[NutmegCarrera]

Looks to me like there was some inclusion in the base metal. Center of picture, and there appears to be a "path" leading to it from somewhere around the 5:00 position in the photo.

A crack can only propogate if there is a void. Usually they start on the outside, unless there was a flaw in the material such as this example has.

We had a very important shaft fail in my first job. Caused a paper machine roll to break under operation. Similar material. About 12" diameter. Under a LOT of tensile pre-load to counteract bending of the body of the roll. Long story short - lots of pieces of roll body material throughout the rest of the machine. Machine down for about 3 weeks to repair damage. Root cause of failure was a similar inclusion - however, ours was on the outside of the shaft.