sodium-cooled fast reactor (SFR) system. A series of stress relaxation data was obtained through stress relaxation tests (SRT) conducted under a constant strain of 1.0% at 500, 550, 600, and 650℃. Herein, two models of exponential model and Peleg’s model were considered to describe the stress relaxation curves of Grade 91 steel. Each parameter value used for the two models was optimized through the nonlinear least square fit (NLSF) method to experimental stress relaxation data. In the results, the exponential model was not in accordance with the experimental stress relaxation curves for all temperatures of 500, 550, 600, and 650℃. Meanwhile, Peleg’s model was found to be in good accordance with all curve data of four temperatures, and its each parameter followed the temperature dependence well. It is suggested that Peleg’s model is reasonable for describing the stress relaxation curves of Grade 91 steel.

신재 및 가동이력 Gr.91강의 재료강도 거동에 미치는 동적변형시효의 영향

남기언(Ki-Ean Nam),이형연(Hyeong-Yeon Lee),어재혁(Jae-Hyuk Eoh),김형모(Hyungmo Kim),홍현욱(Hyun-Uk Hong) 한국압력기기공학회 2024 한국압력기기공학회 논문집 Vol.20 No.1

This study investigates the effects of temperatures and strain rates on the strength and ductility of Gr.91 (ASME Grade 91) steel which is widely being used as a heat-resistant material in Generation IV nuclear and super critical thermal power plants. The tensile behavior of modified 9Cr-1Mo (Gr.91) steel was studied for the three strain rates of 6.67×10<SUP>-5</SUP>/s, 6.67×10<SUP>-4</SUP>/s and 6.67×10-3/s over the temperature range from room temperature (RT) to 650℃. Experimental results showed that at specific combinations of temperatures (300~400℃) and strain rates, serrations appeared in the stress-strain curves. Concurrently, abnormal behaviors such as a plateau in yield strength and tensile strength, a minimum in ductility and negative strain rate sensitivity were observed. These phenomena were analyzed as significant characteristics of dynamic strain aging (DSA). Since this abnormal behavior in Gr.91 steel affects the material strength, it is judged that a correlation analysis between DSA and material strength should be crucial in the design and integrity evaluation of Gr. 91 steel pressure vessel and piping subjected to high-temperature loading.

와이블 분포를 이용한 Grade 91강 용접재의 크리프 수명 예측

박재영(Jae-Young Park),김우곤(Woo-Gon Kim),I.N.C Kusuma,김응선(Eung-Seon Kim) 대한기계학회 2018 대한기계학회 춘추학술대회 Vol.2018 No.12

Grade 91 steel is a promising candidate for the structural materials such as steam generators, intermediate heat exchangers, and hot pipes in a 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 very important to determine the allowable design stress of the SFR structures. In this study, a number of creep rupture data were collected through world-wide literature surveys and the data tested in KAERI. Using these data, the long-term creep life was predicted in terms of two methods of typical Larson-Miller (L-M) parameter and Weibull distribution method. Weibull distribution method was proposed as one of creep life prediction methods. The L-M method showed overestimation for longer time of low stress region, but weibull method revealed a reasonable prediction which gradually decreased in creep strength for longer time of low stress region.

Grade 91 강의 고온 균열진전 실험 결과와 설계 물성치의 비교

이형연(Hyeong-Yeon Lee),김우곤(Woo-Gon Kim),김낙현(Nak-Hyun Kim) 대한기계학회 2015 大韓機械學會論文集A Vol.39 No.1

본 연구에서는 피로 하중 및 크리프 하중을 받는 Mod.9Cr-1Mo (ASME Grade 91)강 시편에 대한 일련의 실험결과로부터 재료물성치인 고온 균열진전 모델을 개발하였다. 이 균열진전 모델은 크리프-피로하중을 받는 균열체의 결함평가에 사용되는 물성치이다. 한국원자력연구원이 수행한 일련의 피로 균열진전(FCG) 속도 실험 및 크리프 균열진전(CCG) 속도 실험 결과로부터 균열진전 모델을 결정하고, 이를 프랑스의 고온 설계 기술기준인 RCC-MRx 와 비교함으로써 설계 물성치의 보수성에 대해 검토하였다. RCC-MRx 는 FCG 모델 및 CCG 모델을 Section III Tome 6 에서 제공하고 있는데, 실험으로부터 결정한 균열진전 모델과 비교한 결과 RCC-MRx 의 FCG 모델은 보수적인 것으로 나타난 반면 CCG 모델은 비보수적인 것으로 나타나 동 물성치에 대한 검증이 필요한 것으로 나타났다. 또한 본 연구에서는 기계적 강도 및 크리프 시험결과에 대해서도 RCC-MRx 의 물성치와 비교 및 분석하였다. The material properties of crack growth models at an elevated temperature were derived from the results of numerous crack growth tests for Mod.9Cr-1Mo (ASME Grade 91) steel specimens under fatigue loading and creep loading at an elevated temperature. These crack growth models were needed for defect assessment under creep-fatigue loading. The mathematical crack growth rate models for fatigue crack growth (FCG) and creep crack growth (CCG) were determined based on the test results, and the models were compared with those of the French design code RCCMRx to investigate the conservatism of the code. The French design code RCC-MRx provides an FCG model and a CCG model for Grade 91 steel in Section III Tome 6. It was shown that the FCG model of RCC-MRx is conservative, while the CCG model is non-conservative compared with the present test data. Thus, it was shown that further validation of the property was required. Mechanical strength tests and creep tests were also conducted, and the test results were compared with those of RCC-MRx.

Grade 91 강의 크리프 균열성장 거동에 대한 천이영역과 정상상태영역의 상관 관계

박재영(Jae-Young Park),김우곤(Woo-Gon Kim),I. M. W. Ekaputra,김선진(Seon-Jin Kim),김응선(Eung-Seon Kim) 대한기계학회 2015 大韓機械學會論文集A Vol.39 No.12

제 4 세대 원자로의 고온 구조재료로 사용되는 Grade 91 강의 크리프 균열성장 거동에 대한 천이영역과 정상상태영역에서의 상관 관계를 조사하였다. 이를 위해 600℃ 의 동일한 온도 및 동일한 하중조건에서 1/2” CT 시편을 사용하여 크리프 균열성장 시험 데이터를 얻었다. 크리프 균열성장 속도식은 C*-파괴매개변수를 사용하여 천이영역과 정상상태영역에서의 평가 식을 각각 도출하였다. C*와 da/dt 의 관계에서 천이영역의 크리프 균열성장 속도는 시험 데이터의 산포가 크지만 정상상태영역의 크리프 균열성장 속도와 비슷한 기울기로서 상관성이 있었으며 천이영역 균열성장속도는 정상상태 균열성장속도에 비해 약 5.6 배 낮았다. 본 결과를 이용하면 짧은 시간의 천이영역 균열성장속도로부터 장시간의 시험으로 얻을 수 있는 정상상태 균열성장속도를 예측할 수 있다. A correlation between the transient regime and steady state regime on the creep crack growth (CCG) for Grade 91 steel, which is used as the structural material for the Gen-IV reactor systems, was investigated. A series of CCG tests were performed using 1/2” CT specimens under a constant applied load and at a constant temperature of 600 °C. The CCG rates for the transient and steady state regimes were obtained in terms of C* parameter. The transient CCG rate had a close correlation with the steady-state CCG rate, as the slope of the transient CCG data was very similar to that of the steady state data. The transient rate was slower by 5.6 times as compared to the steady state rate. It can be inferred that the steady state CCG rate, which is required for long-time tests, can be predicted from the transient CCG rate obtained from short-time tests.

Grade 91 강 용접재의 등응력 크리프 파단 데이터의 와이블 분포를 이용한 장시간 크리프 수명 예측

박재영(Jae-Young Park),김우곤(Woo-Gon Kim),박진솔(Jin-Sol Park),장진성(Jinsung Jang) 대한기계학회 2016 대한기계학회 춘추학술대회 Vol.2016 No.12

Grade 91 steel is a promising candidate for the structural materials such as steam generators, intermediate heat exchangers, and hot pipes in a 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 very important to determine the allowable design stress of the SFR structures. In this study, to accurately predict a long-term creep life, a new approach using Weibull distribution is proposed. To do so, a large body of creep rupture data for Grade 91 weld metal is collected through world-wide literature surveys and KAERI’s experimental data. Using the data, long-term life prediction are carried out by means of three methods, Larson-Miller (L-M) and Manson-Haferd (M-H) methods which have been typically used until now, and the Weibull-distribution method which proposed as a new approach here, and the predicted results are compared. From the results, the Weibull-distribution method is found to be superior to the L-M and M-H methods.

Monte-Carlo Simulation 을 이용한 Grade 91 강의 크리프 균열 성장속도의 확률적 예측

박진솔(Jin-Sol Park),김우곤(Woo-Gon Kim),박재영(Jae-Young Park),김선진(Seon-Jin Kim),김응선(Eung-Seon Kim) 대한기계학회 2017 대한기계학회 춘추학술대회 Vol.2017 No.11

This paper focused on a statistical analysis for evaluating creep crack growth rate (CCGR) of Grade 91 steel which is used in Gen-IV nuclear reactors. A series of CCG data was obtained from CCG tests conducted using 1/2” compact tension (CT) specimens under various applied loads at 600℃. The CCGR was statistically analyzed in terms of the empirical equation between the CCG rate (da/dt) and C* parameter. Three methods such as the least square fitting method (LSFM), the mean value method (MVM), and the probabilistic distribution method (PDM) were compared using the relationships between da/dt and C* parameter, which were obtained using the Monte Carlo Simulation (MCS) technique under various load conditions. It was identified that the predicted data followed the LSFM distribution well. The CCGR of Grade 91 steel could be successfully estimated in the viewpoints of probability.

Grade 91 강의 모재와 용접재에 대한 천이영역의 크리프 균열 성장 속도 비교

박재영(Jae-Young Park),김우곤(Woo-Gon Kim),I.M.W.Ekaputra,김선진(Seon-Jin Kim),김민환(Min-Hwan Kim) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.12

This paper presents a comparison of creep crack growth rate (CCGR) in transient regime for Grade 91 base metal (BM) and weld metal (WM). The CCG data for BM and WM were obtained by the CCG tests performed on 1/2 compact tension specimens under various applied loads at 600℃. The CCG rates were obtained using the empirical equation between da/dt and C<SUB>*</SUB> parameter. The B and q values were determined, and the results of normalized time and transient CCGR behaviors were compared for BM and WM.

Influence of data size on the reliability assessment of creep life of grade 91 steel

김우곤,박재영,B. K. Choudhary,김선진,김민환,장진성 대한기계학회 2014 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.28 No.11

Influence of data size on a reliability assessment of the long-term creep life of Grade 91 steel was investigated for two different sets oflarge body of creep-rupture data comprising 690 (Set-1) and 1072 (Set-2) data points. The Z-parameter on the Larson-Miller parameter ofcreep rupture data supported a normal distribution well. Based on the normal distribution, the reliability of the predicted creep life for thetwo data sets was assessed to obtain variations in reliability owing to fluctuations in the service conditions and the dispersion of the creepdata using a service condition-creep rupture property interference (SCRI) model. A large number of random variables for Z-parameterwere generated using Monte-Carlo simulation. A reliability assessment for the two data sizes under specific service conditions has beencomparatively demonstrated. It was found that the size of the creep-rupture data influenced the reliability of the creep life prediction.