Strukturní stabilita a žárupevnost ocelí pro použití v energetice

Abstract

There is a lack of information about some of heat resistant steels due to big costs and especially due to big time requirements for creep tests. Thesis is focused on the evaluation of microstructure evolution, which is closely connected with creep properties in dissimilar welds of P23/P91 steels, and also on the study of a modified Z-phase in austenitic AISI316LN+V+Nb steel. Dissimilar welds are widely used in tubular systems of power plant boilers to compensate for different operating conditions there. Welds generally represent critical parts of constructions and namely dissimilar welds are sensitive to microstructural changes during creep especially due to the redistribution of interstitial elements across the fusion boundary. Consequently, a part of the thesis is focused on the dissimilar welds of P23/P91 steels. Steel properties can be heavily affected by the minor phase evolution during creep. A convenient steel microstructure can be achieved by optimization of the chemical composition of the steel and by proper heat treatment as well. Despite of that, minor phases can coarsen or can dissolve during creep, which can lead in degradation of the steel properties. It is, therefore, necessary to know not only the thermal stability of the minor phases but also the kinetics of its formation in a particular steel grade. The second part of thesis is devoted to thermal stability and a crystallographic study of the modified Z-phase in austenitic AISI316LN with niobium and vanadium additions, because it has been well described in (9-12)%Cr steels but not in austenitic steels where the amount of information about the modified Z-phase is limited.