Mikrostrukturní stabilita pokročilých kovových materiálů

Abstract

This doctoral thesis focuses on the study of microstructural stability, degradation processes, and phase transformations in welded joints of creep-resistant steels used in modern USC (Ultra Super-Critical) coal-fired power plants. The aim of the work was to deepen the understanding of the behaviour of these materials under long-term service at elevated temperatures and to contribute to the optimisation of their heat treatment and operational reliability. The research was divided into two main parts. The first part was devoted to heterogeneous welded joints of the Super304H–P92 and HR3C–P92 types, investigated after long-term creep exposure at a temperature of 650 °C. It was demonstrated that failure of these joints predominantly occurred in the intercritical region of the heat-affected zone of the martensitic P92 steel (type IV cracking). Microstructural analyses complemented by simulation annealing and thermodynamic-kinetic modelling confirmed a significant effect of the residual vanadium content in austenitic steels on the formation of a modified Z-phase ((V,Nb)CrN), whose thermal stability is lower than that of the conventional NbCrN phase. The second part of the thesis focused on the investigation of the causes of crack initiation in homogeneous welded joints of T24 steel during long-term service exposure in a power boiler operating under critical steam parameters. Analysis of single-pass fillet welds between fins and tubes after exposure at 494 °C and a pressure of 291.4 bar for approximately 22,000 and 31,000 hours revealed that longitudinal cracks were initiated on the flue-gas side at the weld root. Detailed microstructural analysis showed that the microstructure in critical regions of welds without post-weld heat treatment approached the thermodynamic equilibrium state only very slowly. Short-term simulation annealing and kinetic simulations performed in the temperature range of 494–760 °C confirmed that no risk of additional secondary hardening occurs and enabled the proposal of a local heat treatment procedure for welded repairs of membrane wall sections. The obtained results contribute to a deeper understanding of degradation mechanisms in creep-resistant steels and provide a significant practical benefit for the energy sector. The proposed local heat treatment approach represents an alternative method for repairing membrane walls of USC boilers, enabling an extension of their service life without the need for extensive operational shutdowns.