The thermal cracks generated in the Decanter centrifuge all occur and develop along the junction of the dendrites in the weld metal. The most common situation is that cracks in the middle of the weld along the length of the weld are sometimes distributed between two dendritic grains inside the weld.
Decanter centrifuge hot cracks are all produced at the grain boundary, which shows that the grain boundary is a “weak zone” during the crystallization process of the weld.
The reason for the formation of this weak zone is because in the metal crystallization process, there are more brittle impurities enriched in the grain boundary, and these impurities have a lower melting temperature. For example, FeS can be formed when the metal to be welded contains high sulfur content, and FeS and iron form a low-melting eutectic with a melting point of only 988 degrees. For example, in the later stage of the solidification process of the welded metal, the low melting point eutectic is pushed to the grain boundary to form a so-called “liquid interlayer.” In the process of metal transition from liquid to solid, the weld is subjected to tensile stress due to volume shrinkage. Under the action of tensile stress, cracks may be formed in this liquid interlayer, that is, thermal cracks.
Therefore, the reason for the thermal cracking of the centrifuge is that the weld is subjected to the existence of a liquid interlayer, and the weld is subjected to tensile stress during the crystallization process. The existence of a liquid interlayer is the root cause of thermal cracks, and tensile stress is a necessary condition for thermal cracks. From the above analysis, it can be seen that not the entire crystallization process will produce hot cracks, but only in the later stage of the crystallization process, near the solidus line is the dangerous temperature zone for hot cracks.
