The capacity to fill up thin and broad cavities depends on the fluidity of molten metal, which is a function of the gap between the pouring temperature and the liquidus curve of the concerned alloy. The Fe-C diagram below illustrates the fundamental differences between cast iron and steel. Steel generally requires pouring temperatures that are much higher than those required by cast iron.

The fluidity of molten metal also depends largely on the amount of P involved. However, in practice, control over the fluidity can only be exerted exceptionally by such means since the amount of P must be minimized for other reasons.