The article presents the results of a study on the structure and properties of the heat-affected zone (HAZ) in welded joints of low-alloy steel 10HSND, obtained under different welding heat inputs at subzero ambient temperatures. Particular attention is paid to the overheated region of the HAZ, which is characterized by a coarse-grained structure and low impact toughness. Experiments were conducted using UONI 13/Moroz electrodes, welding parameters were recorded, and mechanical tests were performed, including impact bending at −40 °C. The results showed that the impact toughness of the HAZ in the welded joints is significantly lower than that of the base metal but still meets the requirements of GOST standards. The best impact toughness values were observed when welding with the lowest heat input, which is associated with the formation of a more refined microstructure. The studies indicate that 10HSND steel has a wide range of permissible cooling rates, and welding at low temperatures with reduced heat input does not lead to embrittlement of the HAZ. The obtained data can be used to optimize welding processes under low climatic temperature conditions.