I have a engineering degree, so have spent a lot of time taking tests on that. When you heat to cherry red the metal becomes amorphous, or without grain structure. If you cool fast by quenching, the grain structure becomes very small and interlocked. That hardens the metal, making it strong but but too brittle to be useful. If the metal cools slowly, then it forms a large grain structure that has less internal stresses. Large grains unstressed is called "dead soft". To "heat treat" a metal you heat to amorphous and then quench in water or oil (or other fancy liquids depending on the alloy) to its hardest state. The next step is to heat the hardened metal to a temperature slightly below the temp to make it amorphous, and hold it there for a specific time. This relaxes the internal stress and somewhat softens the metal while retaining the strength from the small grain structure. When you buy metal, the "T4" or "T6" tells you the level of heat treating it has received. T4 would be less strong but softer than T6.
The other way of affecting hardness is to "work harden" a metal. This stretches the grain structure through bending the metal, once again making it harder, stronger, and more brittle. "H" on the metal designation tells you the amount of work hardening it has received, like H3.
Now lets relate this to our copper. It comes dead soft. Once you crimp it under a bolt head it is soft enough to conform to the steel bolt and engine block. This also work hardens the copper into a harder state. If you re-use it, it is not soft enough to conform, so if the surfaces of the bolt head and block are too rough, it cannot seal a second time. Thus the need to anneal.
Annealing is done by heating to amorphous and slowly cooling to allow the grain structure to re-form into nice large and unstressed grains. Some jet engine fan blades are 3 feet long, but carefully cooled in a vacuum and very slowly into a single grain! Large grain is soft but very fatigue resistant. Fortunately, some metals, pure copper being one, are not heat treatable. That is how you can get away with quenching it in water while red hot. But the proper way of annealing any metal is to slow cool it to dead soft. Copper, although not heat treatable, is work hardenable.
The key takeaway is that every metal and its alloy behaves differently when subjected to work and temperature hardening. You can get away with quenching pure copper in water. If you do the same to steel it will turn into the hardest, brittle and unusable metal...martensite.
Hi Guest!
Hey - did you know if you click on the title of a thread it will take you to the first unread post since you last visited that thread?
but were afraid to ask: 
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