Can you tell us a bit more about your intentions? You must have access to an old machine or table that is too large to cut with some sort of saw or abrasive blade. But, I will point out that cast iron's propensity to fracture could work in your favor. For instance, even on a fairly thick piece of iron like maybe 6 inches it might be possible to use an abrasive saw to deeply score an it inch or two along the desired cut line and then (as people have done for thousands of years in stone cutting) drive a series of shallow angle wedges into the crack along the length of the score. You can develop enormous forces that way and the score will serve as a stress riser. Understand that I have never done this and, yes, if you were nearby I'd drive over to give it a go because I'd love to see how it would work.
However, as I do have a small cast iron foundry where I cast straight edges I, unfortunately, have had occasion over the years to break up many failed Grey Iron castings. (Ductile or malleable iron will never crack using this method.) It has always impressed me how just a small scored stress riser makes the iron break so much easier simply using a sledge to bash it. If I were serious about a thicker section, I sure would make up some wedges and try it.
Whatever distortions in crystal structure you might induce in the iron by flame cutting they should not extend deeply into the iron. So, if you intend to just separate a convenient-sized chunk from a larger mass, I would guess changes in hardness and stress would not extend more than an inch into the metal. So, if you could cut out your piece into a manageable over-size piece and then saw it or mill it to spec, you'd probably be ok. Caution: I would expect you would form white iron at the burned interface. That iron will be in the 60+ neighborhood on the Rc scale. So, you may need to grind away or abrasive saw a kerf to begin saw cuts. Otherwise you will ruin your band saw blade. Simply testing with a sharp segment of an old file will reveal when you have gotten through the hard layer.
Simple 1150F stress relief should do for the net piece though full-on annealing at 1750 would be sure to remove stress and white iron.
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