As superligas a base de níquel apresentam limitações para uso em temperaturas acima de 600 °C. Por este motivo, os revestimentos protetivos podem ser usados como barreiras para evitar a propagação de nucleação e fissuras. O presente trabalho tem por objetivo avaliar as propriedades mecânicas em altas temperaturas de amostras nitretadas por plasma da superliga INCONEL 625 foram realizados ensaios de tração e fluência em amostras com e sem tratamento. Nos ensaios de tração, nas temperaturas de 200 a 700 °C, o material apresentou uma oscilação nos valores de tensão a partir do início da deformação plástica. Este comportamento da curva tensão-deformação serrilhada pode estar associado com a difusão de um soluto intersticial ou pela difusão de átomos substitucionais ou pelo envelhecimento da deformação dinâmica (DSA). Em 600 °C, o aumento da taxa de deformação promoveu um aumento da amplitude e da frequência de oscilação da tensão. Sob níveis de tensões intermediários ou elevados o mecanismo de fluência dominante da liga Inconel 625 foi por escorregamento de discordâncias mostrando-se semelhante ao de muitos metais puros e ligas de soluções sólidas. / Nickel-based superalloys have limitations for use at temperatures above 600 °C. For this reason, protective coatings can be used as barriers to avoid both nucleation and crack propagation. Thus, to evaluate the mechanical properties at high temperatures of plasma nitrided samples of the INCONEL 625 superalloy, tensile and creep tests were performed on samples with and without treatment. In the tensile tests at temperatures of 200 to 700 °C the material showed an oscillation in the tension values from the beginning of the plastic deformation. This behavior of the serrated strain-strain curve may be associated with the diffusion of an interstitial solute or by the diffusion of substitutional atoms or by dynamic strain aging effect. At 600 °C, the increase in strain rate promoted an increase of the amplitude and oscillation frequency of the stress. Under levels of intermediate of elevated stresses, the dominant creep mechanism of the Inconel 625 alloy was like that of many pure metals and alloys of solid solutions which is by slippage of dislocations.

The objectives of this work is residual lifetime of heaters in petrochemical industry. There is no comprehensive publication about this theme. This paper lists and describes the most of main phenomena that has to be taken into account in the process of determination of residual lifetime. This paper focuses on creep damage of a real heater based on deformation measurement. It describes some heater failures that could cause unplanned operation interruption however, appropriate intervention made possible safe heater operation until planned heater shutdown. No compact method is given how to determine heater residual lifetime, but a direction is given how a complex method of residual lifetime assessment of refinery heaters could be created.

This thesis is focused on the application of advanced methods for evaluating damage to boiler tubes, specifically temperature related damage. The aim of this work is to develop an improved damage evaluation procedure utilizing capabilities of modern approaches. This work describes various types of industrial boiler damage. The main focus is on the most exposed and often the most damaged parts of boilers, which are tube bundles (for example, superheaters). Equipment damage is undesirable and often leads to leakages or even to the boiler shutting down. Therefore, it is necessary to find the problem as soon as possible and make the required changes to prevent further damage. The damage types are divided into five categories based on the damage mechanism. Temperature has one of the biggest influences on damage and it may cause short-term or long-term overheating in the tube bundles. This type of damage occurs when the designated temperature is exceeded and results in reduced creep life. It is necessary to know the real surface temperature history of the tube bundle to estimate temperature related damage, however this is often not available. Therefore, it is necessary to calculate those temperatures based on the available data (i.e. inlet and outlet temperatures and pressures). This is real challenge due to the combination of complex flows of the working substances (mainly flue gasses) and heat transfer. Considering available data, new approach is proposed in order to obtain information required for residual creep life estimation. In the first step, thermal – hydraulic calculation is performed followed by a thermal load estimation of a superheater tube bundle in a natural gas fired boiler, using CFD simulations. In the next step, the surface temperature is evaluated and used to determine the temperature related damage, specifically the creep life estimation. The life expectancy is in some ways influenced by imperfections, and therefore at the end of this thesis the influence of the oxide layer on the inner side of tube and fouling on outer side of tube is described.

Theoretical part of this thesis presents summary of material and testing parameters which influence SCG and consequently the duration of the FNCT test. The experimental part focuses on modification of testing conditions of FNCT test in order to minimize the test time. FNCT tests were performed for four types of surfactants: Arkopal N110, Sodium dodecylbenzensulfonate (NaDBS), Disponil LDBS55 and Simulsol SL8. The most pronounced accelerating effect was observed for NaDBS surfactant, which in comparsion with commonly used Arkopal N110 doubled the test speed at the same concentration (11 %) and other test conditions. The accelerating effect of NaDBS was also observed for two different PE grades: Liten VB85 and VL 20N. Under the test conditions 80 °C and 4 MPa, 5% NaDBS water solution caused for VL 20N almost 12,5-fold acceleration compared to conventional 2% Arkopal N110 and almost 8,5-fold acceleration for VB 85. The correlation between wettability of surfactant solution and FNCT time to fracture was found.

This work deals with the effects of high temperature exposition of low alloyed creep resistant steels on their structural stability and structural stability of their weld joint. The theoretical part of this work gives an overview of creep resistant steels considering their chemical and phase composition and their connection to high temperature mechanical properties. The CALPHAD method is presented as generally accepted approach for equilibrated calculations and Thermo-Calc and DICTRA software packages are introduced as CALPHAD applications. Thermo-Calc software is used for calculation of phase composition of the examined materials and for calculation of the phase diagrams. Software DICTRA is used for simulations of diffusion controlled phase reactions in the analysed weld. Experimentally one weld joint of the steels 15 128 and 15 313 has been studied after four different annealing procedures. The experimental results include chemical profiles across the welds, microhardness profiles and metallographic analysis of significant regions of the welds. These experimental results are compared with the simulation results. The comparison shows a reasonable agreement between experimental and simulation data.

Thermal power stations operate at elevated temperatures and pressures in order to attain maximum available steam energy. At these high temperatures creep becomes a dominant mechanism that needs to be considered. However, for many components, the locations where peak stresses occur are unreachable to apply the commonly used Non-Destructive Testing (NDT) techniques. This encourages the use of Finite Element Analysis (FEA) to better predict the creep state in these complex components. Commonly, creep damage models are used in conjunction with accelerated creep tests to develop material models that can be implemented into a FEA to determine failure. These approaches are often infeasible for industrial decision-making, leaving a gap for more accessible commercially available models to be developed. This paper focuses on using openly available creep data from the Japanese National Institute for Material Science (NIMS). A creep strain model capable of modelling only the primary and secondary creep regimes was then chosen from the ANSYS database to fit this data. In order to fully characterise the experimental data a multi-creep-model approach was adopted that uses a family of creep models, instead of a single creep material model, to characterise the probable range of responses. This methodology was applied to an industrial application, namely an Intermediate Pressure (IP) valve operating under creep-prone conditions. The multi-creep-model approach was incorporated into FEA to analyse the variation in stress distributions. It was interesting to see that a variation of 153% in the creep strain models only resulted in a 21% variation in the relaxed stress. Worst case scenario life time calculations were then conducted using both a time-based Larson-Miller approach and a strain-based ASME code approach. Both sets of results showed that, for the specific component of interest, creep rupture lifetimes were in excess of 3000 years. It was therefore noted that, for the IP valve of interest, the operating temperature and pressure combination were such that no worrisome creep damage occurred. In conclusion, for the specific component analysed, the operating conditions are such that creep based failure will not occur. / Dissertation (MEng)--University of Pretoria, 2017. / NRF / EPPEI / Mechanical and Aeronautical Engineering / MEng / Unrestricted

Considering the unlikely core melt down scenario for a light water reactor (LWR) a possible failure mode of the reactor pressure vessel (RPV) and its failure time has to be investigated for a determination of the loadings on the containment. Worldwide several experiments have been performed accompanied with material properties evaluation, theoretical, and numerical work. At the Institute of Safety Research of the FZR a finite element model has been de-veloped simulating the thermal processes and the viscoplastic behaviour of the ves-sel wall. An advanced model for creep and material damage has been established and has been validated using experimental data. The thermal and the mechanical calculations are sequentially and recursively coupled. The model is capable of evalu-ating fracture time and fracture position of a vessel with an internally heated melt pool. The model was applied to pre- and post test calculations for the FOREVER test se-ries representing the lower head RPV of a PWR in the geometrical scale of 1:10. These experiments were performed at the Royal Institute of Technology in Stock-holm. The results of the calculations can be summarised as follows: # The creeping process is caused by the simultaneous presence of high tem-perature (>600 °C) and pressure (>1 MPa) # The hot focus region is the most endangered zone exhibiting the highest creep strain rates. # The exact level of temperature and pressure has an influence on the vessel failure time but not on the failure position # The failure time can be predicted with an uncertainty of 20 to 25%. This uncer-tainty is caused by the large scatter and the high temperature sensitivity of the viscoplastic properties of the RPV steel. # Contrary to the hot focus region, the lower centre of the vessel head exhibits a higher strength because of the lower temperatures in this zone. The lower part moves down without significant deformation. Therefore it can be assumed, that the vessel failure can be retarded or prevented by supporting this range. # The development of a gap between melt crust and vessel wall could not be proofed. First calculations for a PWR geometry were performed to work out differences and commonalities between prototypic scenarios and scaled experiments. The results of the FOREVER-experiments cannot be transferred directly to PWR geometry. The geometrical, mechanical and thermal relations cannot be scaled in the same way. Because of the significantly higher temperature level, a partial ablation of the vessel wall has to be to expected in the PWR scenario, which is not the case in the FOREVER tests. But nevertheless the FOREVER tests are the only integral in-vessel retention experiments up to now and they led to a number of important insights about the behaviour of a vessel under the loading of a melt pool and pressure.

To get an improved understanding and knowledge of the processes and phenomena during the late phase of a core melt down accident the FOREVER-experiments (Failure of Reactor Vessel Retention) are currently underway. These experiments are simulating the lower head of a reactor pressure vessel under the load of a melt pool with internal heat sources. The geometrical scale of the experiments is 1:10 compared to a common Light Water Reactor. During the first series of experiments the Creep behaviour of the vessel is investigated. Due to the multi-axial creep deformation of the three-dimensional vessel with a non-uniform temperature field these experiments are on the one hand an excellent possibility to validate numerical creep models which are developed on the basis of uniaxial creep tests. On the other hand the results of pre-test calculations can be used for an optimized experimental procedure. Therefore a Finite Element model is developed on the basis of the multi-purpose commercial code ANSYS/Multiphysics®. Using the Computational Fluid Dynamic module the temperature field within the vessel wall is evaluated. The transient structural mechanical calculations are performed applying a creep model which is able to take into account great temperature, stress and strain variations within the model domain. The new numerical approach avoids the use of a single creep law with constants evaluated for a limited stress and temperature range. Instead of this a three-dimensional array is developed where the creep strain rate is evaluated according to the actual total strain, temperature and equivalent stress for each element. Performing post-test calculations for the FOREVER-C2 experiment it was found that the assessment of the experimental data and of the numerical results has to be done very carefully. A slight temperature increase during the creep deformation stage of the experiment for example could explain the creep behaviour which appears to be tertiary because of the accelerating creep strain rate. Taking into account both - experimental and numerical results - gives a good opportunity to improve the simulation and understanding of real accident scenarios.

[pt] O presente trabalho tem como objetivo investigar o efeito da adição de fibras dispersas de polipropileno e de curauá na fluência básica a compressão e na retração autógena e por secagem de pastas de cimento nas primeiras idades. O comprimento das fibras de polipropileno e de curauá foi de 12 mm e a fração volumétrica variou de 0,03 a 0,30 por cento da massa do cimento. As pastas de cimento tinham relação água/cimento de 0,44. A retração autógena foi estudada pelo método do tubo corrugado, acompanhando a variação do comprimento desde as primeiras horas. A avaliação da retração por secagem livre se deu por meio da medição da variação de comprimento de primas durante os primeiros dias de idade. Utilizou-se o método do teste do anel para observação do comportamento das pastas sob retração restrita no intuito de se obter a idade de fissuração de cada pasta. A abertura dessas fissuras também foi acompanhada com auxílio de um microscópio. Testes de fluência a compressão foram realizados para determinar a deformação por fluência em idade jovem. Foi observada uma redução significativa da retração autógena nas pastas reforçadas com 0,30 por cento fibras de curauá. A adição de ambas as fibras reduziu ligeiramente a retração por secagem livre, não tendo diferença significativa entre os teores de fibras. Também foi observado atraso no desenvolvimento de fissuras e redução na abertura de fissuras para ambas as fibras independente do teor. As pastas com adição de fibras exibiram, no geral, uma ligeira melhora na deformação por fluência. / [en] The present research aims to investigate the effect of the addition of discrete polypropylene and curauá fibers on the early age autogenous and drying shrinkage and basic creep in compression in cement pastes. The length of the polypropylene and curauá fibers was 12 mm and the volume fraction varied from 0.03 to 0.30 percent of the mass content of cement. The cement pastes had water to cement ratio (w/c) of 0.44. Autogenous shrinkage tests were carried out following the Corrugated Tube Method, monitoring the length changes since casting. The evaluation of free drying shrinkage was given by measuring the length changes during the first days of age. The ring test method was used to determine the age when cracking occurred. The crack opening of the samples was also monitored by a microscope. Compressive creep tests were performed with cylindrical specimens to determine the early age creep strain. A significant reduction of the autogenous shrinkage was observed in the specimens with 0.30 perent of curauá fibers. The addition of both fibers slightly reduced the free drying shrinkage of the specimens and the fiber content did not appear to influence significantly the shrinkage behavior of the specimens. A delay in the development of cracks and a reduction in crack opening for specimens with both fibers were also observed. This behavior was similar for all fiber content. Pastes reinforced with fibers exhibited, overall, a slight improvement in creep strain. The addition of 0.30 percent of polypropylene fibers showed greater improvement in creep strain.

I höga hus, i storleksordningen 100 m och högre, med vertikalbärande stomme av stål och betong kan differentialsättningar uppstå då de olika materialen deformeras olika. Detta ger med tiden lutande bjälklag. I det här arbetet har ett beräkningsprogram tagits fram med hjälp av Mathcad för att räkna ut de slutliga deformationerna och hur mycket man behöver kompensera för skillnaderna i deformation genom att gjuta bjälklagen med fall. Programmet är tänkt att användas av WSP i projekteringen av höga hus. Med hjälp av programmet har parametriska studier gjorts och det visade sig att genom att ändra på vissa parametrar kan differentialsättningarna utjämnas. I vissa fall kan detta medföra att gjutning med fall ej är nödvändigt. / When high buildings are built with steel and concrete collaborating, some problems may occur because the materials deform differently. The differences in the final deformations make horizontally casted floors start to lean over time. In this study a program has been developed in Mathcad to calculate the final deformations and differences. The differences can then be compensated by casting the floor with a slope. The program is supposed to be used by WSP to project high buildings. The program has been used for some parametric studies to show how different parameters can decrease the differences. If this makes the differences small enough it will not even be necessary to cast floors with a slope.