I have looked at a few cannon balls, one from the Tower of London, a white cast iron with cementite and a little steadite. A cast iron cannonball from Sandal Castle, this time a typical grey cast iron with graphite well distributed and fine pearlitic matrix with steadite regions and some ferrite. Another corroded fragment from the Tower of London with very high phosphorus content and much remnant steadite in a corroded mass of iron oxides, and a few more.......one I remember with extensive decarburization at the surface of the ball and a white cast iron structure towards the interior, the altered surface zone extending over 900 microns.
I think one or two of these were mentioned in my general metallography book from 1991.
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