Desktop electric centrifugeHot cracks occur and develop along the boundary of dendritic crystals in the weld metal. The most common situation is cracking along the length of the weld seam in the middle, sometimes distributed between two dendritic grains inside the weld seam.

　　Desktop electric centrifugeHot cracks all occur at grain boundaries, indicating that grain boundaries are a "weak zone" during the crystallization process of the weld seam. The reason for the formation of this weak zone is that during the metal crystallization process, there are more brittle impurities enriched at the grain boundaries, and these impurities have lower melting temperatures. For example, when the sulfur content of the welded metal is high, FeS can be formed, and FeS forms a low melting point eutectic with iron, with a melting point of only 988 degrees. For example, in the later stage of the solidification process of the welded metal, low melting point eutectic is displaced to the grain boundaries, forming a so-called "liquid interlayer". During the transition of metal from liquid to solid state, the weld seam experiences tensile stress due to volume shrinkage. Under the action of tensile stress, cracks may form in this liquid interlayer, known as thermal cracks.

Therefore,Desktop electric centrifugeThe reason for the occurrence of hot cracks is that the weld seam is subjected to the presence of liquid interlayer and the tensile stress during the crystallization process. The existence of liquid interlayer is the fundamental cause of thermal cracking, and tensile stress is a necessary condition for thermal cracking. From the above analysis, it can be seen that not the entire crystallization process will produce thermal cracks, but only in the later stage of the crystallization process, near the solidus line, is the dangerous temperature zone for the occurrence of thermal cracks.