Neither the quantity of carbon dissolved in iron nor its condition as combined or uncombined carbon is much affected by phosphorus, but this element has the effect of hardening iron slightly. It is, however, highly objectionable in forged iron and only slightly less so in foundry iron for castings, as it causes brittleness when iron is cold, a condition known as "cold short," and may cause the metal to break, when worked cold or when receiving repeated shocks in use. It should not exist in iron for important forgings beyond .06%, nor in foundry iron for strongest castings beyond .5%, although it is used in railroad rails to harden them against wear. Iron high in phosphorus does not make good grate bars nor other castings subjected to high heat, as it renders them spongy.

Phosphorus renders molten iron very fluid and causes it to take an excellent impression of the mould, hence in small castings where strength is not the first requisite, iron may be used containing up to 1% of phosphorus.

Neither phosphorus nor sulphur can be avoided in iron making, and both are difficult to eliminate. The best means of keeping them below objectionable amounts is to select ore fuel and flux for smelting which are as free as possible from them, although materials for the smelter are seldom ideal and cannot always be selected as desired.