Supersaturated Solid Solution

A supersaturated solid solution forms when, during rapid cooling, more alloying elements remain dissolved in the lattice than would be possible in the equilibrium state. This happens easily during welding when cooling is too fast and the atoms can no longer diffuse properly. The problem is that this state is not stable. When heated later or due to stresses, the structure can react, leading to embrittlement or cracks in the heat-affected zone. You have to be careful, especially with higher-alloy steels. In practice, this means: you have to slow down the cooling after welding so that the structure can transform properly. Depending on the material, this means preheating, welding at a lower speed, or insulating after welding instead of cooling quickly. If you cool the parts too fast, you get a hard, brittle structure that will cause trouble later.