Black Iron Pipe (BIP) is an extremely rugged material and before commencing any replacement I would have to see the damage to determine if the damage actually warrants replacement or if it is more cosmetic than functional damage.

Having said that, if it is determined that the pipe must be replaced, ALL connections on BIP MUST BE made by means of approved NPT (national pipe taper) threaded fittings.

When it is necessary to cut out a section of BIP pipe the options are to:

1. Locate the nearest fittings on either side of the section that must be removed. Cut the damaged pipe so it will be able to rotate freely, then holding the fitting with one pipe wrench, unscrew the pipe from the fitting leaving the fitting open where you will be able to attach the new pipe.

2. Cut the damaged section of the pipe out, then using a pipe threading die, cut new threads on the end of the servicable pipe.

You can generally rent a set of pipe threading dies at a local tool rental facility and at some big box home supply centers, a word of caution here, You must use cutting oil very liberally when cutting threads otherwise you run a serious risk of damaging the pipe thread dies and making a ragged thread which will not properly seal.

Also note that both ends of a pipe have a right handed thread so while you are tightening the pipe on one end, it will be unscrewing from the opposite end, therefore when installing a repair section you will need to install a union so that the pipe can be screwed into the fittings on both ends, then the final connection made by tightening the union.

If you can post some photos I may be able to offer some further advise.