Impact
An instantaneous blow communicated from a moving body to another body either moving or at rest.
Impressions on Cast Iron.--See EMBROIDERY IMPRESSIONS ON CAST IRON; HANDWRITING IMPRESSIONS IN CAST IRON.
Impurities in Cast Iron.--The chief impurities contained in cast iron are manganese, sulphur, phosphorus, and silicon. Manganese tends to the formation of combined carbon, reduces tensile strength, produces brittleness, and makes slag. Except in very strong castings, manganese should not exceed 0.5 per cent of the mixture. Sulphur contributes to retain the carbon in the combined state, and promotes the formation of combined carbon. Foundry irons should not contain more than 0.1 per cent of this element. Phosphorus causes hardness by lowering the separation of graphite, but increases fluidity. From 0.3 to 0.5 per cent of this element is all that should be allowed in foundry mixtures, unless cases where great fluidity, regardless of strength, is the chief desideratum. Silicon increases fluidity, reduces hardness and shrinkage, by changing combined into graphitic carbon. Any addition of silicon after the bulk of the carbon has become graphitic hardens the casting. Cast-iron mixtures may contain from 1.75 to 2.5 per cent of silicon. See CAST IRON; SILICON; SOFTENERS.