Cast iron really is the time-proven material of choice for steam heating. Steam puts the system under a lot of stress: large temperature swings that cause the metal to expand and contract with every heating cycle; acidic conditions; and, if the system isn’t well designed or maintained, intense shocks from steam hammer. All of this goes against using thin-walled steel radiators with welded joins, they just don’t last.
We only offer cast-iron radiators for steam systems, never steel. When it comes to valve connections on steam, we only recommend threaded, mechanical joins with steel or brass pipe. While compression fittings are perfect for hydronic systems, we prefer the time-tested, heavy-duty security of a threaded join.