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.

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.

Elucidation of the High Cycle Fatigue Damage Mechanism of Modified 9Cr-1Mo Steel at Elevated Temperature

2015 ◽  
Author(s):  
Takuya Murakoshi ◽  
Motoyuki Ochi ◽  
Ken Suzuki ◽  
Hideo Miura
Keyword(s):  
Fatigue Strength ◽  
High Cycle Fatigue ◽  
Elevated Temperatures ◽  
Surface Hardness ◽  
Diffraction Method ◽  
Fatigue Loading ◽  
Damage Mechanism ◽  
Fatigue Tests ◽  
Electron Back Scatter Diffraction ◽  
Cycle Fatigue

Modified 9Cr-1Mo steel is one of the heat-resistant steels developed for steam generator in a FBR (Fast Breeder Reactor). When it is used in a FBR, the lifetime of the steel under HCF (High Cycle Fatigue) and V-HCF (Very-High Cycle Fatigue) caused by flow-induced vibration has to be considered for assuring its long-term reliability up to 1011 cycles. Since previous studies showed that the fatigue limit did not appear up to 108 cycles, it is necessary to investigate the fatigue strength of this alloy in cycles higher than 108 cycles. In this study, in order to clarify high cycle fatigue strength and fracture mechanism of the modified 9Cr-1Mo steel, the change of the lath martensitic strengthening structure was observed in detail on the surface of specimens fractured by rotary bending fatigue tests by using EBSD (Electron Back-Scatter Diffraction) method. The Kernel Average Misorientation (KAM) value obtained from the EBSD analysis was used for the quantitative evaluation of the change of the lath martensitic texture. It was found that the average KAM values clearly decreased on the surface areas of the fractured specimens after the application of 107-108 cycles of fatigue loading at temperatures higher than 550°C. This result indicates that degradation of the lath martensitic texture occurred around the surface of specimens tested at the temperature higher than 550°C. In order to quantitatively evaluate the decrease of its strength, a hardness test was performed at room temperature by using a nanoindentation method. It was confirmed that the surface hardness of specimens decreased drastically in the specimens fractured at temperatures higher than 550°C. From these results, it was concluded that the effective 0.2%-proof stress decreased during the fatigue tests by the degradation of the lath martensitic texture caused by the fatigue loading at elevated temperatures. Further analyses are indispensable for explicating the damage mechanism more in detail.

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.

Creep Life Prediction of Waspaloy Using the Initial Strain Parameter Technique

2007 ◽  
Vol 353-358 ◽  
pp. 2644-2647
Author(s):  
Seon Jin Kim ◽  
Yu Sik Kong ◽  
Won Taek Jung ◽  
Jong Taek Yeom ◽  
Nho Kwang Park
Keyword(s):  
Life Prediction ◽  
Elevated Temperatures ◽  
Prediction Equation ◽  
Rupture Time ◽  
Initial Strain ◽  
Creep Life ◽  
Creep Tests ◽  
Strain Parameter ◽  
Creep Life Prediction ◽  
Good Agreement

The purpose of this study is to investigate the high temperature creep life of Waspaloy using the Initial Strain Parameter Technique (ISPT). The creep tests were performed at the elevated temperatures from 550oC to 700 oC. Constant stress creep tests were carried out in the experiment. The initial strain was measured for one minute after loading. The creep life of Waspaloy was calculated using the creep life prediction equation of ISPT. The confidence level between the experimental rupture time and the calculated rupture time using the ISPT is within 95%. So, the results show that the creep life prediction by the ISPT was a good agreement with LMP method.

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.

Impression creep of lead

1994 ◽  
Vol 9 (4) ◽  
pp. 903-908 ◽  
Author(s):  
Donyau Chiang ◽  
J. C. M. Li
Keyword(s):  
Creep Test ◽  
Elevated Temperatures ◽  
Constant Load ◽  
Constant Stress ◽  
Tensile Creep ◽  
Indentation Creep ◽  
Impression Creep ◽  
Creep Tests ◽  
Creep Measurements ◽  
Cylindrical Punch

The impression creep behavior of lead was investigated using a 100 μm diameter punch at ambient and elevated temperatures (433 K-563 K) under punching stresses of 6–70 MPa. The results were compared with the data obtained from conventional creep tests reported in the literature. Unlike the indentation creep test, the impression creep test showed a steady-state velocity after a short transient period when the flat-end cylindrical punch was pushed against the lead surface by a constant load. Both the temperature and stress dependences were comparable to those of the constant stress tensile creep tests under similar conditions. A master curve for lead was established by collecting data from the impression creep tests and the constant stress tensile creep tests. The indentation creep measurements for lead were included also. However, the indentation data depend on the load applied.

Creep Tests of Rotating Disks at Elevated Temperature and Comparison With Theory

1954 ◽  
Vol 21 (3) ◽  
pp. 225-235
Author(s):  
A. M. Wahl ◽  
G. O. Sankey ◽  
M. J. Manjoine ◽  
E. Shoemaker
Keyword(s):  
Elevated Temperatures ◽  
Experimental Program ◽  
Test Results ◽  
Rotating Disks ◽  
Creep Tests ◽  
Shear Theory ◽  
Heat Treated ◽  
Long Time ◽  
Average Tension ◽  
Creep Deformations

Abstract A theoretical and experimental program involving methods of calculating creep in rotating disks at elevated temperatures is described. This program consisted primarily of the following: (a) Obtaining forged disks from the same ingot of 12 per cent chrome steel, all disks being forged and heat-treated in the same manner; (b) making spin tests at 1000 F on three of these disks for periods up to about 1000 hr; ( ) making long-time tension-creep tests at 1000 F on many specimens cut out circumferentially from several of the other disks at stresses approximating those of the spin tests; (d) investigating theoretical methods of calculation of creep deformation in such disks; and (e) comparison of spin-test results with those calculated theoretically using average tension-creep data. It was found that available methods of calculating rotating disks based on the Mises criterion gave creep deformations too low compared to the test values, i.e., on the unsafe side for design. Considerably better agreement between test and theoretical results is obtained if the latter is based on the maximum-shear theory. Some discussion is given of the reasons for the better agreement obtained using the latter theory; these are believed to be related in part to the anisotropy of the forged material tested. Further tests on other materials are necessary before general conclusions can be drawn; however, in the absence of test data it is suggested that a conservative course in design for such disks is to apply the maximum-shear theory.

Structure and mechanical properties of cast quasicrystal-reinforced Mg–Zn–Al–Y base alloys

2004 ◽  
Vol 19 (5) ◽  
pp. 1531-1538 ◽  
Author(s):  
Guangyin Yuan ◽  
Kenji Amiya ◽  
Hidemi Kato ◽  
Akihisa Inoue
Keyword(s):  
Mechanical Properties ◽  
Elevated Temperatures ◽  
Cast Alloy ◽  
Strengthening Mechanism ◽  
Icosahedral Quasicrystal ◽  
Compression Tests ◽  
Strengthening Phase ◽  
Creep Tests ◽  
Cast Alloys ◽  
Bulk Alloys

The structure and mechanical properties of Mg–Zn–Al–Y base cast alloys containing an icosahedral quasicrystal phase (i-phase) as a main strengthening phase were investigated. Mg–8Zn–4Al–xY base bulk alloys containing the i-phase were prepared by casting into a copper mold at moderate cooling rates. The Y addition was effective for decreasing the size of the i-phase and the increasing the homogeneity of its dispersed state. The mechanical properties examined by compression tests at room temperature were much superior to those of a conventional AZ91 Mg alloy. The creep tests at elevated temperatures indicated a promising high temperature creep resistance of the quasicrystal-reinforced Mg–Zn–Al–Y cast alloy. The strengthening mechanism was also discussed.

scholarly journals The Effects of Temperature on the Leaching Behavior of Cement Waste Forms - The Cement/Sodium Sulfate System

1989 ◽  
Vol 176 ◽  
Author(s):  
Mark Fuhrmann ◽  
Richard Pietrzak ◽  
John Heiser ◽  
Eena-Mai Franz ◽  
Peter Colombo
Keyword(s):  
Sodium Sulfate ◽  
Elevated Temperatures ◽  
Effective Diffusion ◽  
Finite Cylinder ◽  
Kcal Mole ◽  
Experimental Conditions ◽  
Waste Forms ◽  
Diffusion Controlled ◽  
Increasing Temperature ◽  
Relationship Of

ABSTRACTThe leaching mechanisms of simulated low-level radioactive waste forms are being determined as support for development of an accelerated leach test. Two approaches are being used: (1) comparison of leaching data with results of a model that describes diffusion from a finite cylinder, and (2) observation of the leaching process at temperatures between 20°C and 65°C. To provide results that can be used for modeling, leaching at elevated temperatures must change neither the leaching mechanism nor the structural controls of leaching such as the porosity. Releases of 137Cs, 85Sr, calcium, sodium and potassium from portland cement containing sodium sulfate, as a simulated evaporator sludge, have been determined under a variety of experimental conditions. Data from the leach tests were compared to model results for diffusion from the finite cylinder. While most leaching appears to be diffusion controlled, notable exceptions occur. For all samples, activation energies ranging between 6 and 11 Kcal/mole have been calculated from the relationship of the effective diffusion coefficient to increasing temperature, close to the expected value of 5 Kcal/mole for diffusion.

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