<p>The susceptibility to damage of welded joints in power units of thermal power plants operated for more than 270–280 thousand hours under creep and fatigue conditions largely depends on their structural and phase state. The primary damageability of welded joints, compared to the damageability of steam pipes that have not been affected by welding heat, is caused by the presence of structural, chemical, and mechanical heterogeneity formed as a result of such heating. During long-term operation of welded joints, the initial heterogeneity develops further. Physical and chemical processes occur in the metal of welded joints, including tempering and recrystallization, which generally contribute to its damage susceptibility. Studying the course of such processes makes it possible to improve the assessment of the reliability of welded joints and their residual life.</p>