CGI is used to create engine block, cylinder heard and bearing cap castings for diesel trucks. It has half the weight of conventional gray cast iron, while providing twice the strength and stiffness. This means engine block wall thicknesses can be minimised, providing savings in weight and fuel consumption.
The strength of the material also means it is harder to machine. With two to three times the tensile strength of normal iron, higher cutting forces are needed for milling. Up to 25% more machining power is required.
The low thermal conductivity of CGI means machining-generated heat is localised, damaging the tool. The material also tends to stick to the cutting edge of the tools, and the presence of titanium also makes it tougher to machine and reduces tool life.
As a result tools used to machine CGI last half as long as those used for normal cast iron. The process also takes three times as long if conventional processes are used.
However, the material provides 50% better milled surface finish than cast iron, so fewer machining passes may be required, or finishing can be eliminated altogether.
According to Sandvik, carbide coated with thick layers of titanium carbon nitride and aluminium oxide works best for milling CGI. Tuning and boring operations can be optimising by using tools made of a carbide substrate with high abrasive wear characteristics and wear-resistant thick coatings. Coolants should not be used when operating on CGI.