Probably not by heat treatment.  As NDIY says, Copper work-hardens, but having to hammer and bend a finished item is usually bad news.

Dave

<p style=”text-align: left;”>Give it time and it will age harden naturally.</p>
Andy

I am not sure why you need re-harden it. It would make it slightly tougher than in its annealed state but not stronger, and as others have pointed out the forming and age will bring it back nearer to its original state.

Manipulating it after annealing it may have work hardened it to some degree already. If you need to harden it further you can rub it repeatedly with a jeweller’s burnisher.

I can see a case for hardening annealed copper tube if the tube is carrying gas or water in any place where it might be vulnerable to forces that could push the tube out of its wanted position.

Plumbing joints in copper that have been brazed are a little more vulnerable to being pushed out of position in this way than soft-soldered joints.