PHOSPHORUS.
Phosphorus, Phosphorus confers both beneficial and harmful properties on cast iron. Cast iron generally contains from 02 to 1.5 per gent phosphorus, but on account of its injurious effect it should be kept below 1 per cent for light thin castings, and below 0.5 per cent in heavy castings.
Condition of Phosphorus in Cast Iron.— Up to a proportion of 1.7 per cent, phosphorus dissolves in iron forming an alloy. Above this proportion free iron phosphide is formed, and at 15.6 per cent of phosphorus, the entire mass of the iron has been converted into iron phos phide. Between the two percentages named, the grains of the iron-phosphorus alloy become coated with a pellicle of iron de cidedly weakening its structure. The presence of carbon also in the iron militates against the presence of the contained phosphorus, and may eject it, altogether; so that it is common to find both free iron carbide and free iron phos phide in a sample of cast iron. A percentage of 0.89 carbon is compatible with the maximum of 1.7 per cent of phosphorus in alloy with the iron. In strong castings the proportion of phosphorus is held down to 0.4 per cent; in ordinary castings 0.7 per cent is allowed.
Effect of Phosphorus on the Properties of Cast Phosphorus makes iron fluid, or at least prolongs the fluidity, and thus aids the iron in taking the exact form of the mold. It
makes iron weak, brittle and liable to break un der shocks. It is advantageous in light, thin castings where fluidity and exactness of outline are important and where strength can be neglected. It is very detrimental in large ir regular-shape castings, especially where strength is required.
Action of Phosphorus on Cast Iron.— Phosphorus by prolonging the fluidity of the iron gives the graphite a further opportunity to separate and thus tends to reduce shrinkage. By prolonging the iron in a plastic condition just after it sets, phosphorus allows the separating graphite to force the iron into the minutest corners of the mold. The phosphorus prolongs the cooling of the iron as it crystallizes out and thus allows it to separate into larger crystals, which weakens the iron and causes it to .be brittle. When phosphorus is present in large quantities it forms a eutectic with the iron which fills up the crevices between the particles which have previously solidified. When the eutectic solidifies later it causes strains in the iron which makes it short and liable to break under shock.