Possibility of hot cracks formation in welded seam is determined by next factors:
1) Size and character of plastic deformations, formed in metal of welded seam during crystallization;
2) The speed (rate) of increasing these deformations;
3) Deformational capacity of welded seam metal in temperature range of hot cracks formation.
Size and character of plastic deformations during crystallization of welded seam depend on relative change of metal volume. The volume of metal decreases with decreasing of the temperature, it is also increasing in the process of martensitic transformation into austenite (pearlite), and during metal transfer from liquid to solid state, it is unevenly decreased.
If metal volume shortening occurred in free state, no strains or deformations will originate. However, under conditions of welding the changing of metal volume in welded seam area is controlled by surrounding sector of basic metal.
The determination of actual deformations of crystallizing and cooling metal of welded seam has essential meaning for evaluation of hot cracks formation possibility, since the last one can form only in the case if in metal of welded seam deformations of tension occur, that exceed deformational capacity of material under conditions of current temperature.
Since the metal of welded seam is connected to basic metal, its actual deformation can significantly differ from temperature deformations for separately marked section of welded seam metal.
Under influence of welding source of heat, the temperature of basic metal is also changed, at that few its sections at this moment of time are cooling and because of this fact they are shortening, and more distant areas continue to heat up and, consequently, enlarge.
If in each moment of time of deformation enlargements would be equal to shortening deformations, the welded seam would not experience tensions. The necessary condition for hot cracks formation is existence of significant difference between actual deformations of crystallizing and cooling metal of welded seam and those heat deformations, which could appear in the case, if metal of welded seam endures shortening, which is proportional to temperature decreasing during the process of their cooling.