Just to note: Pure copper, [as pure as that used for conductors at any rate] is not hardenable at all by the application of heat, nor by the rate of cooling, however fast or slow - even by quenching in water. It will start to anneal if heated to somewhere between one third and one half of its melting point in degrees kelvin [k], [ = 300DEG F - 760DEG F] depending on how much work hardening it has been subjected to. Work hardening is an effect caused by the distortion of the metal 'grains' by drawing, bending or hammering and is not to be confused with the hardening of steel, which is caused by specific alloys of carbon and iron retained in the steel alloy by supercooling. When copper is heated, distorted grains are simply 'healed' back to a more normal shape, [in layman's terms], so hardening is just not possible by heating.
It is possible to embrittle copper if it is heated for long enough and hot enough in the presence of oxygen, because copper oxide in the metal reduces its ductility or 'pitch'. This effect is sometimes seen when insulation is burned off to obtain scrap in large batches. Interestingly, annealed copper has a better conductivity than work hardened metal, [ the usual condition in drawn wire], so that the conductivity of the cable in the vicinity of a Cadweld would be marginally improved. An alloy of the Cadweld and the copper wire will very likely form, in a very thin layer a few atoms thick, at the boundary between the weld and the wire surface. This is what makes the good electrical connexion, and gives the improved resistance to corrosion when buried.
