Nanomodification is an effective way of forming a fine-grained structure of a weld metal. Part I. Factors determining the stability of the weld seam against embrittlement

Steel constructions are the most important objects of the construction industry. The main method of connecting individual elements and assemblies of such structures is fusion welding (electric arc, plasma, electron beam). One of the main problems of metal structures is their tendency to brittle fractures that occur suddenly, without noticeable previous deformations, often at below zero temperatures and loads. In this case, 70–80% of such damage is associated with welding. The nature of the destruction depends not only on temperature, but also on the type of crystal lattice. Metals with a body-centered cubic (bcc) lattice (tungsten, molybdenum, α-iron) are plastic at a relatively high temperature, but become brittle when negative. And metals with a face-centered cubic (fcc) lattice (aluminum, nickel, copper and γ-iron) are plastic even at temperatures close to absolute zero. With decreasing temperature, the mobility of dislocations and vacancies decreases, the yield strength σs increases, that is, the ductility of the metal decreases. Using the A.F. Ioffe model showed that the critical temperature of the transition from viscous to brittle fracture with decreasing temperature depends on the rate of increase of σs. In austenitic steels (fcc lattice with a period a = 0.3645 nm), the impurity mobility (paired with a vacancy) through the internode of the lattice is higher, and the growth rate σs is lower than that of low-carbon low alloy steels (bcc lattice with a shorter period a = 0, 2861 nm). Therefore, the low carbon steels from which structures are made are more sensitive to lower temperatures. In addition, resistance to brittle fracture depends on the number and size of metal continuity defects (pores, inclusions, low-plastic phases in the structure, etc.), which are stress concentrators. Welds are the main suppliers of such dangerous defects and sources of crack initiation. During crystallization of the weld pool, a coarse, columnar dendritic structure with reduced ductility is formed. The formation of a fine-grained structure of the weld during crystallization helps to increase its ductility. The grain size in the cast metal (in the weld) depends on the crystal growth rate and the rate of their nucleation. When welding, the most effective way to grind the grain in the seam is to increase the rate of nucleation. The introduction of refractory particles of nanoscale order into the weld pool (nanomodification) allows obtaining a fine-grained weld structure.