Impurities, similar to those found in iron, are present in steel. Sulphur in the steel tends to make it red-short and to interfere with its welding and forging properties. Steel should not contain more than .1% of sulphur, and when possible the amount should be reduced to .03 or .04%. Manganese tends to counteract the effects of the sulphur.

Phosphorus increases the tensile strength and raises the elastic limit of low carbon or structural steel, but detracts from its ductility and strength to resist shocks. Phosphorus is, therefore, considered a dangerous ingredient, and should never exceed .2%.

Silicon influences the form which carbon takes in cast iron and its rate of cooling; it increases the tensile strength and reduces the ductility of steel. The process of manufacture, however, usually removes all silicon, and therefore that element gives very little trouble.

Manganese increases the elastic limit, the tensile strength, and the ductility of steel. It also counteracts the sulphur and phosphorus. It is thus an important factor in preventing red-shortness. The proportion of manganese necessary to produce good results is between .2 and .5%.