Toby comments.:- I stand by what I said, if you wish to soften copper to make an efficient head gasket then heat it until just cherry red then submege in water.
If you wish to soften steel just heat it up until dull red then let it cool in still air.
I note your comments regarding annealing, but in my Industry, (Pipeline Steel), we use a lower temperature for stress relief heat treatment than the steel annealing temperature.