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Process irregularities in remelting furnace operation have an impact on the ingot solidification structure which depends on the local thermal conditions and the alloy concerned. In this work, a laboratory investigation into the structures resulting from interruptions in solid growth is presented in order to understand the cause and effect relationship between the solidification interruption and banding. The results demonstrate the range of structure, segregation and precipitation changes which are experienced by a range of alloys including Inconel 718, Nimonic 80A, Waspaloy andM50. In the case of alloys which form primary precipitates, the interruption period is shown to give rise to structure changes. Segregations are found due to the interruption. We also comment on the differences observed between industrial examples of banding and the laboratory samples which may be due to the absence of liquid movement in the latter technique. It is concluded that the interruption in solidification condition during a directionally solidification itself can produce banding through its effects on the morphology of precipitates.

Process irregularities in remelting furnace operation have an impact on the ingot solidification structure which depends on the local thermal conditions and the alloy concerned. In this work, a laboratory investigation into the structures resulting from interruptions in solid growth is presented in order to understand the cause and effect relationship between the solidification interruption and banding. The results demonstrate the range of structure, segregation and precipitation changes which are experienced by a range of alloys including Inconel 718, Nimonic 80A, Waspaloy andM50. In the case of alloys which form primary precipitates, the interruption period is shown to give rise to structure changes. Segregations are found due to the interruption. We also comment on the differences observed between industrial examples of banding and the laboratory samples which may be due to the absence of liquid movement in the latter technique. It is concluded that the interruption in solidification condition during a directionally solidification itself can produce banding through its effects on the morphology of precipitates.

No description available.

Economic and environmental considerations have resulted in a worldwide drive to increase the cycle efficiency of fossil fired power plant. Boiler designs able to achieve significant efficiency increases already exist; the limiting factor is the performance of materials. As a result, much effort is currently being focussed on the development of enhanced materials to increase their operating temperature and/or pressure. The requirement that such materials should possess good thermal fatigue performance in addition to good creep performance dictates the selection of ferritic and martensitic steels for many components. Thus, most of the development effort in this field is currently focussed on martensitic steels that can operate beyond the current maximum plant design of 290 barg/580°C up to 335barg/630°C or even beyond. The most advanced conventional ferritic steels such as E911, P92, P122 and NF12 are 9-12% Cr martensitic steels and gain their creep strength from the tempered martensite structure and the precipitated carbides and nitrides. Their long term creep performance is ultimately limited by the rate at which these precipitates coarsen or otherwise transform over time at elevated temperature. This research work presents the development of an alternative alloy which aims to increase the high temperature long term creep performance by replacing the relatively low stability carbides and nitrides present in conventional ferritic steels with a thermodynamically more stable dispersion of titanium nitride particles. To overcome the solubility limitation on precipitating a significant level of fine titanium nitride and to remove the dimensional constraints of gas phase nitriding, the innovative technique being developed here is one of solid state nitriding using a nitride donor. The microstructure and properties of the titanium nitride strengthened steels have been assessed at each stage of the alloy development using a range of optical and electron microscope examination techniques and hardness, tensile and creep mechanical assessment techniques. The results have shown that the processing route plays an important role in the development of the titanium nitride particles and these in turn play an important role in the development of the grain structure. The initial evaluation of the creep rupture properties found them to be very poor, below that of the base material. This was due to two factors; relatively coarse titanium nitride particles and very fine grain size (due to the titanium nitride particles pinning) which resulted in extensive grain boundary sliding. This research, therefore, investigates the development of the entire processing route, including the development of powder metallurgy and spray forming procedures with the aim of achieving a homogeneous dispersion of fine titanium nitride particles to resist dislocation creep and the development of a coarse interlocking grain structure to resist grain boundary sliding. The achievements in the creep properties are presented in comparison with conventional ferritic creep resistant steels and advanced ferritic steels such as PM2000. The properties achieved are discussed, not only in relation to the beneficial aspects such as creep strength and the effect this has on boiler cycle efficiency, but also in relation to deleterious effects that are a consequence of reduced creep ductility. Finally, possible mechanisms to improve the properties as well as methods of reducing the production costs are assessed with a view to achieving the overall objective of developing a commercially viable material.

Made available in DSpace on 2014-06-11T19:27:12Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-12-16Bitstream added on 2014-06-13T19:35:01Z : No. of bitstreams: 1 dias_gcb_me_guara.pdf: 1781054 bytes, checksum: 9aebafdc1f0f4dac5ad0f6b86d3c63ff (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O presente trabalho estudou as variáveis de fabricação no processo na linha de galvanização contínua por imersão a quente, simulando o processo de conformação e a cura da pintura nas montadoras, visando avaliar as condições de deformação, tempo, temperatura e o envelhecimento desde o tempo de estocagem até a conformação dos aços ultra baixo carbono semi-estabilizado ao Nióbio para obtenção de índices de bake hardening (IBH) satisfatórios, visando o atendimento e a garantia das especificações dos aços BH180 e BH220. Para a execução das análises, utilizou-se o laboratório de controle da qualidade da empresa, onde foram retirados corpos de prova para ensaios de tração e do índice bake hardening seguindo os procedimentos conforme a norma requisitada. O ensaio padrão de bake hardening (BH) consiste em, deformar o corpo de prova até 2%, interromper o ensaio, aquecer o material na estufa por 20 minutos a 170ºC e após resfriamento, dar continuidade ao ensaio. O cálculo do índice de BH foi especificado segundo a norma DIN BS EN 10325. Este trabalho visou a comparação entre dois testes de envelhecimento, um em condição natural e outro artificial. Na condição natural, os corpos de prova foram deixados em temperatura ambiente durante o período de seis meses e, na condição artificial, as variáveis tempo e temperatura foram analisadas. A garantia do tempo de estocagem está sendo discutida entre as montadoras e as usinas. A condição padrão para a simulação do ensaio de aços bake hardening, avaliou a influência de cada variável e verificou que o índice de BH está dentro do especificado em norma, sendo encontrado o valor de 49,0 MPa para o aço BH180 e de 45,0 MPa para o aço BH220. As variáveis mencionadas são importantes para o incremento do índice de BH principalmente a pré-deformação... / This paper studied the variables in the manufacture process in continous hot dipped galvanizing line, simulating the process of drawing and curing of paint in the automakers, to evaluate the conditions of deformation, temperature, time and aging from storage time to drawing of ultra low carbon steel semi-stabilized to achieve a reasonable bake hardening index (BH) in order to find the specifications and warranty BH180 and BH 220 steels. For the application of tests, it was use the quality control laboratory of the company`s, where specimens were removed for tensile tests and bake hardening index (BH) following the standard procedures as required. The bake hardening (BH) pattern test consists of deforming the specimen until 2%, stop the test, after heating the material in an oven for 20 minutes at 170 ° C and after air cooling, continue the trial. The calculation of a reasonable bake hardening index (BH) was specified according to DIN BS EN 10325. This study aimed to compare two aging tests, one in natural and the other one in artificial condition. In natural condition, the specimens were left at room temperature for a period of six months, and in artificial condition, time and temperature variables were analyzed. The warranty of the storage time is being discussed between the automakers and steel plants. The default condition for the simulation of the bake hardening steels test, evaluated the influence of each variable and found out that bake hardening index (BH) is within the specified standard, and with values of 49.0 MPa for BH180 steel and 45.0 MPa for BH220 steel. The variables mentioned are important for increasing the bake hardening index (BH) mainly pre-deformation has a significant influence. The steels studied seem not to have great influence on aging after the production in the time period evaluated.

The microstructure of high strength steel is susceptible to delayed failure caused by the absorption of hydrogen produced either during cathodic charging or electroplating. When 0.8%C and AISI 4340 steel are subjected to constant load testing, a wide range of failure times is observed. By applying Weibull statistics small changes in experimental parameters such as heat-treatment are detected readily and are explained in the terms of hydrogen trapping at microstructural defects. During the electroplating of steel in a double-cell, quantitative measurements are made of the amount of hydrogen permeated. Current densities are measured in the range 2- 40 mAcm2 and it is shown that, although the lowest current density produces the most mechanically sound plate, it also causes the largest amount of hydrogen absorption. The nickel deposit is found to act as a reservoir for reversibly trapped hydrogen allowing diffusion to continue into the steel after the cessation of plating. Permeation measurements were taken on AISI 4340 steel using an electrochemical probe developed from the Barnacle Electrode. The effects of cathodically charging and electroplating with nickel are compared. Exposure of the steel to the atmosphere is shown to have an important influence on the hydrogen content after a period of time due to a limited occurrence of corrosion. Various post- plating treatments are commonly used to remove a damaging concentration of hydrogen and the quantitative effects of such treatments are described. Finally, a mathematical model is proposed which explains the reason for the wide spread of delayed failure times. It is found that if the stress intensity necessary to initiate a crack is known and, provided either the nominal stress or the crack size is known, it is possible to calculate either the allowable defect size or the allowable stress below which cracking is not expected to occur.