I used a 4" grinder with a metal-grinding disc in it to cut a score around the pipe about 1/8" deep and as narrow as possible. Then take a masonry chisel (not a woodworking chisel) and place the tip in the score and hit it sharply with a hammer. The pipe should break cleanly along the score line. There is a tool that breaks cast iron pipe in one easy step...I'm not sure what it's called (a "pipe breaker?") but it's probably not worth buying or renting one if you're only making a few cuts.