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18: A typical creep curve indicates three different regions: the primary, secondary and the tertiary creep region [49]. Once the material experiences an instantaneous strain, ɛ0, which is composed of elastic (recoverable on release of load), anelastic (recovers with time) and plastic (nonrecoverable) strain, as a result of sudden loading, the primary creep region begins only after that. As the name suggests primary creep region describes the initial stage of creep deformation. This region is characterized by a decreasing strain rate with time. This continues untill the secondary stage starts. The strain rate of deformation remains constant during the secondary creep region. The strain rate in secondary stage is the minimum strain rate of a creep deformation. The creep life of any material can be estimated through the knowledge of the creep strain rate in secondary stage. The last stage of creep deformation is the tertiary creep regime. In this stage, the material undergoes very high strain rate deformation and eventually fractures. The substantial amount of the total strain experienced by a material is contributed by the secondary stage creep regime.

18: A typical creep curve indicates three different regions: the primary, secondary and the tertiary creep region [49]. Once the material experiences an instantaneous strain, ɛ0, which is composed of elastic (recoverable on release of load), anelastic (recovers with time) and plastic (nonrecoverable) strain, as a result of sudden loading, the primary creep region begins only after that. As the name suggests primary creep region describes the initial stage of creep deformation. This region is characterized by a decreasing strain rate with time. This continues untill the secondary stage starts. The strain rate of deformation remains constant during the secondary creep region. The strain rate in secondary stage is the minimum strain rate of a creep deformation. The creep life of any material can be estimated through the knowledge of the creep strain rate in secondary stage. The last stage of creep deformation is the tertiary creep regime. In this stage, the material undergoes very high strain rate deformation and eventually fractures. The substantial amount of the total strain experienced by a material is contributed by the secondary stage creep regime.

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A modified 9Cr-1Mo steel received in normalized and tempered condition has been separately subjected to different normalization treatments varying the austenitization temperatures (1100°C / 1025°C / 950°C) and different hot-rolling treatments varying the finish rolling temperatures (1050°C / 1000°C / 950°C / 875°C). A combination of both these proc...

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... Fig. 2.12: Characteristic points on schematic load-time profile in the transition ...
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... appeared as good alternatives to the austenitic ones during mid-seventies. These steels were then introduced into the international fusion material programs in the early eighties. By now, ferritic martensitic grade P91 steels are one of leading candidate material for fusion structural applications as well, particularly for fuel tubes, Fig. 2.1. Fig. 2.1: Application of 9Cr-1Mo steel in fuel tubes for nuclear reactor ...
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... the so-called ductile-to-brittle- transition temperature (DBTT) by measuring the total energy necessary to fracture a V-notched specimen (see for instance [49]). Recently, Charpy impact testers are instrumented so that the load-time curve can be obtained and evaluated. A typical Charpy curve for a tempered martensitic steel is presented in Fig. 2. 1 [3]. The effect of irradiation on the Charpy curve is also depicted in Fig. 2.2. In general, irradiation induces a shift of the DBTT to higher temperature and a decrease of the upper shelf ...
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... the total energy necessary to fracture a V-notched specimen (see for instance [49]). Recently, Charpy impact testers are instrumented so that the load-time curve can be obtained and evaluated. A typical Charpy curve for a tempered martensitic steel is presented in Fig. 2. 1 [3]. The effect of irradiation on the Charpy curve is also depicted in Fig. 2.2. In general, irradiation induces a shift of the DBTT to higher temperature and a decrease of the upper shelf ...
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... thermomechanical processing primarily applied to 9Cr-1Mo grade steel with ferritic-martensitic microstructure can be broadly categorized into three different temperature domains, Fig. ...
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... and initial microstructure, as well as on the physical configuration of the specimen, some variants can be favoured over others during transformation. [53][54][55]. However, if no variants have been preferred during transformation following Kurdjumov-Sachs orientation relationship, the {002} pole figure of product martensite will be as shown in Fig. 2.6. Rules or criteria for variant selection must therefore be postulated, and in order to be successful in predicting the product from the parent texture, models of texture transformation are needed that can also take into account the relative weights of all the variants. Several variant selection models have been proposed over the past ...
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... hand BCC materials have only twelve primary slip systems. Hence, the contribution of the glide stress is higher in BCC material as compared to the dislocation interaction stress on the yield strength. As, the Peierls-Nabarro stress is dependent on the temperature, the yield strength of BCC material is also highly sensitive to the temperature, Fig. 2.9. The variations in yield stress and fracture stress as function of temperature are shown in Fig. 2.9. The temperature (TGY), where yield stress intersects with the fracture stress is known as the general yield temperature [64]. The significance of this temperature is that below this temperature the brittle fracture occurs without ...
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... is higher in BCC material as compared to the dislocation interaction stress on the yield strength. As, the Peierls-Nabarro stress is dependent on the temperature, the yield strength of BCC material is also highly sensitive to the temperature, Fig. 2.9. The variations in yield stress and fracture stress as function of temperature are shown in Fig. 2.9. The temperature (TGY), where yield stress intersects with the fracture stress is known as the general yield temperature [64]. The significance of this temperature is that below this temperature the brittle fracture occurs without ...
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... A list of different approaches followed in the literature to define the crystallographic grain size in steel is summarized in 12° on Misorientation angle Ferrite-Pearlite [29] In order to determine the ' effective grain size', Bhattacharjee et al. [17,29] have carried out EBSD scan on the cleavage fracture surface of the broken Charpy samples, Fig. 2.16. They have demonstrated that a single facet can comprise of more than one grain having less than 12° misorientation between them. In support, they have theoretically shown that a crack loose only 5% of its energy it if deviates by ...
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... factor [86]. Therefore, fracture stress is dependent on the texture of the microstructure, which eventually determines the nature of impact fracture (brittle or ductile) and provides a direction for improvement of impact fracture resistance. Ghosh et al. in their model considered a crystal with certain orientation ahead of the crack tip, Fig. 2.17. The angle between the cleavage planes 1, 2 and 3 (as marked on Fig. 2.17) and the fracture planes are h1, h2 and h3, ...
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... out lined in Section 2.2, the primary challenge for the application of 9- 12%Cr-Mo steel is its nature of transition from ductile fracture mode to brittle transgranular fracture mode when this steel experiences irradiation, Fig. 2.2. In this section, a short review on various works done to investigate the ductile-to-brittle transition behaviour of 9-12%Cr-Mo steel using Charpy impact tests is ...
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... is a time dependent plastic deformation that a material experiences at a constant load or stress. This phenomenon usually occurs in a material at temperatures higher than room temperature, unless room temperatures are high homologous temperature for that material. Generally creep is represented by strain-time curve, as shown in Fig. 2.18. The creep curve is a result of microstructural level changes occurring in a material. This curve indicates a competition between the processes of strain hardening and recovery. Materials usually get strain hardened during plastic deformation. For further plastic deformation the applied stress must exceeds the increase in flow ...
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... mechanism of creep, it is necessary to consider the microstructure to be constant. Even though thermal stabilization establishes a constant microstructure during the course of a test, as the materials are usually heat treated at temperatures higher than the test temperature, stress assisted processes altering the microstructure cannot be ruled, Fig. 2.20. Moreover, in case of non-equilibrium structures such as nanocrystalline materials undergo stress assisted microstructural changes, which prevent these materials to attain a constant creep microstructure. Therefore, it is recommended that creep tests on nanocrystalline material should be carried out at a stress level that is lower ...
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... micrographs of single-pass and multi-pass deformed samples are shown in Fig. 4.2 and Fig. 4.3, respectively. The EBSD boundary maps showing the high- angle boundaries (>15° misorientation indicated in black) and low-angle boundaries (2- 15° misorientation indicated in red) inside single-pass and multi-pass deformed samples are given in Fig. 4.4 and Fig. 4.5, respectively. Comparing between the optical micrographs ...
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... fraction of coarse precipitates was highest in As-received sample, followed by 950-Reheat sample, 1025-Reheat sample and 1100-Reheat sample, Fig. 6.2(c-h). ...
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... temperature (i.e. 550 ºC) regime, the 1025NT-550 has shown the slowest minimum creep strain rate (order of 10 -12 s -1 ) followed by 950NT-550 and 1100NT-550, Fig. 9.2e. Although the minimum creep rate exhibited by the 950NT-550 sample is lower than that by the 1100NT-550, the former test sample has not shown better creep life than the latter, Fig. 9.2(b,e). While 950NT-550 was ruptured after 620 hours, the 1100NT-550 reached steady state creep regime after 1200 hours, Fig. 9.2b. This phenomenon can be attributed to the absence of clearly distinguishable steady state regime for 950NT-550 sample during creep deformation, Fig. 9.2b. Therefore, analysing the creep-resistant ability of any ...
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... test sample has not shown better creep life than the latter, Fig. 9.2(b,e). While 950NT-550 was ruptured after 620 hours, the 1100NT-550 reached steady state creep regime after 1200 hours, Fig. 9.2b. This phenomenon can be attributed to the absence of clearly distinguishable steady state regime for 950NT-550 sample during creep deformation, Fig. 9.2b. Therefore, analysing the creep-resistant ability of any material based on minimum creep rate may sometime mislead, rather time to rupture should also be considered in this regard, Fig. 9 ...
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... being subjected to at low temperature (550°C) creep test, 950NT-550 had the lowest creep life, whilst the 1025NT-550 reached the steady state creep regime at a much later stage with the minimum creep rate being the lowest among all, Fig. ...
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... in the 1100NT as compared to 1025NT, smaller inter-particle spacing between coherent precipitates (following Eqn. 9.1) provide more retardation of dislocations to circumvent the particles by local climb ( Fig. 9.6(d,e)) in the former specimen [334,335], which results in higher creep life for the 1100NT-600 as compared to that in 1025NT-600, Fig. 9.2f. On the other hand, both Cr23C6 and MX type precipitates could have coarsened during creep at higher test temperature (650°C), and become incoherent. In the 1025NT-650 sample, the evidence of MX particle-dislocation interaction at the detachment end of the particle could be observed in Fig. 9.6f. Srolovitz et al. have also indicated ...

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A modified 9Cr-1Mo steel has been exposed to three separate austenitization temperatures, i.e., at 950, 1025, and 1100 °C for normalization. After subsequent tempering at 750 °C, the normalized and tempered samples were creep-tested at temperatures of 550, 600, and 650 °C. The creep strength of the investigated samples was evaluated in terms of minimum creep strain rate and time to rupture. The effects of microstructure, precipitate and boundary misorientation on the creep behavior of the samples have been studied with TEM and EBSD analyses. Further, the evolution of crystallographic texture after creep tests has also been studied. The presence of an intermediate size of martensitic microstructural units (i.e., prior-austenite grain, martensitic packets, etc.) and combination of fine coherent and incoherent Nb(C,N) precipitates has provided superior creep strength for the samples normalized at 1025 °C, when these are subsequently subjected to low-temperature (i.e., 550 °C) and high-temperature (i.e., 650 °C) creep tests, as compared to other conditions of normalizing heat treatment.