According to dimensions and influence on metal density are there two categories of cracks (local ruptures): microcracks (their dimensions are comparable to dimensions of crystal lattice parameters; are available almost always in real crystals) and macrocracks which appear in weld joint in the course of welding or use of product (are dangerous defect – reason for structural failure).
Formation of cracks of the second category becomes possible only when plastic deformations which appear in metal due to non-uniform heating and cooling exhaust its deformation capability, and stresses increasing will reach ultimate strength values, i.e. onlyin that case when the value of inherent stresses in weld will reach ultimate strength values of metal at this temperature.
It is possible to specify two temperature intervals when the formation of cracks is possible:
1) Near solidus temperature (hot cracks);
2) In the area of lower temperatures (for steel 750°С) where is observed a sharp reduction of ductility due to structural transformations (cold cracks).
Hot (crystal) cracks usually appear in metal of weld at the closing stage of crystallization process at the temperatures close to solidus. Cold cracks mostly appear as a result of structural transformations in metal.
Hot cracks have intergranular character, i.e. they pass along the boundaries between crystals and the cold cracks mostly cross the crystals and boundaries between them.