The following is presented as an approximate guide to the dimensional accuracies to be expected in Ductile Iron castings.

APPROXIMATE DIMENSIONAL TOLERANCES ON DUCTILE IRON CASTINGS IN GREEN SAND

Additional points to be considered:

1. The values given refer to 80-to-90 mold hardness range (green sand). Softer molds yield lower dimensional accuracy.
2. Accuracy can be increased at additional cost through the use of very hard molds such as dry sand, chemically bonded sand, etc. Even more accurate castings can be produced in semi-precision and precision molds. The investment casting process provides about the utmost accuracy obtainable with approximate tolerances of ±0.003 x specified dimension. Production costs increase with increased demand for accuracy.
3. Accuracy can be improved by a factor of approximately two if the (large) number of castings to be produced permits an experimental production run followed by reworking of the pattern equipment.

PRINCIPLE NO. 6-

THE EFFECT OF SECTION SIZE ON MECHANICAL PROPERTIES

Data included in this work, like most published data on Ductile Iron properties, are valid for approximately 20 to 50 mm thick castings. Standard specifications are normally set for 25 mm keel block or Y-block test castings. Some standard specifications recognize the effect of section size (cooling rate) by lowering minimum required values, usually for elongation, with larger test castings or for samples cut from the casting.

The effect of section size on properties is the result of changes in microscopic structure as the latter is influenced by cooling rate. Three prominent effects of cooling rate on microscopic structure are:

1. Very high cooling rates do not permit all the insoluble carbon to precipitate in the form of spheroidal graphite. Instead, various amounts of a hard and brittle component, iron carbide (Fe3C) will form.
2. Very slow cooling results in large diameter, irregularly-shaped spheroids of graphite up to 1.5 mm in diameter.
3. Varying the cooling rate in the 1560-480°F (850-250°C) temperature range from very fast to very slow produces different structures from martensite (very fast cooling) through pearlite, pearlite-ferrite to all ferrite (slow cooling).