When maximum ductility and good machinability are needed and high strength is not required, ductile iron castings generally are given a full ferritizing anneal.
Heat and hold at 705 to 720°C (1300 to 1330°F), one hour per inch of section. Furnace cool at 55°C/h (100°F/h) to 345°C (650°F). Air cool.
The graph 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).
This information is from https://dl.asminternational.org/books/book/9/chapter/109877/Heat-Treating-and-Properties-of-Ductile-Iron To find this article, scroll down under Chapter Contents to Annealing Ductile Iron.
