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A diamond bladed angle grinder is useful and make much less CI dust
and
sparks.

having used diamond to cut ferrous, I found it almost useless. I was
told the diamonds react with the iron and dissolve into it, making
diamond discs ineffective on iron and steel. And in fact, hardening the
steel a bit in the process.

That sounds a bit unlikely given the quench from red heat needed to
harden carbon steel but ISTR cast iron is already stuffed full of carbon
so if diamond did dissolve in it it shouldn't make any difference.

--
Roger