OK - but the answer gets kinda far into the weeds! The foundry that I owned made 100% of the Motor Wheel hubs that they supplied to Ford & Chrysler passenger cars in the 80s. Also, I'm a metallurgical engineer - so here goes.
Ductile iron should be made from its own base iron. High production gray iron base has too many tramp elements in it which can hurt the elongation of as cast ductile iron. Also, gray iron melters often use scrap iron for melt stock that has too many tramp elements in it. Melt stock that contains thing like old bathtubs and steam radiators contains relatively high amounts of phosphorous. If used for ductile iron, phosphorus in the metal raises the brittle transition temperature too much. In which case - ductile iron becomes strain rate sensitive and can experience brittle failure at room temperature. Even at lower phosphorus levels - traumatic brittle impact can occur in severe cold weather environments.
The excellent elongation of ductile iron (versus gray cast iron) is achieved by adding magnesium as an alloy to the liquid iron post melt. The magnesium causes the graphite that precipitates out of the liquid iron as it goes through the solidification process to have a spheroidal morphology (round "nodular" graphite in the ferritic matrix). These spheroids of graphite don't interrupt the metal matrix of the iron as much as the "flake" graphite that is seen in gray iron. As a result - ductile iron can stretch before fracture a lot more than gray iron.
Gray typically has more pearlite stabilizers in the chemistry. So in addition to the graphite "flakes" interfering with its ability to "stretch", it also has a matrix that is just not conducive to ductility to begin with.
We didn't cast cranks, but to my knowledge - GM crankshafts from the late 60s were either cast or forged. Actually, one of the advantages of cast iron is that it has great anti vibration qualities. Also, after a million cycles (doesn't take long for a reciprocating part) it generally has an infinite fatigue life. Steel, on the other hand, will continue to propagate fatigue cracks to eventual failure even after a million cycles. I'm sure that many of you have noted these fatigue cracks when crack detecting old forged cranks. So, don't use forged cranks with significant cracks thinking that they aren't too bad. Steel fatigue cracks will continue to propagate to eventual failure. Cracks in cast cranks generally stop propagating when they hit the flake graphite structure (kinda like drilling a hole at the end of a crack to stop it from propagating).
