Peculiarities of manifestation of basic positions of metal science in conditions of welding heating and cooling of metal are considered. Information on the steels most widely used for the production of welded structures is given. Calculation methods for determination of welding thermal cycle parameters are described. The effect of the main characteristics of heating and cooling under the conditions of the welding process on the course of structural-phase transformations in the weld metal and in the heat-affected zone is shown. The issues of weldability and subsequent heat treatment of welded joints of steels of various structural groups are described.