Sulphur.— Sulphur acts in two ways in cast iron. First through the carbon and second by uniting directly with the iron. On account of its injurious action sulphur is generally kept below 0.15 of 1 per cent, and for light work below 0.08 of 1 per cent.
Condition of Sulphur in Cast Iron.— Sul phur exists in cold cast iron as a sulphide of iron, as a compound of this sulphide with iron, and when manganese is present, as a sulphide of manganese. In molten cast iron it is prob ably dissolved as gaseous sulphur.
Effect of Sulphur on the Properties of Cast Iron.— Sulphur makes iron hard, red short, weak and liable to contain blow holes. Its only beneficial effect is to give a hard wear ing quality to iron subjected to frictional wear.
Action of Sulphur on Cast Iron.— Sul phur by shortening the time of cooling of cast the separation of graphitic car bon. The carbon which has not been able to
separate as free carbon remains in the combined condition, and consequently sulphur tends to harden the iron. It is moreover heavily affected by the content of silicon. It is customary with iron founders to add silicon to iron with a sulphur content above the normal, with the effect of releasing the carbon which the sulphur retains in the iron.
When cast iron solidifies sulphur is still in a gaseous condition and surrounds each particle of iron with a covering of gaseous sulphur. When the iron is sufficiently cold, the sulphur solidifies, or unites with the iron as a sulphide of iron. In this condition it occupies less space than in the gaseous condition and thus leaves the crystals surrounded by minute spaces which makes the iron very brittle and weak