Microstructure Evolution and Creep Properties of 2.25Cr-1Mo Ferrite-Pearlite and Ferrite-bainite Steels After Exposure to Elevated Temperatures

Abstract

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.