Steam pipes in electric power plants are subjected in service to the long-term action of mechanical loads and high temperatures. Since emergencies can arise due to slow crack growth and subsequent sudden fracture of a structural element, the optimum design and reliable use of power plants require data on the crack resistance of the structural materials used. The goal of the present study is to evaluate the change in the resistance of the metal of steam lines in heat and electric power plants to brittle fracture in relation to structural changes in service. Microstructural analysis showed the initial ferrite-pearlite structure is transformed during service into a ferritecarbide structure. It is found that the structural and phase transformations which occur in the metal of steam pipes during service cause a significant reduction in resistance to brittle fracture. There is a simultaneous deterioration in the strength properties of the steel. The study results show a need to allow for the embrittlement of steam-pipe metal during service when performing periodic cold hydraulic pressing. This pressure-testing may, according to the authors, cause the nucleation and growth of brittle cracks capable of leading to pipe fracture.