Gray or Ductile iron??

One thing I've run into in machining castings is gray iron is very nice to machine, ductile can be mean. Having worked at Moline iron foundry where we cast ductile, malleable, and gray irons we had to turn test bars from every melt to test the tensile strength. Never had a problem turning the gray bars but the ductile -- wow, if you found a hard spot, toss the sucker in the garbage. It would tear up a carbide insert faster than you could shut the feed off and send our poor South Bend lathe jumping across the floor. Milling was the same story. Not even a carbide endmill would last once you hit a hard spot in a ductile iron casting. Gray iron filled the cavities better and gave better detail than ductile. And the hot ductile casting was easy to tell from others - spit on it and you could smell the sulphur. Had more problems with gas holes in ductile castings, venting and gating were a trial and error process on the molds.

The main difference between gray, malleable, and ductile is the graphite.

Graphite is a practically pure form of free carbon in the iron.

Gray iron contains graphite in thin flat flakes that are distributed though a matrix of pearlite and some ferrite. These flakes are responsible for giving gray iron its excellent machinability and vibration damping qualities but also to limiting its mechanical properties. A 40 class gray iron has 40,000 psi tensile strength, 80 class has 80,000 psi.

Malleable iron has nodular graphite distributed through a ferritic matrix. These nodules have less effect on the mechanical properties than do the flakes of gray iron. The nodules are composed of an aggregate of fine flakes.

Ductile iron has graphite in the form of spheroids which are single polycrystaline particles. This structure is obtained by adding magnesium (nickel-magnesium alloy containing 50-80% nickel) in the ladle just before pouring. Tensile strengths above 150,000 psi have been achieved for ductile iron.

Thanks to my metallurgy text book.

Steve