Well they're certainly used, personally I wouldn't feel comfortable with them, especially on heating pipes. Unbending the copper though could potentially work harden the copper.
Work hardening means making the metal harder and more brittle, this happens when you repeatedly bend metals. I'm not aware of any way of reversing this.
Annealing will definitely soften copper but I don't know how it can be applied here.
Basically the metal is heated red hot (difficult to tell, naturally!) and then quenched in cold water.
John
Plastic pipe has been used on heating systems for probably 40 years. I haven't heard of any failure of pipe or fittings.
Copper will corrode through by various means.
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