Yes, it’s okay to take soldering irons, and other devices that generate heat onto the plane, but only if approved by your airline, and only if the battery does not exceed 160Wh.
Items that generate extreme heat may present a serious risk to passengers, crew and the plane.
You will be asked to relinquish all dangerous or prohibited items found in your carry-on luggage. If you refuse:
We x-ray bags going on planes to make sure there is nothing dangerous in them. If something comes up on the x-ray that needs checking, the bag will be opened, searched and any dangerous or prohibited item will be removed.