Effects of pre-heating treatment on the annealing behaviour of cold-drawn fire-refined coppers

The influence of hydrogen, oxygen and lead contents on the thermo-mechanica l properties of fire-refined copper has been studied and related to the dec rease of its hydrogen and oxygen content by the heat treatment known as pre -heating. Fire-refined coppers contain between 0.5 and 0.7 weight ppm of hy drogen, and usually more than 50 weight ppm of lead. The decrease in the hy drogen contents of this material through pre-heating at their optimal pre-h eating temperatures, leads to a strong decrease of their annealing temperat ure for 30% elongation (epsilon (30%)). Fire-refined coppers with a lead co ntent higher than 20-50 weight ppm (and preferably higher than 250 weight p pm) exhibit a microstructure before pre-heating with a low number of voids, as compared to the pre-heated fire-refined copper. Pre-heating also caused a decrease of the hydrogen contents to no detectable values and a decrease of the amounts of dissolved oxygen. These results are related to the possi bility that lead increases the solubility of hydrogen in copper. Pre-heatin g at the optimal temperature develops faceted voids and decreases the hydro gen content to undetectable values by its reaction with the dissolved oxyge n, thus forming water. This theory is confirmed by the poor results of pre- heating treatment obtained for oxygen-free high-purity copper, oxygen-hydro gen-free fire-refined coppers and coppers with lead contents lower than 20- 50 weight ppm. (C) 2001 Kluwer Academic Publishers.