Tom Lieb: "Cast crankshafts are manufactured using the most basic production methods. Molten metal is poured into a mold, then machined into the final shape of the crankshaft. This method is the most inexpensive and also the least durable. Forged cranks start out as a round bar of metal, are heated up, and then pressed into shape with hydraulic presses and dies. In a forging, this compressive force squeezes the molecules together and creates one uniform grain flow. On the other hand, the grain structure in a cast crank looks like beach sand. That's why forged cranks are significantly stronger than cast cranks.
"Billet cranks are the strong ones, which is why they're used extensively in NASCAR Sprint Cup and Top Fuel. Visually, billet cranks are indistinguishable from forged cranks, so the difference between them is in the grain's structure and metal. If you're making a forged 4.000-inch-stroke small-block Chevy crank, you start with a round bar of metal that's 4.75 inches in diameter. After the forging process, the total width of the crank ends up being 6.75 inches. That means grain that was flowing parallel to the crank has been forced to negotiate a series of 90-degree turns to make the rod journals and the mains. What was once the centerline of the crank has now been offset by stretching, sharing, and weakening the grain. On the other hand, a billet crank has uniform grain structure than runs parallel throughout the length of the crank. Instead of being pressed into shape, a billet crank is formed from a round bar of highly refined forged steel that's much larger in diameter. Compared to a forged crank, the metal blanks used in a billet crank weigh more than twice as much. To make a 4.000-inch-stroke crank, you'd start with an 8-inch diameter piece of steel, then whittle it down into the shape of a crank. This method eliminates all stress risers, yielding a stronger end product than a forged crank."
Alan Davis: "The most basic material used to manufacture cranks is iron, and numbers such as '4340' refer to what's mixed in with that iron to make steel alloys. Steel is 95 percent iron, and the difference in the various types of alloys is in the remaining five percent. The American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE) have established a set of standards that determines the content of the different grades of metal. As you add carbon to a basic iron crank, you end up with cast steel. Add more carbon, and then you start getting into the different alloys like 1013. The materials added to iron to make alloys isn't expensive, but the labor to mix all the ingredients together is what drives up costs. Generally, 4340 is considered the best alloy to make cranks and rods out of. Next on the list is 4130, followed by 5140. Factory forged cranks were made from alloys like 1013 or 1053. While they're much stronger than iron, they're not nearly as strong as 4340."
Shawn Mendenhall: "It's hard to generalize how much power a crank can handle, because there are other variables besides whether a crank is cast or forged. Bigger main and rod journals will obviously take more abuse in a cast crank. For small-blocks, a safe limit for a cast crank is 500 hp and a maximum of 6,500 rpm. With big-blocks, their bigger mains will handle 650 hp. We've seen cast cranks in turbo motors take over 2,000 hp, but we don't recommend it. These days, 4340 is the standard for forged cranks. Just a few years ago, 5140 and 4130 cranks were sold alongside 4340 cranks. However, they aren't much cheaper, so it makes no sense to use them over a 4340 forging. For 99 percent of people, a 4340 crank is more than they will ever need. If you can break one somehow, then something else was probably wrong with the motor. Once you get into very high rpm and tons of power, then billet may be the best option."