RESUMOOs aços 2,25Cr-1Mo são largamente utilizados no parque de geração termelétrica no Brasil, estando em boa parte das caldeiras atualmente em operação, e podem apresentar microestrutura composta de ferritaperlita ou ferrita-bainita. Submetido à fluência, esse material tem suas propriedades degradadas em serviço através de modificações microestruturais típicas, como o coalescimento dos carbonetos originalmente presentes. A extensão das mudanças microestruturais representa, nesse caso, perda de resistência mecânica no material.Como existe uma aceleração no grau de deformação, que depende da tensão, da temperatura e do tempo, com a degradação, a extensão da deterioração microestrutural pode ser usada como uma medida de dano. Dessa forma, é importante conhecer essas modificações para fornecer suporte às técnicas de previsão de vida residual de componentes fabricados com esse tipo de material.A degradação do material ferrítico-perlítico é bastante conhecida, mas a literatura ainda não apresenta resultados consistentes quanto à do ferrítico-bainítico. Trata-se, aqui, de um estudo comparativo da evolução microestrutural do aço 2,25Cr-1Mo com as duas microestruturas, perlítica e bainítica, em temperaturas entre 550 e 600°C até 2.000 h. Os resultados mostraram que a microestrutura do aço ferrítico-bainítico é mais estável que a do ferrítico-perlítico. Entretanto, a estrutura bainítica não obedece aos mesmos estágios de degradação, identificados por Toft e Marsden utilizados para classificar a perlita.Palavras chaves: degradação microestrutural, envelhecimento, materiais para altas temperaturas, bainita. Microstructural evolution and hardness changes in bainite and pearlite in Cr1Mo 2.25 steels after aging treatment ABSTRACTThe 2.25Cr-1Mo steels are widely used in boilers currently in operation in thermal generation plants in Brazil, and can present a microstructure consisting of ferrite-pearlite or ferrite-bainite. Subjected to creep in operation, the material properties of this steel degrade due to typical microstructural changes such as the coalescence of the carbides originally present in the material. In this case, these microstructural changes reduce the mechanical strength of the material.Since strain increases with microstructural degradation and strain depends on the stress, temperature and time, the extent of microstructural degradation can be used as a damage measurement method. Thus, it is important to know the microstructural changes in the material to provide technical support for residual life prediction of components.The degradation of the ferritic-pearlitic steels is well known, but the literature does not present consistent results regarding the ferritic-bainitic steels. This is a comparative study of the microstructural evolution at temperatures between 550 and 600°C to 2,000 h of pearlitic and bainitic 2.25 Cr-1Mo steels. The results showed that the ferritic-bainitic steel is more stable than the ferritic-pearlitic. However, the bainitic structure does not present the same stages of degradation iden...

2.25Cr-1Mo steels are widely used in thermoelectric power plants. Under operational temperatures, their properties degrade due to microstructural changes related to carbide coalescence and stoichiometric transformations. The extent of such microstructural changes is controlled by stress, temperature and time. Therefore, these factors can be used to evaluate damage and as life assessment tools for the individual component. In the past, ferrite-pearlite was the predominate microstructure in commercial Cr-Mo steel products, owing to the well-known methodologies for remaining life assessment based degradation. Currently, the ferrite-bainite microstructure obtained through a more economical route is most commonly used for this steel grade. However, there is no consensus in the literature about microstructural changes that can be used as a degradation pattern for ferrite-bainite steels. This paper compares the aged microstructures and creep properties of ferrite-pearlite and ferrite-bainite 2.25Cr-1Mo steels. Aging was conducted at 500, 575 and 600ºC until 2,000 h, and creep tests were performed at 575ºC under a stress of 100 MPa. Microstructural changes were characterized by optical microscopy scanning electron microscopy. Metallographic observations of the ferrite-bainite steel show a more stable behavior at the ageing temperatures and time considered. However, creep tests revealed that the ferrite-pearlite microstructure possesses a better rupture time performance. Carbide size distribution and stoichiometric evolution of the carbides provided by transmission electron microscopy support the creep behavior. These results show that the current techniques for evaluating microstructural degradation of 2.25Cr-1Mo steels must be reconsidered.