posted September 19, 2003 01:02 AM
To understand what nodular iron is, you have to know what basic steel, and cast iron is:
Steel has disolved carbon in it. Less then 2%. 4140 steel has .4% carbon, 1045 steel has .45% carbon. With low % of carbon steel does not cast well at all, it's like slush. That is why most steel parts are rolled (plate, bar ect), or forged (con rods, cranks ect). Steel has the same tensile strength as compressive strength. Steel also transfers harmonics very well. A steel crank will ring like a bell if you tap on a counterweight with a wrench.
Grey cast iron (engine block) has approx 4% carbon in it. This is about the right percentage of carbon that makes it pour well into molds. With that much carbon, it cannot be disolved with all the iron. The carbon settles out and makes what looks like tiny cracks all throughout the cast iron. All cast iron is pre-cracked. The cast iron has good compressive strength, but poor tensile strength due to all of the tiny cracks in the cast iron. The cracks are imperfections, stress risers where the material pulls apart. However, the tiny carbon flakes absorb harmonics and dampen vibration. Alot of heavy machine bases ect are made from cast iron. It also has poor ductility. Just like concrete.
Nodular iron aka ductile iron. Has a small amount of magnesium added. This makes all the tiny carbon flakes turn into round nodules. This gives the iron alot more ductility and higher tensile strength then plain old grey cast iron. You also have the carbon nodules that help dampen harmonics similar to cast iron. The GM cast cranks are made from nodular iron.
So the nodular iron cranks your talking about would probably be similar to a stock gm crank, material wise.