Sulfide Shape Control via Rare Earths

Rare earths like cerium or lanthanum bind the sulfur in steel and turn elongated manganese sulfides into round, globular inclusions. That's good for welding because it makes the structure more stable and less prone to cracking. The addition is done in the melt, the exact amount depends on the analysis and doesn't need to be overdosed. Then let it cool normally, that's all there is to it.