Casting defect that results in material separation caused by mechanical or dynamic overload.
Differentiation is made between cold and hot cracks (also referred to as warm cracks or solidification cracks) depending on the temperature range at which they occurred during production or shaping of the workpiece.
With cold cracking, the material compound is separated by clearly visible cleavages (Fig. 1). The sharp edges delimit a clearly visible fracture, mostly along the entire cross section.
Formation of cold cracks is not exclusively limited to completely cooled conditions; but the differ from hot cracks in terms of the crack cause since hot cracks only form in the solidus range as long as residual melt is still present. In contrast to that, cold cracks occur in solid condition, are clearly visible to the human eye, and generally result in rejection of the casting.
Cold cracking takes places if stresses occurring in the respective casting are greater than the strength of its material. In most cases, these types of critical stresses occur in castings through constraint of material shrinkages after solidification, e.g. due to incorrect or unsuitable designs (Fig. 2).
Regardless of the material, for castings produced by the method of sand casting, roughly the same root causes apply as for e.g. shrinkage constraint due to over-compaction of mold material and/or over-solidified cores, or incorrect selection of the coremolding process for the respective material. Moreover, optimum wall thickness ratios and accordingly favorable, uniform cooling conditions play a critical role with regard to prevention of sensitivity of the casting to cold cracking.
Selection of the type of alloy or improper modification may have an impact on shrinkage cracking, much the same as quenching the part in unsuitable media after heat treatment( e. g. in too cold water) or uneven quenching.
Inspection of constructive or design measures (even wall thicknesses, uniform cooling, low degree of shrinkage constraint, no sharp transitions) and improvement of all technical production measures for all casting processes helps prevent cold cracking.