Hi Jim ,
Quick answer is that there is no easy way of re-hardening copper tubes after annealing .
Copper will age harden eventually but with the nearly pure copper commonly used in model work this can take several months .
Copper can be hardened rapidly by work hardening it – ie stressing the metal and releasing again several times . Not usually any practical way of doing this with most model engineering components though it does seem to happen of its own accord in copper boilers ..
In practical terms use slightly thicker tube to get a bit more strength in the annealed condition .
For interest only :
There are ways of rehardening annealed copper which are used just occassionally in specialist industries . All these rely on work hardening .
(1) Using hydraulics – pipes are cyclically pressurised and de-pressurised in a controlled way .
(2) Using ultrasonics to generate thousands of small stress / release cycles ..
(3) Using shot blasting and machine peening/planishing .
Michael Williams .