Phosphide Eutectic
A brittle, hard constituent in phosphorus-containing cast irons, consisting of steadite and iron carbide, which impairs machinability.
Phosphorus is undesirable in steel. During solidification, it migrates to the grain boundaries and forms a low-melting phosphide eutectic there. This makes the material brittle and prone to cracking, which is also called cold shortness.
In welding, this is a problem because the heat input can further increase this brittleness. So if you have to weld material with a high phosphorus content, expect cracks in the weld area and poor formability.
Best to avoid such material altogether or use a filler material that binds the phosphorus. If there's no other way, weld with as little heat input as possible and let the weld cool slowly.