Back in my hometown they would make the fire plugs glow cherry red trying to thaw water pipes for its customers. During one of the coldest winters on record they actually destroyed a lot of them trying to thaw them.
The real threat was burning the house down. If the grounds weren't properly removed/isolated and the welder could find an alternate ground path to/from the house wiring it would start fires. With no water to put out the fire alot of houses burned to the ground that year.
If you don't know what you are doing it can be very dangerous to thaw pipes with a welder.
RSlater,
RSmike