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The materials under study are 9–12% Cr ferritic/martensitic steels, alternative candidate materials for application in core components of nuclear power reactors. This work involves: (1) investigation of high temperature fracture mechanism during slow tensile and limited creep testing at 600°C; (2) extensive study of solute element segregation, theoretically using proposed model of combined equilibrium and non-equilibrium mechanisms; and experimentally using field emission gun scanning transmission electron microscopy; (3) investigation of effects by thermal ageing and irradiation on microstructural developments in relation to high temperature mechanical behaviour. Quenching rates, tempering temperature and tensile test temperature variations are included in the thermomechanical treatments.

The corrosion fatigue crack propagation behaviour of a high strength low alloy steel, N-A-XTRA 70, in a synthetic sea water solution was tested using S. E. N. specimens subjected to a loading frequency of 0.1 Hz and a load ratio of 0.6. In order to simulate the conditions encountered by a thumbnail type crack several specimens from each of the microstructural types tested, namely parent plate, heat affected zone and heat treated material, had their crack sides covered by transparent plastic covers. Severe overprotection and slight underprotection conditions were produced using cathodic protection potentials of -1400, -1300, -1200 and -700 mV (S. C. E. ). The Paris relationship da/dN = CLKm was found to be a useful tool in describing the crack propagation rate data. Results obtained, presented in the form of plots of log da/dN against log AK, show that for parent plate, H. A. Z. and heat treated material, covering the crack sides of specimens produces enhanced corrosion fatigue crack propagation rates, at cathodic protection potentials of -1400 and -1300 mV (S. C. E. ), when compared to non covered specimens. This trend was also true for H. A. Z. specimens at a potential of -700 mV (S. C. E. ). For parent plate specimens, however, covering the crack sides at a potential of -700 mV (S. C. E. ) produced reduced crack propagation rates over non covered specimens. It is believed restriced oxygen access may account for these results. Plots of the Paris exponent m and constant C for the three microstructures tested produced three lines of the form m= alnC +b where a and b were found to be dependent upon material parameters. Comparison of results with BS 4360: 50D revealed that N-A-XTRA 70 exhibited superior fatigue performance when tested in air but behaved worse under conditions of free corrosion.

Orientador: Tomaz Manabu Hashimoto / Banca: Marcelino Pereira do Nascimento / Banca: Cristina de Carvalho Ares Elisei / Resumo: 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... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: 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. / Mestre