Constitutional Supercooling
Constitutional undercooling is a local increase in melting temperature due to concentration gradients, which destabilizes the planar interface and promotes dendritic growth.
Physico-chemical mechanism of action
Constitutional undercooling occurs when unequal amounts of compounds or elements (e.g., carbon and silicon) arrange themselves in the melt. These concentration differences lead to local temperature increases because the melting energy differs for different additives. This destabilizes the planar interface between melt and solid and favors the growth of dendritic structures.
Metallurgical relevance & risks during welding
Constitutional undercooling can cause various problems. It can increase hardness of the weld, leading to crack-sensitive martensite. Furthermore, it can coarsen the microstructure and impair toughness. In extreme cases, it can cause hot cracking or stress formation, shortening the weld's service life.
Workshop practice & instructions
To avoid constitutional undercooling, careful selection and processing of welding materials is important. This includes controlling filler materials and following the correct melting and cooling paths. For heavy steels or in high-stress areas, more precise temperature control is required to optimize microstructure formation and minimize risks of cracking and hardness issues.