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[en] The ferritic-martensitic steels containing 9% Chromium are favoured structural materials for steam generator applications in power plants. The creep strength of the steel is derived from solute strengthening due to the presence of molybdenum/ tungsten in the matrix; dislocation strengthening arising from the initial high dislocation density of the order of 1 x 1014 m-2; boundary hardening from fine lath/subgrains and precipitation strengthening from finely distributed M23C6 and MX type (niobium/ vanadium carbide/carbonitride) precipitates. Creep strength of 9% Cr steels breaks down in the long-term creep regime due to the recovery of subgrains caused by coarsening of M23C6 precipitates and the conversion of finely distributed useful MX precipitates and consequent formation and growth of complex nitride Z-phase i.e. Cr(V,Nb)N precipitates. In common, Z-phase grows very rapidly at the expense of MX precipitates. These observations strongly indicate that evolution of microstructural features (softening of subgrains accompanied with the decrease in dislocation density and the variations in the number density of different precipitates) influence the creep behaviour of 9% Cr steels and these variables need to be incorporated into the kinetic rate relationships for the development of constitutive models. In general, understanding and modelling of creep behaviour of 9% Cr steels are important for safe life design and remnant life assessment of components operating at high temperatures. The continual improvement in understanding of underlying microstructure variables in 9% Cr steels has led to the development of new constitutive models or modification in existing models. In this regard, microstructure based continuum creep damage model and its applicability for the plain 9Cr-1Mo and modified 9Cr-1Mo steel have been presented
[en] The paper objective was to study the phenomenon of the mechanical stress relaxation in the vicinity of a V notch (blunt flaw) on the inner surface of the CANDU pressure tube. This kind of notches model the Debris Fretting Flaws found by in-service inspection on the fuel channels. The analyses are done inside of a small zone around the flaw, which is called “the process zone”. The Neuber’s total strain energy density rule is applied to obtain the stress relaxation and the total strain due to the creep phenomenon in the process zone. Further, the paper describes the formalism to obtain the numerical solutions for the stress-strain field in the process zone, where the high stresses are developed due to the flaw tip that play the stress intensifier under tensile stress. (author).
[en] Deposition of 60 % NiCr + 30 % Cr3C2 + 10 % Al2O3 and 60 % NiCr + 40 % Cr3C2 metal matrix composite coating has been done on SUS 304 steel surface by the High Velocity Oxygen Fuel (HVOF) Thermal Spray Coating technique. The purpose of the coating was to improve SUS 304 steel creep resistance and cyclic oxidation on boiler tubes of steam power plant application. The treatments include creep test at 550 oC with a load of 0.4 and 0.6 σy (yield strength) for 30240 minutes (21 days), cyclic oxidation with various temperatures of 600, 700 dan 800 oC in tubes furnace for 1 hour and cooling for 15 minute in 1 cycle. Measurement was done every one cycle to observe weight change of the oxide, and the measurement was done for 5 cycles. The recommended coating specimen for boiler tubes of steam power plant application is coating specimen of 3 three components of MMC with the lowest creep rate of 0.000148917 %/hour and cyclic oxidation resistance at 600o (which has a lowest change in oxide weight, coating thickness and oxide compounds formed on coating). (author)
[en] This study investigates the effects of addition of Carbon nanotube (CNT) at different volume ratios (0.5- 5%) into Ti6Al4V matrix by mechanical alloying in terms of the density, microstructure, hardness and creep under dynamic load. As a result of the good bonding of carbon nanotubes powders with the main matrix, Ti-6Al-4V/CNT composites have experienced change both in microstructure and mechanical properties (such as hardness, density) and, correspondingly, qualitatively creep behaviour of Ti-6Al - 4V matrix alloy has been improved compared to the lean one. The density of CNT reinforced Ti6Al4V composites sintered at 1300°C for 3h decreases with increasing CNT content. The hardness tests indicated that the hardness of composites increased with CNT addition. In addition, although creep strain is decreased continually with CNT content until 5%, creep life increased with increasing CNT content until 4% of CNT but decreased above 4%. After sintering at 1300 °C under vacuum for 3 hours the density of the composite material reached to a level of 98.5 %, the microhardness to 538 HV and the creep behaviour was improved. The characterization of Ti6Al4V / CNT composites after mechanical alloying was carried out using scanning electron microscopy (SEM), energy dispersive x-rays spectroscopy (EDS) analysis and Xray diffraction (XRD) methods. Although Ti–6Al–4V alloys are used as biomaterial, this study aimed at using MWCNTs containing Ti-6Al-4V composites at high temperature applications. Because MWCNTs reinforced Ti-6Al-4V composites are cheaper and have lower weight than the other materials used in this kind of applications. (author)
[en] Alloys D9 and IFAC-1SS are used as fuel clad and wrapper material in Prototype Fast Breeder Reactor (PFBR), India. Mechanical properties of these alloys are controlled by their carbon content and chemical activity. Carbon activities of alloy D9 and IFAC-1SS were determined as a function of carbon content by foil equilibration method in liquid sodium at 923 K. (author)
[en] Bivariate log-normal distribution analyses coupled with the cumulative hazard function method were conducted on the specific output class of 8 steam turbine units with components such as rotors, moving blades, nozzles, casings and other auxiliary equipment of high-, intermediate- and low-pressure turbines. The damage phenomena were classified into erosion, crack, deformation, corrosion, creep void formation and material degradation with corresponding components. Operation time and start up cycles for damage incidence in respective units were collected and statistically analyzed adding the non-failed data as well as failed data. After applying the bivariate log-normal distribution regression to those data sets, the prescribed failure probability was imposed to construct the equal probability ellipse contours as the quadratic function of operation time and start-up cycles. To determine whether the events were time dependent or cycle dependent, the shape and inclination of the contours were utilized. The order of event incidence was determined by using the lower end values of the major axis of equal probability contours. Although the order of event incidence could show variations according to the prescribed failure probability values, the examples for 90% probability ellipse contours were demonstrated here. The assessment results showed that the statistical analyses were effective for investigating the damage incidental scenario making and maintenance planning for actual plants. (author)
[en] The creep behavior of Ni–Cr–W alloy at 950 °C has been investigated by a novel creep testing system which is capable of in-situ measurement of strain. Tubular specimens were pressurized with argon gas for effective stresses up to 32 MPa. Experimental results show that the thermal fatigue reduces the creep life of the tubular specimens and with the introduction of thermal cycling fatigue the primary stage disappears and the creep rate higher than the pure thermal creep (without thermal fatigue). Also the creep behavior of Ni–Cr–W alloy doesn't consist in the secondary stage. A new creep equation has been derived and implemented into finite element method. The results from the finite element analyses are in good agreement with the creep experiment
[en] • In-Vessel Retention (IVR) of molten core debris via water cooling on the external surfaces of the reactor vessel is an inherent severe accident management feature of the AP1000/CAP1400 passive Nuclear Power Plants (NPPs). • Main failure modes associated with IVR: Thermal Failure / Structural Failure. • The Structural Failure assessment is also necessary because high temperature induced creep damage are the immediate threats to the RPV, especially with the sustained pressure loads existence.
[en] There is a need for Tantalum (Ta) design data for ISIS spallation target design, especially for high cycle fatigue. The target operates under a high duty cycle of pulsed heat load. Very little such information exists in the literature for this material. HIPed (Hot Isostatic Pressed) Ta is used in the target manufacture and so data for As Received (A/R) and HIPed material is required. Following a comprehensive tensile test programme to establish batch properties, suitable fatigue test specimens have been carefully prepared to ASTM specifications. Great care has been taken in the selection of the fatigue test, load/strain control, frequency and monitoring in order to gain good quality and useful data. A small number of specimens have been cycled at various loads up to 480 million cycles before failure. Cold creep has been found to be an issue and investigated. The preparation, test methods and results for the first two A/R specimens are presented. (author)
[en] The effect of taking a miniature sample scoop on the creep life of ASME Grade 91 (9Cr steel) piping was experimentally assessed. Internal pressure tests were conducted at 650degC on tubular specimens of virgin and long-term used 9Cr steels having scoop on the outer surface, which simulate taking a sample scoop. When the ratio of scoop depth to wall thickness of the specimen was 8%, the creep life decreased to 85% of that of scoop-free specimen. On the other hand, no creep life reduction was observed in the materials with the depth ratio of 5%. Finite element analyses were conducted on the tubular specimens under the test conditions and actual-size piping under actual service conditions, where the ratio of the scoop depth was about 5% in both cases. The stress and strain concentration states near the scoop of the actual-size piping were almost the same as those of the tubular specimens, indicating that the test results were valid for the actual piping. Thus, it was concluded that taking a miniature sample to a scoop of up to 5% of the thickness of the steel has a negligible effect on the creep life of the piping. (author)