Low carbon structural steels are highly weldable metals and are widely used in many welded structures. According to the adopted regulatory and technical documentation, this group includes structural steels, which include 0.25% carbon. In addition to these steels, low-carbon steel castings and forgings used for the manufacture of welded forged and welded cast structures are characterized by a similar composition. When choosing a welding technology for low-carbon structural steels, specialists are guided by such requirements as the absence of defects in the welded joint and the ability to ensure the uniform strength of the welded joint. It is very important that during welding the heat-affected zone of the weld metal and the welded joint, as well as the base material, have the appropriate mechanical properties.
The physical and mechanical properties of the weld metal largely depend on its structure, which is characterized by such parameters as chemical composition, cooling conditions of the welded structure and heat treatment. If low-carbon structural steel is used in welding, the weld metal practically does not differ in its physical and chemical composition from the base material. Minor differences are explained by a decrease in the carbon content in the weld metal and an increase in the content of silicon and manganese. Along with this, the decrease in the strength of the seam due to the reduced content of carbon in it during the welding process is compensated by an increase in the rate of its cooling. The huge advantage of using mild structural steel for arc welding is that it can easily achieve consistent weld metal strength.
The cooling rate of the weld metal is greatly influenced by the thickness of the welded metal, the initial temperature of the product and the welding mode. Studies have shown that the change in the mechanical properties of the weld metal is influenced not only by the cooling rate, but also by the plastic deformation that occurs in it under the action of the welding stress and causes a significant increase in the yield stress. Moreover, a significant effect of the cooling rate was noted during arc welding of single-layer fillet welds. With an increase in the thickness of the base metal, the cooling rate of the weld metal changes. For a multilayer weld, a lower critical temperature of transition to the brittle state is inherent than for a single-layer weld, which is associated with the refinement of the weld structure under the action of high temperature, which increases when the welding layers are applied. In electroslag welding of fillet welds with full penetration of walls and butt welds, the cooling rate is low, therefore, changing the mode does not affect the physical and mechanical characteristics of the weld metal.
