Cast iron is very abrasive, so the edge on HSS will not last very long, but it will last about 4 hrs. assuming correct speed and feed.. You should use cobalt alloy HSS for that reason. The tool relief angles should be 7 degrees. I find that works best. It is adequate to reduce tool load and yet provide plenty of edge support. It is also very important to have plenty of back clearance to prevent the chips from getting trapped between the tool and the work piece. This is a common cause of poor finish. Also you should use a nose radius of at least 1,5 times the feed rate per revolution for a good finish.