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The main aim of our paper is to analyze the influence of temperature and service time on the fracture resistance of welded joint constituents for new and exploited low-alloyed A-387Gr.B steel (Cr–Mo type) under the action of dynamic load and changes in the mechanical properties. The exploited parent metal, as a part of the reactor mantle, is in the damage repair stage, i.e., a part of its mantle is replaced with a new material. Wohler’s curves were constructed, i.e., the fatigue strength as material resistance to crack initiation, was determined at the room and working temperatures. Testing of the welded joint and microstructural analyses of the parent metal, weld metal, and the heat-affected zones were carried out. Based on the testing results, the analysis of the fracture resistance, and the microstructural changes, represent the comparison of values obtained for the characteristic areas of the welded joint and the justification of the selected technology of welding. © 2021, Springer Science+Business Media, LLC, part of Springer Nature.