Its a 1 inch steam radiator pipe coming out of an elbow.

Its in a floor between joists.

Tried a propane torch. Can get a bit of a swing with a hammer. Doesn't budge.

Will WD40 work with something like this? Some other product?

Can't get at it from below.

chances are that it is a cast elbow.
cast is brittle and will crack if you hit it with a hammer.
using 2 hammers, one held behind the fitting and the other to swing, give it a couple of sharp raps and crack it. then it will unscrew by hand.
replace it and the riser nipple as well..

"In floor between joists" and "can't get to it from below" would usually indicate that they CANNOT use a hammer to break the elbow. A bigger wrench and a piece of pipe for a "cheater" may break it loose.

Yeah, I'm going to try a bigger hammer. And maybe a cheater pipe if I can find something. I was working by myself and using a regular hammer. Maybe a 2 pound sledge with a helper or a full size sledge with a helper.

as hj said....a big wrench and a cheater.

Thanks. That's what I will try tomorrow.

"Give me a lever long enough and a fulcrum on which to place it, and I shall move the world." - Archimedes

I once needed a 3' and real muscle to unscrew a piece of 1".

Try unscrewing the valve first.

Insert something into the 1" to prevent crushing.

Good luck.

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638