Woo-Gon Kim

Creep Characterization of Type 316LN and HT-9 Stainless Steels by the K- R Creep Damage Model

The Kachanov and Rabotnov (K-R) creep damage model was interpreted and applied to type 316LN and HT-9 stainless steels. Seven creep constants of the model,A, B. k, m, λ, γ, andq were determined for type 316LN stainless steel. In order to quantify a damage parameter, the cavity was interruptedly traced during creep for measuring cavity area to be reflected into the damage equation. For type 316LN stainless steel, λ=εR/ε* and λf=ε/εR were 3.1 and increased with creep strain. The creep curve with λ=3.1 depicted well the experimental data to the full lifetime and its damage curve showed a good agreement whenr=24. However for the HT-9 stainless steel, the values of A and A/ were different as λ=6.2 and λf=8.5, and their K-R creep curves did not agree with the experimental data. This mismatch in the HT-9 steel was due to the ductile fracture by softening of materials rather than the brittle fracture by cavity growth. The differences of the values in the above steels were attributed to creep ductilities at the secondary and the tertiary creep stages.

Role of dynamic strain aging on low cycle fatigue and crack propagation of type 316L(N) stainless steel

Tensile, strain-controlled low cycle fatigue (LCF), and crack propagation tests were conducted at RT and 600 °C in air atmosphere for type 316L(N) stainless steels containing 0.04% N and 0.10% N. Nitrogen increased tensile strength without the reduction of elongation. LCF life increased with nitrogen content and the magnitude of increase was higher at 600 °C than at RT. Fatigue crack propagation rate increased with temperature but decreased with the addition of nitrogen. Nitrogen retarded dynamic strain aging (DSA) and decreased crack propagation. The larger increase of LCF life at 600 °C than that observed at RT with the addition of nitrogen was attributed to the retardation of DSA by the addition of nitrogen.

Analysis of the Creep Rupture Data of Alloy 617 for a High Temperature Gas Cooled Reactor

Since very high temperature gas-cooled reactor (VHTR) components for a hydrogen production are designed for a lifetime of 30 to 60 years at a high-temperature of 950°C, a material selection is important, and also one of the most important properties in these metallic components is their longterm creep behavior. In this study, for selecting a suitable material for the intermediate heat exchanger (IHX), the hightemperature strengths of several candidate alloys approved for use by ASME VIII (boiler and pressure vessel code) were reviewed. From this review, alloy 617 was immediately selected as one of the best materials. In addition, to provide design data for a long duration for alloy 617, lots of creep data was collected through world-wide literature surveys of European Alloy-DB and available literature surveys. Using these data, a long-term creep life for alloy 617 was predicted for up to 106 hours based on the Larson-Miller (LM) parameter. Also, the creep rupture data was analyzed by a statistical treatment. Creep master curves for the creep-life prediction were presented for various standard deviations; 1σ, 1.65σ, 2σ and 3σ. A minimum design value for the creep-rupture time was found to be 10 times lower than the average value. Failure probability of the creep rupture data followed a lognormal distribution well.Copyright

Evaluation of Monkman-Grant Parameters for Type 316LN and Modified 9Cr-Mo Stainless Steels

The Monkman-Grant (M-G) and its modified parameters were evaluated for type 316LN and modified 9Cr-Mo stainless steels prepared with minor element variations. Several sets of creep data for the two alloy systems were obtained by constant-load creep tests in 550-650°C temperature range. The M-G parameters,m, m’,C, andC’ were proposed and discussed for the two alloy systems. Them value of the M-G relation was 0.90 in type 316LN steel and 0.84 in modified 9Cr-Mo steel. Them’, value of the modified relation was 0.94 in type 316LN steel and 0.89 in 9Cr-Mo steel. Although creep fracture modes and creep properties between type 316LN and modified 9Cr-Mo steels showed a basic difference, the M-G and its modified relations demonstrated linearity quite well. Them’ of modified relation almost overlapped regardless of the creep testing conditions and chemical variations in the two alloy systems, and the parameterm’ was closer to unity than that of the M-G relation.

Reliability Prediction of Long-term Creep Strength of Gr. 91 Steel for Next Generation Reactor Structure Materials

This paper focuses on reliability prediction of long-term creep strength for Modified 9Cr-1Mo steel (Gr. 91) which is considered as one of the structural materials of next generation reactor systems. A “Zparameter” method was introduced to describe the magnitude of standard deviation of creep rupture data to the master curve which can be plotted by log stress vs. The larson-Miller parameter (LMP). Statistical analysis showed that the scattering of the Z-parameter for the Gr. 91 steel well followed normal distribution. Using this normal distribution of the Z-parameter, the various reliability curves for creep strength design, such as stress-time temperature parameter reliability curves (σ-TTP-R curves), stress-rupture time-reliability curves (σ-tr-R curves), and allowable stress-temperaturereliability curves ([σ]-T-R curves) were reasonably drawn, and their results are discussed. (Received January 24, 2011)

Creep Properties of Hastelloy-X Alloy for the High Temperature Gas-Cooled Reactor

The creep properties for the Hastelloy-X alloy which is one of candidate alloys for a high temperature gas-cooled reactor are presented. The creep data was obtained with different stresses at 950oC, and a number of the creep data was collected through literature surveys. All of the creep data were combined together to obtain the creep constants and to predict a long-term creep life. In the Norton’s creep law and the Monkman-Grant relationship, the creep constants, A, n, m, and m’ were obtained. Creep master curves based on the Larson-Miller parameter were presented for the standard deviations of 1σ, 2σ and 3σ. Creep life at each temperature was predicted for a longer-time rupture above 105 hours. Failure probability was also estimated by a statistical process of all the creep rupture data.

Modeling of a Long-Term Creep Curve of Alloy 617 for a High Temperature Gas-Cooled Reactor

This study aimed to model the long-term creep curves above 105 hours by implementing a nonlinear least square fitting (NLSF) of the Kachanov-Rabotnov (K-R) model. For this purpose, the short-term creep curves obtained from a series of creep tests at 950oC were used. In the NLSF of their full creep curves, the K-R model represented a poor match to the experimental curves, but the modified K-R one revealed a good agreement to them. The Monkman-Grant (M-G) strain represented the behavior of a stress dependency, but the parameter was constant with a stress independency. The value in the modified K-R model was 2.78. Long-term creep curves above 105 hours from short-term creep data were modeled by the modified K-R model.

Application and Standard Error Analysis of the Parametric Methods for Predicting the Creep Life of Type 316LN SS

To predict the creep-rupture life of type 316LN stainless steels which are major structural components of liquid metal reactors, a number of creep-rupture data were collected through literature survey and experimental data of KAERI. Using the data, the creep-rupture life was analyzed by means of the Larson-Miller, the Orr-Sherby-Dorn and the Manson-Haferd parametric methods. Polynomial equations for predicting the creep life were obtained. In order to analyze the acceptance and use of the parametric methods, standard error values were accurately investigated by statistical process of the creep data. As for the results, the three parametric methods are found to be favorable in predicting the creep life of type 316LN stainless steel. Each method did not generate a large error in the standard error of the estimate with variations of the temperatures, but the Orr-Sherby-Dorn and the Manson-Haferd methods showed a better agreement than the Larson-Miller one. Especially, at higher the 700oC, the Manson-Haferd method conformed well to the experimental data. The reason is because the Manson-Haferd method includes two constants of ta and Ta.

Cyclic Stress Response and Fracture Behaviors of Alloy 617 Base Metal and Weld Joints under LCF Loading

Cyclic stress response and fracture behaviors of Alloy 617 base metal (BM) and Alloy 617 weld joints (WJ) are investigated under strain controlled low cycle fatigue (LCF) loading. Axial fully reversed total-strain controlled tests have been conducted at room temperature with total strain ranges of 0.6, 0.9, 1.2, and 1.5%. At the all testing conditions, weld joint specimens have shown higher peak stresses than the base metal specimens, whereas the plastic strain accumulation of the base metal specimens is comparatively higher than those of the weld joint specimens. The cyclic stress response behavior of both base metal and weld joint specimens revealed initial cyclic hardening during first small number of cycles followed by progressive softening to failure. Higher strain amplitudes decreased the fatigue lives for both base metal and weld joint specimens; subsequently weld joint specimens had lower fatigue resistances relative to base metal specimens. Furthermore, the cracking in weld joint specimens initiated in the weld metal (WM) region. The crack initiation and propagation showed transgranular mode for both base metal and weld joint specimens; especially weld joint specimens showed a wedge type crack initiation about 45 degrees to the loading direction because of the dendritic structure.

Development of Materials for a High Temperature Gas Cooled Reactor in Korea

The very high temperature reactor (VHTR) materials R&D started as an objective of the development of key technologies for a nuclear hydrogen production funded by the Korea government in 2006. We are performing materials R&D for 5 components: a reactor pressure vessel (RPV), a reactor/process intermediate heat exchanger (IHX) and hot gas duct, a control rods component, a reflector and support structures in the core region and reactor materials for the sulfur-iodine (SI) process. The scopes of our works are focused on a material screening/selection and qualification, codifications of a high temperature structural design to a very high temperature region, and to support the licensing of a system design, material characterizations and database establishments. Current target materials are modified 9Cr-1Mo, alloy 617, graphite, ceramic fiber reinforced composite, Fe-Si and SiC.Copyright