Phosphorus-induced temper embrittlement

Phosphorus can segregate at grain boundaries during welding and embrittle the microstructure. This issue mainly occurs with slow cooling because the phosphorus then has time to gather there. To avoid this, you need to cool the heat-affected zone quickly after welding so that the phosphorus remains bound in the steel and doesn't migrate. If you feel the cooling process is too slow, it helps to cool the weld with compressed air or a damp cloth—but be careful not to quench the steel suddenly, as that would make it hard and prone to cracking.