BBQ cast iron plate for sharpening?

hi, I am looking at getting an old cast iron BBQ plate 10mm thick and cutting it into rectangles 8" x 3" to use as a sharpening plate with diamond paste after getting them machined and flattened



Can anyone please advise on whethere a plate like this would be suitable for this purpose, considering the annealing, whatever, that could come from exposure to a flame on the one side?