The accepted theory of solidification cracking in welds as well as castings is that a coherent, interlocking solid network separated by essentially continuous thin liquid films is ruptured by tensile stresses arising from thermal contraction. If sufficient liquid metal is present near the cracks when they form, they can backfill or heal. If not, the cracks appear as open tears. For this reason, solidification cracks are also called hot tears, or, most commonly, hot cracks.
Several factors favor solidification cracking, some of which are metallurgical in origin and some of which are mechanical in origin. Metallurgical factors include:
(1) Freezing temperature range, the wider the freezing range for an alloy, the larger the area that is weak and susceptible to weld solidification cracking. The freezing range of an alloy can be increased either by intentional additions (e.g., Cu, Mg, or Zn in Al) or by the presence of undesirable impurities (e.g., S and P in steels or Ni-based alloys).
(2) Presence of low-melting eutectics, the effect of impurities, such as S or P in steels, is to widen the freezing range considerably, but without producing large volumes of liquid to permit backfilling. Such residuals tend to segregate to grain boundaries, where diffusion rates are almost always higher due to the more open nature of lattice structure in these areas, and form low-melting compounds that lead to cracking. In plain carbon steels, for example, FeS films would form and lead to severe cracking if not for intentional additions of Mn leading to preferential formation of harmless globules of MnS. For this reason, Mn/S ratio is carefully controlled in steels.
(3) Grain or subgrain structure in the fusion zone, coarse columnar grains are more susceptible to solidification cracking than fine equiaxed grains for several reasons. First, fine, equiaxed grains can better deform to accommodate contraction stresses, relieving stress across interdendritic boundaries. Second, liquid can be more effectively fed to incipient cracks to promote healing in fine-grained materials, where grain boundaries run in all directions. And, third, when the grain boundary area is greater, the concentration of potentially harmful low-melting point segregates is reduced
(4) Surface tension of the grain boundary liquid, If the surface tension between the solid grain and the grain boundary liquid is very low, a thin liquid film forms by wetting the grain boundaries. For this situation, susceptibility to cracking is very great. If, on the other hand, the surface tension of the liquid is high, it will tend to bail up to form globules and will not coat the grain boundaries with a continuous film. This greatly reduces the susceptibility to cracking. The effects of different surface tensions on liquid wetting and distribution is shown in figure below,
(1) Contraction stresses, two internal sources of tensile stresses are generated are shrinkage as liquid transforms to solid and contraction due to the thermal coefficient of expansion.
(2) Degree of restraint, for the same joint design and material, the greater the degree of restraint of the workpiece by fixturing or other structure in the weld assembly, the more likely that solidification cracking will occur.
Reference: Principles of Welding, Robert W. Messler, Jr.
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