Much more chemically heterogeneous metal weld in a narrow area near the boundary of fusion. Here, there is diffuse infiltration of some elements from the molten metal weld pool in solid base metal. Band width of the diffusion penetration may reach 0,1 0,4 mm. Additional changes in the composition in this region may be caused by melting of the metal along the grain boundaries, leading to the weld pool and penetration of these channels molten metal weld pool in the base metal. Another reason for the inhomogeneity of the metal near the fusion – the diffusion of elements from solid phase into the liquid. Most of the alloying additives and impurities more soluble in the molten metal than a solid.
Contact with the molten metal firm contributes to the displacement of dopants and the establishment of the concentration profile corresponding to the equilibrium state. In the process of cooling after complete solidification of the molten pool may be some diffusion of the alignment in this area (dashed line). This character of the redistribution of impurities at the boundary fusion observed in welding of steels, where the molten pool moves sulfur, phosphorus, carbon, copper welding, where possible enrichment of the weld pool oxygen received from the base metal. When welding metals and alloys, with polymorphic transformations in the areas of welding heat generated plots with different phase compositions. Such transformations are observed, for example, when welding carbon steels and titanium alloys. Low carbon steel (carbon content up to 0.25 wt.%) Are polymorphic. In the initial state (normalized) steel is a mixture of grains of ferrite (a-Fe) with the crystal lattice in the form of body-centered cube and pearlite (a mixture of crystal a-Fe and cementite Fe3C). When heated above 723 are C (designation of the temperature during the heating – cooling Ag – Acj) begins the transformation of this mixture into austenite (a solution of carbon in y-Fe, which has lattice in the form of face-centered cube). At temperatures of N2 (AC2), the process ends (= 880 C), ie structure of the steel is fully austenitic. When heated above 1100 C, austenite grains grow rapidly and steel, heated above this temperature, has the structure of overheating.