[MeneMeneTekelUpharsin]

Repeated thermal cycling. Impact extractors. Make sure not a lefthanded thread.

Correct on all three counts. You might also hit them by putting something into the allen wrench socket and hitting the end of them to break them loose (don't hit too hard) which is called "warming them up". The impact extractors would also do that for you. Also, think about the concept of reverse thread. Best wishes.

[cpdiii]

When I was roughnecking on the drilling rigs on occasion we would have a joint that would not break loose.

These were five inch tubes of steel and we were pulling on them with several thousand pounds of force. We would hit them with a very big sledgehammer and they would break free. What was happening was the metal had become galled.

[Windflier]

You might also hit them by putting something into the allen wrench socket and hitting the end of them to break them loose (don't hit too hard) which is called "warming them up".

Thanks for the tech tip. I'll try that.