When maximum ductility and good machinability are needed and high strength is not required, ductile iron castings generally are given a full ferritizing anneal. The microstructure thus is converted to ferrite, and the excess carbon is deposited on the existing nodules (Fig. 3a). This treatment produces ASTM grade 60-40-18. Amounts of manganese, phosphorus, and alloying elements such as chromium and molybdenum should be as low as possible for superior machinability, because these elements retard the annealing process.
For subcritial anneal, heat and hold at 705 to 720 °C (1300 to 1330 °F), for one hour per inch of section. Furnace cool at 55 °C/h (100 °F/h) to 345 °C (650 °F). Air cool. The Figure shows the influence of subcritical annealing at 705 °C (1300 °F) for various periods of time on the hardness of four ductile irons. When alloys are present, controlled cooling times through the critical temperature range down to 400 °C (750 °F) must be reduced to below 55 °C/h (100 °F/h).
