The purpose of the paper is to present the mechanism of deformation and failure of welded joints during the process of retrofit. The considered joints are made of conventional steels used in the power industry, as well as new-generation steels. The problems of failure and the design of such joints are considered in the paper. The model of welded joint was built with properties of low alloyed chrome-molybdenum steel and a new-generation steel of P/T91 type. All metallurgical zones, i.e. the weld and the heat-affected zone were considered, modelled and discussed. A simulation of thermomechanical loading was conducted under various loading conditions to explain the causes of the failure of energy structures, which appears more frequently during their loadings or steam dropping (i.e. in conditions of changes of thermal loadings) than during their stable work. The results of calculations were confirmed by experimental metallurgical studies. It is observed that the brittle and soft zones of coarse-grained ferrite with minimal impact strength, resulting from internal and external operation parameters, appear in regions where oscillation of the highest von Mises and normal stresses takes place. The relevant literature does not describe the analysis of the phenomenon of acceleration of the failure of the welded structure leading to its failure, while taking into account the complex thermomechanical conditions of their operation. This mechanism is based on the accumulation of structural weakening of the weld as the result of the diffusion process within the axially symmetrical joint, the growth of the brittle zone and their additional degradation, and the influence of the oscillation of maximal normal and von Mises stresses. It was noted that in the process of thermomechanical loading, the state of failure initiation in circumferential and longitudinal directions appeared in the weld. A few illustrations presenting the mechanism of the deformation and failure of the welded joint are presented. The effects of the distortions of the weld are also discussed in the paper.