I think thermo-cycling causes hardening too. Copper gaskets on spark plugs get hard with use and won't reseal unless you anneal them soft again.
We used to put them all on a wire and heat with a torch until cherry red, then hang them up and let them cool. Buff the soot off of them and they were like new.
We used to put them all on a wire and heat with a torch until cherry red, then hang them up and let them cool. Buff the soot off of them and they were like new.