There's been a lot of talk lately about whether or not rolling the fillets really improves the strength of the turbo crankshafts over the NA cranks. I found a small book on ductile iron technology (another name for nodular iron), and it has one table where they compare the endurance limits of bending fatigue strengths of several crankshafts. They don't say what the cranks fit, they just call it a "type 202 crankshaft". Anyway, for one grade of ductile iron as cast the endurance limit is listed as 30,000 psi, while the same grade as cast with rolled fillets raised this to 97,000 psi, an improvement of 3.2 times! Using a different type of cast iron followed by a heat treatement process gave 60,000 psi and 143,000 psi, respectively, for austempered ductile iron without and with rolled fillets, for a gain of 2.4 times in strength. The same crank in 1046 steel, quenched and tempered, had a limit of 48,000 psi. So depending on the initial grain structure and composition of the ductile iron rolling the fillets increased the fatigue strength by 2.5 to 3 times, and the cast cranks with rolled fillets were stronger than a moderately high carbon steel crank. Unfortunately they don't give a specific reference for this table so I can't follow it up further. This was from "Ductile Iron Data for Design Engineers", published by Rio Tinto Iron and Titanium, Inc., 1990.
Just fodder for thought.
Just fodder for thought.