Evaluation of Damage Evolution in Nickel-Base Heat-Resistant Alloy Under Creep-Fatigue Loading Conditions

2017 ◽  
Author(s):  
Ken Suzuki ◽  
Takuya Murakoshi ◽  
Hiroki Sasaki ◽  
Hideo Miura
Keyword(s):  
Grain Boundaries ◽  
Fatigue Test ◽  
Damage Evolution ◽  
Power Plants ◽  
Hold Time ◽  
Fatigue Loading ◽  
Fatigue Tests ◽  
Electron Back Scatter Diffraction ◽  
Heat Resistant Alloy ◽  
Creep Fatigue

In this study, interrupted creep and creep-fatigue tests of Alloy 617, which is a candidate alloy for boiler tubes and pipes of A-USC (advanced ultra-supercritical) power plants of the 700°C-class, were conducted to investigate damage evolution process. Also, the change of the micro texture of the alloy was continuously observed at a fixed area to elucidate the mechanism of damage evolution under creep and creep-fatigue loading from the viewpoint of the change of the order of atom arrangement using EBSD (Electron Back-Scatter Diffraction) analysis. The conditions of the creep test were a temperature of 800°C and the stress of 150 MPa in inert gas (99.9999% Ar). The stress-controlled creep-fatigue tests were carried out at 800°C in Ar using stress ratio R = −1 and hold time of 10 minutes at peak tension. IQ (Image Quality) values, which are the average sharpness of the obtained diffraction pattern, were used for evaluating the change of the micro texture during the tests. In both creep and creep-fatigue test, intergranular cracks appeared. The IQ value decreased monotonically in the vicinity of grain boundaries with the decrease of fracture life, indicating that the crystallinity of grain boundaries degraded faster than that of grains. This localized damage around grain boundaries was attributed to the intergranular crack propagation in the creep and creep-fatigue test. In addition, all the grain boundaries with IQ value lower than 85% of IQ value in as-received specimen were found to be cracked during both creep and creep-fatigue test. Therefore, there was the critical IQ value around grain boundaries at which intergranular cracks occurred under creep or creep-fatigue loading condition.

Acceleration of Grain Boundary Cracking in Ni-Base Alloy 617 Under Creep-Fatigue Loading at 800°C

2020 ◽  
Author(s):  
Kenta Ishihara ◽  
Yifan Luo ◽  
Hideo Miura
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Power Plants ◽  
Base Alloy ◽  
Thermal Power ◽  
Fatigue Loading ◽  
Intergranular Cracking ◽  
Alloy 617 ◽  
Effective Lifetime ◽  
Creep Fatigue

Abstract In recent years, in order to solve the global warming issue, the operating temperature of advanced thermal power plants has attempted to improve thermal efficiency and reduce CO2 emissions. Under the creep and creep-fatigue conditions at elevated temperature, however, the effective lifetime of heat-resistant alloys such as Ni-base Alloy 617, which has high strength and good corrosion resistance at about 750°C, was found to decrease drastically. Main reason for this short lifetime was attributed to the change in the crack initiation and propagation paths from transgranular one to intergranular one. Therefore, it is important to understand and express the criteria for grain boundary cracking. In this study, electron back-scatter diffraction (EBSD) analysis was applied to the visualization of the degradation process of the quality of grain boundaries in the alloy. The change in the crystallinity of grains and grain boundaries were continuously monitored during creep and creep-fatigue tests. It was found that accumulation of vacancies and dislocations degraded the crystallinity of grain boundaries and thus, their strength. The accumulation occurred around the specific grain boundaries which consisted of grains with large difference of Schmid factor during creep test. On the other hand, it occurred around all grain boundaries under the creep-fatigue loading. Thus, the accumulation of defects was clearly accelerated under the creep-fatigue loading. The critical image quality (IQ) value of intergranular cracking was almost the same regardless of the loading mode. Once the IQ value of the damaged grain boundaries decreased to a critical value, intergranular cracking started to occur at the grain boundaries.

Effect of Strain Rate and Hold Time in Creep-Fatigue Test

2013 ◽  
Author(s):  
Masakazu Kojima ◽  
Madoka Funai ◽  
Takashi Dozaki ◽  
Osamu Watanabe ◽  
Akihiro Matsuda
Keyword(s):  
Strain Rate ◽  
Tensile Strain ◽  
Compressive Strain ◽  
Hold Time ◽  
Perforated Plate ◽  
Fatigue Loading ◽  
Fatigue Tests ◽  
Holding Time ◽  
Loading History ◽  
Creep Fatigue

The present paper shows the two experimental results for creep-fatigue interaction effects of perforated plate at elevated temperature. (1) The loading history is assumed to be triangle form in fatigue tests, and that in creep-fatigue loading history, the loading rate from compressive strain to tensile strain is assumed to be constant, which vary from fast rate to slow rate in 5 types of strain rate. The slow strain rate loading includes the creep effects to reduce the life span, which is shown to be predicted if the constant loading assumption is assumed. (2) The holding time effect is also investigated. The tensile strain is held to be constant, and holding time is elongated in the present experiments, which is investigated by the fracture analysis.

Application of Ultrasonic Test on Creep-Fatigue Life Evaluation

2006 ◽  
Vol 321-323 ◽  
pp. 476-479
Author(s):  
Bum Joon Kim ◽  
Byeong Soo Lim ◽  
Sung Jin Song ◽  
Young H. Kim
Keyword(s):  
Fatigue Life ◽  
Power Plants ◽  
Hold Time ◽  
Ultrasonic Inspection ◽  
Fatigue Tests ◽  
Ultrasonic Test ◽  
Life Evaluation ◽  
Creep Fatigue ◽  
Fatigue Life Evaluation ◽  
The Relationship

This work investigates the relationship between the creep-fatigue life and ultrasonic test of creep-fatigue damage. Under the creep-fatigue interaction, the main cause of life reduction is the initiation and growth of microvoid with increasing hold time. The number/size of microvoid/cavity, the fraction of cavity area varied with the hold time. Therefore, the life evaluation using the microvoid with the variation of hold time is very informative for safety of components in power plants. In this study, using the heat resisting alloy, P122 steel for USC (ultra super critical) power plant, the creep-fatigue tests with various hold times and their ultrasonic inspection were carried out for the purpose of evaluation for creep-fatigue life. The results obtained by Rayleigh surface wave of backscattered ultrasound were compared and analyzed with the experimental parameters. The good agreement between the experimental life and the predicted life was obtained.

Application of Damage Mechanics and Polynomial Chaos Expansion for Lifetime Prediction of High-Temperature Components Under Creep-Fatigue Loading

2020 ◽  
Author(s):  
Felix Koelzow ◽  
Muhammad Mohsin Khan ◽  
Christian Kontermann ◽  
Matthias Oechsner
Keyword(s):  
High Temperature ◽  
Power Plants ◽  
Damage Mechanics ◽  
Fatigue Loading ◽  
Probabilistic Methods ◽  
Lifetime Prediction ◽  
Model Parameters ◽  
Creep Fatigue ◽  
High Temperature Components ◽  
Mechanics Concepts

Abstract Several (accumulative) lifetime models were developed to assess the lifetime consumption of high-temperature components of steam and gas turbine power plants during flexible operation modes. These accumulative methods have several drawbacks, e.g. that measured loading profiles cannot be used within accumulative lifetime methods without manual corrections, and cannot be combined directly to sophisticated probabilistic methods. Although these methods are widely accepted and used for years, the accumulative lifetime prediction procedures need improvement regarding the lifetime consumption of thermal power plants during flexible operation modes. Furthermore, previous investigations show that the main influencing factor from the materials perspective, the critical damage threshold, cannot be statistically estimated from typical creep-fatigue experiments due to massive experimental effort and a low amount of available data. This paper seeks to investigate simple damage mechanics concepts applied to high-temperature components under creep-fatigue loading to demonstrate that these methods can overcome some drawbacks and use improvement potentials of traditional accumulative lifetime methods. Furthermore, damage mechanics models do not provide any reliability information, and the assessment of the resultant lifetime prediction is nearly impossible. At this point, probabilistic methods are used to quantify the missing information concerning failure probabilities and sensitivities and thus, the combination of both provides rigorous information for engineering judgment. Nearly 50 low cycle fatigue experiments of a high chromium cast steel, including dwell times and service-type cycles, are used to investigate the model properties of a simple damage evolution equation using the strain equivalence hypothesis. Furthermore, different temperatures from 300 °C to 625 °C and different strain ranges from 0.35% to 2% were applied during the experiments. The determination of the specimen stiffness allows a quantification of the damage evolution during the experiment. The model parameters are determined by Nelder-Mead optimization procedure, and the dependencies of the model parameters concerning to different temperatures and strain ranges are investigated. In this paper, polynomial chaos expansion (PCE) is used for uncertainty propagation of the model uncertainties while using non-intrusive methods (regression techniques). In a further post-processing step, the computed PCE coefficients of the damage variable are used to determine the probability of failure as a function of cycles and evolution of the probability density function (pdf). Except for the selected damage mechanics model which is considered simple, the advantages of using damage mechanics concepts combined with sophisticated probabilistic methods are presented in this paper.

Molecular Dynamics Analysis of the Acceleration of Intergranular Cracking of Ni-Base Superalloy Caused by Accumulation of Vacancies and Dislocations Around Grain Boundaries

2020 ◽  
Author(s):  
Ryo Kikuchi ◽  
Shujiro Suzuki ◽  
Ken Suzuki
Keyword(s):  
Molecular Dynamics ◽  
Plastic Deformation ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Fatigue Loading ◽  
Transgranular Fracture ◽  
Dynamics Analysis ◽  
Intergranular Cracking ◽  
Creep Fatigue ◽  
Schmid Factor

Abstract Ni-based superalloys with excellent high temperature strength have been used in advanced thermal power plants. It was found that grain boundary cracking is caused in the alloy under creep-fatigue loading due to the degradation of the crystallinity of grain boundaries and the grain boundary cracking degrades the lifetime of the alloy drastically. In order to clarify the mechanism of intergranular cracking, in this research, static and dynamic strains were applied to a bicrystal structure of the alloy perpendicularly to the grain boundary using molecular dynamics analysis. In addition, the effect of the accumulation of vacancies in the area with high-density of dislocations on the strength of the bicrystal structure was analysed. It was found that the fracture mode of the bicrystal structure changed from ductile transgranular fracture to brittle intergranular one as strong functions of the combination of Schmid factor of the two grains and the density of defects around the grain boundary. The local heavy plastic deformation occurred around the grain boundary with large difference in Schmid factor between nearby grains and the diffusion of the newly grown dislocations and vacancies was suppressed by the large strain field due to the large mismatch of the crystallographic orientation between the grains. The accumulation of vacancies accelerated the local plastic deformation around the grain boundary. Therefore, the mechanism of the acceleration of intergranular cracking under creep-fatigue loading was successfully clarified by MD analysis.

Grain Boundary Cracking of Nickel-Based Alloy 625 Under Creep Loadings at Elevated Temperatures

2019 ◽  
Author(s):  
Yan Liang ◽  
Yifan Luo ◽  
Ken Suzuki ◽  
Hideo Miura
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Power Plants ◽  
Elevated Temperatures ◽  
Thermal Power ◽  
Thermal Power Plants ◽  
Alloy 625 ◽  
Creep Fatigue ◽  
Creep Loading ◽  
Initial Cracks

Abstract Since the operating condition of thermal power plants has become harsher for minimizing the emission of CO2, Ni-based superalloys, such as Alloy 617 and 625, have been used in the plants to replace the conventional ferritic materials. Unfortunately, the increase of coefficient of thermal expansion compared with conventional steels is a concern. In addition, Ni-based superalloys have to suffer creep-fatigue random loading because thermal power plants have to compensate the random output of various renewable energies. It was found that the lifetime of Ni-based superalloys under creep-fatigue loading was much shorter than that under simple fatigue or creep loading. Thus, it has become very important to clarify the crack mechanism and establish the quantitative theory for estimating their lifetime under various loading conditions at elevated temperatures. Thus, the elucidation of the initial damage mechanism of Alloy 625 under various loading is indispensable. Hence, the initial cracking mechanism of Alloy 625 at grain boundaries under creep loading was investigated experimentally. The creep test was applied to small specimens in Argon atmosphere. The change of the micro texture during the creep test was observed by using SEM. It was confirmed that all the initial cracks appeared at certain grain boundaries. The change of the crystallinity was observed by EBSD (Electron Back-Scatter Diffraction) analysis quantitatively. It was found that the local accumulation of dislocations at the cracked grain boundaries caused the initial cracks at those grain boundaries. The initiation of cracks appeared clearly between two grains which had difference of KAM (Kernel Average Misorientation) values larger than 0.2. Therefore, dislocations were accumulated at one side of the grain boundary. By measuring the KAM values near grain boundaries, the appearance of initial cracks can be predicted approximately.

Void Growth Simulation of a Steam Turbine Casing Material Under Creep and Creep-Fatigue Loading

2007 ◽  
Author(s):  
Takashi Ogata
Keyword(s):  
Growth Rate ◽  
Cyclic Loading ◽  
Void Growth ◽  
Fatigue Loading ◽  
Fatigue Tests ◽  
Growth Behavior ◽  
Growth Simulation ◽  
Creep Fatigue ◽  
Turbine Casing ◽  
Damaged Materials

High temperature components in thermal power plants are subjected to creep and creep-fatigue loading where creep voids initiate and grow on grain boundaries. Development of a quantitative evaluation method of the void growth is important for reliable maintenance of these components. In this study, creep and creep-fatigue tests were carried out at 600 °C on a 1Cr-Mo-V casting steel. Creep damaged materials were produced by interrupting the creep tests and microstructure of the damaged materials were observed carefully by a scanning microscope. The creep-fatigue tests were also conducted in a scanning electron microscope, and continuous observation of void growth behavior during the tests was made. From the observations, spherical shape voids initiate and grow up to their length of 2μm on grain boundaries at initial stage of damage, and then these voids change their shape to crack-like to grow until their length reaches around 10μm under both the creep and the creep-fatigue conditions. Under the same stress level, the void growth rate in the creep-fatigue condition was faster than that in the creep condition indicating acceleration of void growth rate by cyclic loading. Previously proposed void growth simulation model, in which the void growth was controlled by diffusion and power law creep, was modified to express acceleration of the void growth by the cyclic loading. Void growth behavior within a certain area under both the creep and the creep-fatigue condition were simulated by the modified program. Predicted void growth behaviors agreed with observed ones. The void growth behavior of an actual turbine casing was also simulated and void growth behavior was discussed based on the result.

Creep-Fatigue Properties of the Candidate Materials of 700°C-USC Boiler

2007 ◽  
Author(s):  
Keiji Kubushiro ◽  
Hiroki Yoshizawa ◽  
Takuya Itou ◽  
Hirokatsu Nakagawa
Keyword(s):  
Hold Time ◽  
Strain Range ◽  
Small Strain ◽  
Fatigue Tests ◽  
Holding Time ◽  
Experimental Results ◽  
Fatigue Properties ◽  
Alloy 617 ◽  
Creep Fatigue ◽  
Cycles To Failure

Creep-fatigue properties of candidate materials of 700°C-USC boiler are investigated. The candidate materials are Alloy 230, Alloy 263, Alloy 617 and HR6W. Creep-fatigue tests were conducted at 700°C and the effect of both strain range and hold time were studied. Experimental results showed that at 1.0% strain range, cycles to failure with 60 min strain holding is about 10% of that without strain holding, but at 0.7% strain range, cycles to failure with 60 min strain holding decreases down to about 1% of without strain holding. It appears that cycles to failure is decreased by increasing strain holding time at all tested strain ranges, and the effect of holding time is emphasized at small strain range. These phenomena depend on the kind of alloys.

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