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AuthorsVivas Méndez, Javier
AdvisorCapdevila, Carlos; San-Martín, D.
KeywordsFerritic/martensitic steels
Power plants
Aceros de alta resistencia
Issue Date23-Sep-2019
PublisherUniversidad Complutense de Madrid
Consejo Superior de Investigaciones Científicas (España)
AbstractNowadays, there is a need to increase the operation temperature of 9Cr ferritic/martensitic steels to improve the efficiency of future power plants. As an example, in the case of coal fired power plants, an increase of 1 % in the efficiency allows the reduction of 2.4 millions of tons of CO2, 2 000 tons of NOX, and 500 tons of particles. The maximum operation temperature for the 9Cr ferritic/martensitic steels is 620 °C due to their low microstructural stability at higher temperatures. The microstructure of these steels consist in tempered martensite with a high dislocation density. During creep, this microstructure evolves to a more stable microstructure, which consists of ferrite and different kinds of precipitates. The evolution towards this microstructure produces a drop in the creep strength by a decrease in the dislocation density and the coarsening of martensitic laths and blocks. The coarsening of these microestructural features and the drop in the dislocation density is hinded by two kinds of precipitates, M23C6 carbides and MX carbonitrides. The coarse M23C6 carbides are rich in Cr and are located on lath, block and prior austenite grain boundaries. The main problem of these carbides is their fast coarsening rate, which limits their ability to inhibit the movement of lath and block boundaries during creep. The another kind of precipitates, the MX, are carbonitrides rich in V and Nb. These precipitates are located within the laths, with a smaller size than that of the M23C6 carbides. The most interesting feature of these precipitates is their high thermal stability. This characteristic makes these precipitates very usefull to pin the dislocations during creep and retard the microstructural degradation. The main objective of this thesis cosists in developing, in these steels, new microstructures with a higher microestructural stability than that of the current microstructures. To achieve this, we have considered the high thermal stability of these MX precipiates and we have assumed that , if we obtain a high number density of MX precipitates within the martensitic laths, the creep strength will be improved considerably. To reach this, we are going to employ two strategies. One of them consists in applying a thermomechanical treatment, in a commercial steel, as an alternative to the existing conventional processing route. The other one consists in developing new steel compositions keeping the existing conventional processing route (which does not include a thermomechanical treatment, as it will be described below). The results obtained by applying the thermomechanical treatment show important improvements in creep strength compared to that for the commercial steel processed by the conventional route. However, linked to this improved creep strength, a considerably drop in creep ductility is observed which limits the use of this processing route. The improvement in creep strength is attributable to the high number density of MX nanoprecipitates. The drop in creep ductility is related to the increase in the prior austenite grain size promoted by the higher austenitization temperature employed during the thermomechanical treatment compared to that used in the conventional processing route.
DescriptionTesis Doctoral
Appears in Collections:(CENIM) Tesis
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