We've got a steel hammock stand that's been outside all summer. The
parts slot together and before assembling them in the spring, I
liberally coated them with grease - however it would appear that it
wasn't liberal enough.

All the joints nicely pulled apart except for one.

I've tried using WD40, pouring hot water over the sleeve part to make
that part expand and clamping one tube in a workmate ( long way along
the length of the vice ) while whacking the joint in the opposite
direction with a large rubber mallet. I've also tried tightening a rope
around it with a strong cross piece to get some leverage. So far the
pipe is winning.

So what other techniques might work ?

The steel tubes are very thick, maybe 35mm diameter with a wall
thickness of about 3-4mm. Most of the pipe is powder coated and I'd
like to keep it that way by not using fierce tools on it. The parts
that slide together are bare metal and they slide into a suitably wider
tube. I guess that some moisture has got in over the summer and started
some rust that fixes the pipes together.

One part has a welded cross member to form feet ( and a convenient way
to get leverage ), but the other part is a 4 foot length of straight
tubing.

I'd rather take it apart as otherwise it's annoyingly bulky to store
over the winter.