The effect of microstructure on fatigue properties and fracture mechanisms of welded joints of steel 10 used in pile foundations has been experimentally investigated. It has been established that the cause of the occurrence and development of repeated cracks in building structures where repair welding work was performed in the cold season is the high cooling rate of the welded joint, leading to the appearance of an overheating of the Widmanstätten structure in the section of the heat-affected zone. Accordingly, such structures are unsuitable for the perception of alternating loads. However, in order to ensure the operability of repair welded joints made in the cold season, it is necessary to perform additional technological operations that reduce the cooling rate.