Crankshafts have been forged for somewhat over a hundred years now, I think. I have a couple of old forging books that show the entire sequence for forging the things, not that I ever plan to do it.
I agree that probably all ordinary crankshafts are forged, and that forged is better than cast, but I wouldn't completely agree that the grain lines follow the forging - at least, not after heat treating. Once the steel has been taken above the A3 point, the grain of the steel becomes random again. It just can't help itself.
Prior to normalizing though, the grain of the steel definitely does flow with the forging forces to some extent, depending on the manner and amount of forging. Open die forging generally will develop more grain flow while closed die forgings develop somewhat less. As a general rule - there are no real absolutes in forging work, in my experience.
It is my understanding from my metallurgist buddy at one of the biggest industrial forging joints in the country, that after forging the cranks are normalized to relieve stresses, then sent out for rough grinding. After rough grinding they are then induction hardened at the bearing and rod journals and finish ground to tolerance. The remainder of the crank is left in the normalized condition so that it can handle the stresses of reciprocation.
I haven't asked him what he thinks the reason is for a crank to break, but my guess would be a bad forging or bad line bore. During the forging process it is possible, though very unlikely, that a "cold shut" could get forged in at one of the transitions, resulting in a stress riser that later cracked under use. It is also possible that the original billet could have been flawed prior to forging, but that is usually caught by X-ray on QC check. In the US, anyway.
It is also possible, and maybe even likely, that the crank was improperly machined after forging, leaving a sharp inside corner at a transition - this becomes another stress riser in use.
With all the possible ways that the crank might have been flawed, it still might well have performed fine if it was operating in a perfectly bored set of journals. But the least little bit of misalignment in the journals would set up stresses and harmonic vibrations that would cause it to self-destruct for sure. Even a perfect crank won't tolerate an out-of-line bearing line for too long.
That's my take on it, anyway. I'm **** sure no metallurgist, mechanic or engineer, just a blacksmith who's been around a bit, so you can take it for what it's worth. I'd definitely have the line bore checked, no matter what else you do. Put a new crank in a bad bore at your peril, in other words.
Rich
