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Doctoral Thesis
DOI
10.11606/T.88.2002.tde-21012011-094354
Document
Author
Full name
César Augusto de Jesus Falcão
E-mail
Institute/School/College
Knowledge Area
Date of Defense
Published
São Carlos, 2002
Supervisor
Committee
Spinelli, Dirceu (President)
Andrade, Arnaldo Homobono Paes de
Bueno, Levi de Oliveira
Ferreira, Itamar
Lirani, Joao
Title in Portuguese
Estudo do comportamento da fadiga de baixo ciclo em altas temperaturas do aço inoxidável AISI 420.
Keywords in Portuguese
Aço inoxidável
Fadiga isotérmica
Fadiga termomecânica
Metodologia
Abstract in Portuguese
Os ensaios tradicionais de fadiga realizados à temperatura constante nem sempre são capazes de reproduzir os mecanismos atuantes na solicitação anisotérmica. Por isto, novas técnicas mais avançadas de ensaios à temperatura variável foram desenvolvidas recentemente como a Fadiga Térmica (FT) e a Fadiga Termomecânica (FTM). A parte experimental deste trabalho incluiu a implementação e a descrição da metodologia de um sistema de ensaio para a FTM. Além disto, também foi determinada a propriedade mecânica de fadiga de baixo ciclo para o aço inoxidável AISI 420, utilizado na fabricação de palhetas de turbinas a gás para a geração de energia em usinas de açúcar e álcool. Os ensaios de fadiga isotérmica foram realizados a 490°C, ou seja, na máxima temperatura do ciclo anisotérmico. Já os ensaios anisotérmicos foram realizados na faixa de temperatura entre 260-490°C e nas condições em fase e fora de fase. A curva tensão deformação cíclica foi obtida pelo método convencional para os ensaios isotérmicos, sendo que o material sofreu endurecimento cíclico. Os ensaios em fase apresentaram vida ligeiramente menor do que os ensaios fora de fase. Não foram observados mecanismos ativados pela temperatura como fluência e oxidação, na faixa de temperatura dos ensaios anisotérmicos, assim, o mecanismo determinante para o processo de falha dos corpos de prova foi a fadiga pura. O comportamento dos resultados de FTM fora de fase foi melhor estimado pelo modelo de acúmulo de dano do que os resultados para a condição em fase. A análise microestrutural e fratográfica após os ensaios de fadiga revelou que os carbonetos de cromo continuaram dispersos na matriz. As trincas nuc1earama partir da superfície externa dos corpos de prova tubulares e se propagaram transgranularmente para o interior do metal.
Title in English
Analysis of the low cycle fatigue behavior at high temperatures in a stainless steel AISI 420.
Keywords in English
Fatigue
Isothermal fatigue
Methodology
Stainless steel
Abstract in English
The traditional fatigue test performed at elevated and constant temperature not always is able to reproduce the mechanisms acting during thermal loading. Therefore, new advanced testing techniques at variable temperatures were recently developed, and they are called Thermal Fatigue (TF) and Thermomechanical Fatigue (TMF). The experimental part of this work includes the TMF methodology implementation and description, as well as the Isothermal Fatigue (IF) and TMF results from an AISI 420 stainless steel, regularly used to produce gas turbine blades for power generation in sugar and alcohol industry. The isothermal fatigue tests were carried out at 490°C, the maximum temperature reached during the thermal cycle, and the TMF tests were performed in the temperature range of 260-490°C, in the inphase and out-of-phase loading conditions. The cyclic stress-strain curve was obtained by the conventional method, and the results showed that this material strain hardens. The TMF in phase tests exhibited a fatigue life shorter than the out-of-phase tests. It was not observed, in this range of temperature, any temperature activated micromechanism of failure, such as creep or oxidation mechanisms. Therefore, it was concluded that the only mechanisms acting during the TMF tests was fatigue. The damage accumulation model better fit the TMF results behavior, for the out-ofphase loading condition than the in-phase results. The microstructural and fractographic analyses showed that the chromium carbides continued generally distributed on the matrix after both types of fatigue testing. It was also noted that the fatigue cracks always were nucleated at the specimen external surface, and propagated transgranularly into the materials substrate.
 
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Publishing Date
2011-01-31
 
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