Eutectic Phosphide Eutectic

Phosphorus in cast iron forms a low-melting eutectic during solidification, the so-called steadite. This is a hard, brittle structure that precipitates at grain boundaries. When welding, this is a problem: the heat melts this eutectic early and can lead to hot cracking. So you have to be careful not to heat the part too much and control the cooling, otherwise you'll get cracks in the weld metal or in the heat-affected zone. In short: this stuff makes cast iron hard and wear-resistant, but when welding it's a risk factor you need to keep in mind.