Patent ID: 7384489

Claim:
A method for heat treating metallic strips inside a heat treatment chamber having a pressure greater than atmospheric pressure, and wherein said heat treatment chamber has no sealed partitions between zones in the heat treatment chamber, said method comprising: a. passing the strip through a heating zone of the chamber; b. moving the strip through at least one cooling zone of the chamber; c. establishing a first protective gas atmosphere containing nitrogen and a first content of hydrogen and/or helium where the said content is from 3% to 5% by volume, in the chamber but not in the said cooling zone(s); d. establishing a second protective gas atmosphere in the said cooling zone(s) by providing a gas containing nitrogen and a second content of hydrogen and/or helium wherein the said second content is greater than the said first content; e. providing at least one introduction of nitrogen into the chamber; f. providing at least one injection of a third protective gas containing a third content of hydrogen and/or helium and optionally including nitrogen into the said cooling zone(s) wherein the said third content is greater than the said second content, whereby a gaseous exchange occurs between at least one zone of the said chamber having the first gas atmosphere and the said cooling zone(s) having the second atmosphere; controlling the flow rate of the said introduction(s) of nitrogen and said injection(s) according to the gaseous exchange; and obtaining the pressure in the chamber and the content of hydrogen and/or helium from said first and second protective gas atmospheres.