Thermoelectric power plants that use mineral coal show high wear in heat exchanger due to the action of several damage mechanisms associated with the impact of hard particles from the residue of burnt coal, the ashes. The employment of coatings should be given into consideration particularly for critical components, which are subject to severe erosive conditions is one of the solutions. However, the choice of material will depend on several factors, including the properties of aggressive ashes. This paper aims to characterize ashes generated by a Brazilian coal-based power plant and a FeCrNbNi-based metallic coating obtained by the electric arc spraying process. No trace of sulfur content was found in fly ashes and it was defined that wear is mainly related to the impact and energy of hard particles are the leading causes of degradation in coal-fired boiler equipment. According to the assessment, applied coating showed (5 ± 2)% by volume of pores and cracks, with 1.6% of oxides after the spraying process and hardness 35% greater than ash particles. Preliminary results in field operation suggest that the material showed relatively low wear compared to the original substrate and showed great applicability in controlling material deterioration for this purpose.

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.

Thermal power plants and refineries around the world share many of the same problems, namely aging equipment, high costs of replacement, and the need to produce more efficiently while being increasingly concerned with issues of safety and reliability. For equipment operating at high temperature, there are many different mechanisms of degradation, some of which interact, and the rate of accumulation of damage is not simple to predict. The paper discusses the mechanisms of degradation at high temperature and methods of assessment of such damage and of the remaining safe life for operation.