Microsegregation in cast structures

Microsegregations occur when, during solidification, alloying elements are no longer properly incorporated into the crystal structure and accumulate at grain boundaries. This happens especially with rapid cooling.

In welding, these are critical. If you cool the structure too quickly or don't control the heat input properly, you get hard, brittle spots and, in the worst case, hot cracks in the weld.

So you have to ensure that the part cools slowly and evenly after welding, so that the elements have time to distribute evenly again. Clean preheating and controlled cooling are the be-all and end-all.