Disappearance of Martensitic Strengthened-Micro-Texture in Modified 9Cr-1Mo Steel Caused by Stress-Induced Acceleration of Atomic Diffusion at Elevated Temperatures

2018 ◽  
Vol 774 ◽  
pp. 31-35
Author(s):  
Taichi Shinozaki ◽  
Ken Suzuki ◽  
Hideo Miura
Keyword(s):  
Power Plants ◽  
Elevated Temperatures ◽  
Thermal Power ◽  
Atomic Diffusion ◽  
Dominant Factor ◽  
Micro Structure ◽  
Creep Tests ◽  
Heat Resistant Steel ◽  
Component Element ◽  
Applied Tensile Stress

Modified 9Cr-1Mo steel is a heat-resistant steel developed for a steam generator in a FBR (Fast Breeder Reactor) and it has been applied to various thermal power plants. Recently, it was found that the fatigue limits did not appear up to 108 cycles at temperatures higher than 500oC. The reason for the decrease of the fatigue life was attributed to the change of the initially designed microstructure of the alloy. The initially dispersed fine lath martensitic texture disappeared at temperatures higher than 500°C, when the magnitude of the applied stress exceeded a certain critical value. In order to explicate the dominant factors of the change quantitatively, the change of the microstructure and the strength of the alloy were continuously observed by applying an intermittent fatigue and creep tests at elevated temperatures and EBSD analysis. It was found that there was a critical stress which caused the microstructure change at each test temperature higher than 500°C, and the activation energy of the change was determined as a function of temperature and the applied tensile stress. The dominant factor of the micro structure change was the stress-induced acceleration of the atomic diffusion of the component element of the alloy.

Stress-Induced Change of the Microstructure and Strength of Modified 9Cr-1Mo Steel Under Fatigue and Creep Loadings at Elevated Temperatures

2017 ◽  
Author(s):  
Taichi Shinozaki ◽  
Ken Suzuki ◽  
Hideo Miura
Keyword(s):  
Activation Energy ◽  
Applied Stress ◽  
Test Temperature ◽  
Elevated Temperatures ◽  
Critical Value ◽  
Atomic Diffusion ◽  
Dominant Factor ◽  
Martensitic Structure ◽  
Creep Tests ◽  
Microstructure Change

The change of the lath martensitic structure in modified 9Cr-1Mo steel was observed in the specimens after the fatigue and creep tests using EBSD (Electron Back-Scatter Diffraction). The Kernel Average Misorientation (KAM) value obtained from the EBSD analysis were used for the quantitative evaluation of the change in the lath martensitic texture. It was found that the average KAM values of the fractured specimens decreased clearly after 107−108 cycles of the fatigue loading at temperatures higher than 500°C when the amplitude of the applied stress exceeded a critical value. This change corresponded to the disappearance of the lath martensitic structure. The critical value decreased monotonically with the increase of the test temperature. This microstructure change decreased the strength of the alloy drastically. It was found that the change of the microstructure started at a certain time at each test temperature as a function of the amplitude of the applied stress. There was the critical stress at which the microstructure change started at each test temperature higher than 500°C, and the activation energy of the change was determined as a function of temperature and the amplitude of the applied stress. The dominant factor of the microstructure change was the stress-induced acceleration of the atomic diffusion of the component elements in the alloy. In order to improve the long-term reliability of the alloy, it is very important to increase the activation energy by modifying the microstructure of this alloy.

Creep-Damage-Induced Deterioration of the Strength of Ni-Base Superalloy due to the Change of its Microstructure

2017 ◽  
Author(s):  
Hayato Sakamoto ◽  
Ken Suzuki ◽  
Hideo Miura
Keyword(s):  
Power Plants ◽  
Elevated Temperatures ◽  
Creep Damage ◽  
Test Sample ◽  
Test System ◽  
Electron Back Scatter Diffraction ◽  
Dominant Factor ◽  
Jet Engines ◽  
Creep Tests ◽  
Kikuchi Pattern

Ni-base superalloys are widely used for various power plants and jet engines. Since the operating temperature of thermal plants and equipment has been increasing to improve their thermal efficiency for decreasing the emission of carbon-dioxide, the initially designed microstructure was found to change gradually during their operation. Since this change of microstructure should deteriorate the strength of the materials, sudden unexpected fracture should occur during the operation of the plants and equipment. Therefore, it is very important to clarify the dominant factor of the change of the microstructure and the relationship between the microstructure and its strength for assuring the stable and reliable operation of the plants and equipment. In this study, the change of the strength of a grain and a grain boundary of Ni-base superalloys caused by the change of their microstructure was measured by using a micro tensile test system in a scanning ion microscope. A creep test was applied to bulk alloys at elevated temperatures and a small test sample was cut from the bulk alloy with different microstructure caused by creep damage by using focused ion beams. The test sample was fixed to a silicon beam and a micro probe, respectively, by tungsten deposition. Finally, the test sample was thinned to 1μm and the sample was stretched to fracture at room temperature. The change of the order of atom arrangement of the sample was evaluated by applying electron back-scatter diffraction (EBSD) analysis quantitatively. In this study, the quality of grains in Ni-base superalloys was analyzed by using image quality (IQ) value calculated by using Hough transform of the observed Kikuchi pattern. It was found that the order of atom arrangement was deteriorated monotonically during the creep tests and this deterioration corresponded to the change of the microstructure clearly. Both the yield strength and the ultimate tensile strength of a grain in the alloys decreased drastically with the change of the microstructure, in other words, the IQ value of the grains. There was a clear relationship between the IQ value of a grain and its strength. Therefore, this IQ value is effective for evaluating the crystallinity of the alloys and the remained strength of the damaged alloys. The change of the microstructure was dominated by the strain-induced anisotropic accelerated diffusion of component elements of the alloys and the activation energy of the diffusion was determined quantitatively as a function of temperature and the applied stress.

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.

Simulation of Creep Deformation and Rupture of Notched Bar Specimens of Grade 92 Steel

2015 ◽  
Author(s):  
Haruhisa Shigeyama ◽  
Yukio Takahashi ◽  
Jonathan Parker
Keyword(s):  
Creep Strain ◽  
Creep Behavior ◽  
Power Plants ◽  
Thermal Power ◽  
Failure Criteria ◽  
Creep Rupture ◽  
Creep Failure ◽  
Creep Tests ◽  
Round Bar ◽  
Grade 92 Steel

Creep strain equations of Grade 92 steel which is used in boilers and piping systems of ultra-supercritical (USC) thermal power plants were developed based on the results of creep tests on smooth round bar specimens of three kinds of Grade 92 steels. In these equations, primary creep behavior was represented by a power-law and tertiary creep behavior was described by an exponential function. Creep parameters were determined as a function of creep rupture times which were calculated from stress and absolute temperature. Additionally, generalized creep failure criteria considering the multiaxial stress were established on the basis of results of creep tests on circumferentially notched round bar specimens. These creep strain equations and creep failure criteria were incorporated into finite element analysis software. Then, creep failure analyses were carried out and the resulting deformation behavior and rupture times were compared with the experimental results. Creep rupture lives were predicted with a good accuracy, within a factor of two in most cases.

Microstructure Evolution in a 3%Co Modified P911 Heat Resistant Steel under Creep Conditions

2010 ◽  
Vol 89-91 ◽  
pp. 295-300 ◽  
Author(s):  
Alla Kipelova ◽  
Rustam Kaibyshev ◽  
Andrey Belyakov ◽  
Izabella Schenkova ◽  
Vladimir Skorobogatykh
Keyword(s):  
Structural Changes ◽  
Elevated Temperatures ◽  
Second Phase ◽  
Resistant Steel ◽  
Creep Tests ◽  
Tempering Temperature ◽  
Heat Resistant Steel ◽  
Heat Resistant ◽  
Static Annealing ◽  
Average Size

The microstructural changes in a 3%Co modified P911 heat resistant steel were examined under static annealing and creep at elevated temperatures. The quenched steel was tempered at temperatures ranging from 673 to 1073 K for 3 hours. The temperature dependence of hardness for the tempered samples exhibits the maximum at 723 – 823 K which is associated with the precipitations of fine carbides with an average size of about 20 nm. The transverse lath size of martensitic structure is  200 nm after air quenching and remains unchanged under tempering at temperatures below 800 K. An increase in tempering temperature to 1073 K resulted in hardness drop. Coagulation of carbides and growth of martensitic laths takes place at these temperatures. The creep tests were carried out at 873 and 923 K up to rupture, which occurred after about 4.5 × 103 hours. The structural changes in crept specimens were characterized by the development of coarse laths/subgrains. The mean transverse size of which was  0.67 and  1.3 m after the creep tests at 873 and 923 K, respectively. On the other hand, an average size of second phase particles of  165 nm was observed in the samples tested at both temperatures.

Silicate Brick with Reduced Density and Thermal Conductivity

2020 ◽  
Vol 1011 ◽  
pp. 37-43
Author(s):  
Oleg Jivotkov ◽  
Vladimir Kotlyar ◽  
Grigorii Kozlov ◽  
Irina Jivotkova ◽  
A. Kozlov
Keyword(s):  
Thermal Conductivity ◽  
Power Plants ◽  
Elevated Temperatures ◽  
Thermal Power ◽  
Thermal Characteristics ◽  
Mineralogical Composition ◽  
Average Density ◽  
Technological Parameters ◽  
Chemical And Mineralogical Composition ◽  
Silicate Materials

Modern housing practice involves the widespread use of silicate brick. The main disadvantage of this building material is its high average density, accompanied by high thermal conductivity, which is accompanied by large heat losses through the enclosing walls of buildings and structures. This article discusses the possibility of improving the thermal characteristics of silicate materials through the use of ash microspheres in the production of piece silicate products, which will significantly reduce the thermal conductivity of the manufactured material. Ash microspheres, being a component of ash and slag waste from the thermal power plants, have a number of valuable properties: firstly, very low density, secondly, closed micro porosity, thirdly, chemical and mineralogical composition prone to reactions under conditions of elevated temperatures and pressures. We have studied the chemical and mineralogical composition as well as physical and mechanical properties of ash microspheres, developed the compositions and technological parameters for the silicate materials production. It has been established that the replacement of quartz sand with ash microspheres as part of the molding mass makes it possible to obtain a silicate brick of medium density class 1.0 and strength sufficient to erect load-bearing enclosing products and structures. Compositions of molding materials using a silicate binder and aluminosilicate ash microspheres using the generally accepted methods were developed and the properties of the obtained silicate material and were studied in accordance with the current GOST requirements for the similar materials.

scholarly journals Diagnostics of the bainite transformation mechanism and the effect of normalizing and tempering on the hardness and microstructure of the new Cr-Mo-V-Ti steel for operation at elevated temperature

2018 ◽  
Vol 182 ◽  
pp. 02006
Author(s):  
Zdzisław Ławrynowicz
Keyword(s):  
High Speed ◽  
Power Plants ◽  
Elevated Temperatures ◽  
Thermal Power ◽  
Bainitic Ferrite ◽  
Residual Austenite ◽  
Low Alloy Steels ◽  
Transformation Mechanism ◽  
Bainite Transformation ◽  
Conventional Heat Treatment

The Cr-Mo-V-Ti based low alloy steels are widely used in thermal power plants because of their ability to withstand elevated temperatures and high pressure under continuous service. In the present work conventional heat treatment like normalizing and tempering of the alloys has been performed. The material used in this study was the laboratory prepared experimental low alloy Cr-Mo-V-Ti steel. Samples were austenitized at 980oC for 0.5 hour air cooled and tempered at 500, 550, 600, 650, 700 and 750oC for 1 hour. Mechanism of bainite transformation has been studied in Fe-C-Cr-Mo-V-Ti steel using high speed dilatometry. These experimental data indicate that bainitic ferrite forms by a displacive transformation mechanism, but soon afterwards, excess of carbon is partitioned into the residual austenite. The changes observed in the microstructure of the steel tempered at the higher temperature, i.e. 750°C were more advanced than those observed at the temperature of 500°C. Performed microstructural investigations have shown that the degradation of the microstructure of the examined steel was mostly connected with the processes of recovery and polygonization of the matrix, disappearance of lath bainitic microstructure and the growth of the carbides. The magnitude of these changes depended on the temperature of tempering.

scholarly journals Interfacial crack behavior in the stationary temperature field conditions

2014 ◽  
Vol 18 (suppl.1) ◽  
pp. 169-178
Author(s):  
Jelena Djokovic ◽  
Ruzica Nikolic ◽  
Katarina Zivkovic
Keyword(s):  
Temperature Field ◽  
Temperature Difference ◽  
Power Plants ◽  
Thermal Stresses ◽  
Elevated Temperatures ◽  
Driving Forces ◽  
Thermal Power ◽  
Interfacial Crack ◽  
Interfacial Fracture ◽  
Stationary Temperature Field

The brittle coatings, made of different materials, when subjected to elevated temperatures and in the heat exchange conditions, are susceptible to delamination. Those coatings, as well as thin films, can be used for various thermo insulating deposits, e.g. in turbines of thermal power plants., In layers made of different materials, due to the environmental temperature change, thermal stresses appear as a consequence of a difference in their thermal expansion coefficients. In this paper driving forces were analyzed causing delamination of one layer from the other, i.e. the interfacial fracture in the two-layered, bimaterial sample. This analysis was limited to considering the sample behavior when exposed to the stationary temperature field. The energy release rate G, which is the driving force for this interfacial fracture, is changing with temperature and that variation is increasing with increase of the temperature difference between the environment and the sample. Analysis of this relation can be used to predict the maximal temperature difference, which the two-layered sample can be subjected to, without appearance of delamination between layers.

Disappearance of Strengthened Micro Texture of Modified 9Cr-1Mo Steel Caused by Stress-Induced Acceleration of Atomic Diffusion at Elevated Temperatures

2018 ◽  
Author(s):  
Yifan Luo ◽  
Hideo Miura
Keyword(s):  
Mechanical Stress ◽  
Creep Test ◽  
Elevated Temperatures ◽  
Effective Diffusion ◽  
Fatigue Loading ◽  
Critical Value ◽  
Prediction Equation ◽  
Electron Back Scatter Diffraction ◽  
Atomic Diffusion ◽  
Creep Tests

The change of the lath martensitic structure in the modified 9Cr-1Mo steel was observed in the specimens after the intermittent fatigue and creep tests using EBSD (Electron Back-Scatter Diffraction) analysis. The Kernel Average Misorientation (KAM) value and the image quality (IQ) value obtained from the EBSD analysis were used for the quantitative evaluation of the change in the lath martensitic texture. It was found that the lath martensitic texture started to disappear clearly after 107–108 cycles under the fatigue loading at temperatures higher than 500°C when the amplitude of the applied stress exceeded a critical value. Similar change also appeared in the creep test. The critical value decreased monotonically with the increase of the test temperature. This microstructure change decreased the strength of the alloy drastically. In order to explicate the dominant factors of the change quantitatively, the changes of the microstructure and the strength of the alloy were continuously measured by applying an intermittent creep test at elevated temperatures. It was found that the effective activation energy of atomic diffusion decreased drastically under the application of mechanical stress at elevated temperatures. The effective diffusion length for the disappearance was about 9 μm, and this value was much larger than the initial pitch of the lath martensitic texture of about 0.5 μm, and smaller than the average size of the initial austenite grains of about 20 μm. Therefore, the stress-induced acceleration of atomic diffusion was attributed to the disappearance of the initially strengthened micro texture. The change of the micro texture caused the drastic decrease in the yielding strength of this alloy. Finally, the prediction equation of the lifetime of the alloy was proposed by considering the stress-induced acceleration of atomic diffusion under the application of mechanical stress at elevated temperatures.

A Review on Utilization of Fly- Ash and Pond-Ash as a Partial Replacement of Cement in Concrete Mix Design

2018 ◽  
pp. 413-418
Author(s):  
Harshkumar Patel ◽  
Yogesh Patel
Keyword(s):  
Fly Ash ◽  
Electricity Generation ◽  
Power Plants ◽  
Renewable Energy Sources ◽  
Thermal Power ◽  
Strength Test ◽  
Partial Replacement ◽  
Pond Ash ◽  
Research Activities ◽  
Ash Disposal

Now-a-days energy planners are aiming to increase the use of renewable energy sources and nuclear to meet the electricity generation. But till now coal-based power plants are the major source of electricity generation. Disadvantages of coal-based thermal power plants is disposal problem of fly ash and pond ash. It was earlier considered as a total waste and environmental hazard thus its use was limited, but now its useful properties have been known as raw material for various application in construction field. Fly ash from the thermal plants is available in large quantities in fine and coarse form. Fine fly ash is used in construction industry in some amount and coarse fly ash is subsequently disposed over land in slurry forms. In India around 180 MT fly is produced and only around 45% of that is being utilized in different sectors. Balance fly ash is being disposed over land. It needs one acre of land for ash disposal to produce 1MW electricity from coal. Fly ash and pond ash utilization helps to reduce the consumption of natural resources. The fly ash became available in coal based thermal power station in the year 1930 in USA. For its gainful utilization, scientist started research activities and in the year 1937, R.E. Davis and his associates at university of California published research details on use of fly ash in cement concrete. This research had laid foundation for its specification, testing & usages. This study reports the potential use of pond-ash and fly-ash as cement in concrete mixes. In this present study of concrete produced using fly ash, pond ash and OPC 53 grade will be carried. An attempt will be made to investigate characteristics of OPC concrete with combined fly ash and pond ash mixed concrete for Compressive Strength test, Split Tensile Strength test, Flexural Strength test and Durability tests. This paper deals with the review of literature for fly-ash and pond-ash as partial replacement of cement in concrete.

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