Chippings, also referred to as disruptions, mainly occur at the mold surface of die casting molds after several thousand fillings if the crack propagation of stress cracks and or firing cracks leads to breakage of small and grainy sections on the surface.
Chipping always occurs if several interleaving cracks or breaks meet under the surface and cause complete breakage of small grainy sections. Respectively, branched or interleaved stress cracks may lead to chippings as illustrated in Fig. 1.
Particularly in an advanced network of firing cracks, secondary cracks in parallel to the surface occur at the position with maximum radial tension. These often origin from the primary firing cracks running vertically to the surface. These secondary cracks cause entire meshes to fall out of the primary network of cracks forming inadmissible deviations in shape at the die cast part (Fig. 2).
Chipping also occurs in gas-nitrified maraging steels. In these steels, grain boundary precipitations occur at the nitrided layer impairing the plasticity of the material. As a result of this, the increasing number of temperature load cycles leads to precipitations at approx. 50,000 fillings (Fig. 3).
Berns H., Examples for damage at hot-forming tools, Druckgusspraxis 2/2005, page 66-72.