Craftsman Cribsheet No. 136: ISO Material Groups: Cast Iron

Cast irons are divided into three main grades for machining. Within those categories, they can be broken down into subgroups of ferritic and pearlitic cast iron. Ferritic cast irons are more ductile and have higher dimensional stability in higher temperatures. Pearlitic cast iron has a higher resistance to fretting corrosion. Fretting corrosion occurs in air when surfaces rub under oscillation, rotation or vibration under load. Pearlitic cast irons also have higher hardness and higher mechanical properties.

Some cast irons can be heat-treated to martensitic with higher hardness values. Hardness numbers in the chart are approximate ranges. Machinability varies widely in each category because cast irons of each type can vary widely in ductility and mechanical properties. The less ductile the material, the better the chip will break. There is little industry standard on the naming of the categories from insert manufacturers. Below approximates the categories and ranges.

Find the specific grade of cast iron in a reference manual to get surface feet per minute (sfm) with high strength steel (HSS). Multiply the HSS number by 2-2.5 and it will provide a reference point for what sfm you should use with carbide. The sfm guidelines given based on material groups above can vary widely, which can be seen in the machinability range.