Method of making safe a heat treatment enclosure operating under a gas atmosphere, the said enclosure comprising a cooling chamber for rapidly cooling a metal strip running from an upstream chamber to a downstream chamber by means of a plurality of guide rollers, wherein said rapid cooling chamber is filled with a controlled atmosphere with a high hydrogen content and said strip is confined within said rapid cooling chamber by means of at least one pressure-balancing duct and of a plurality of gas locks placed between the various chambers, the pressures of the gas atmospheres between the chambers being balanced by means of ducts controlling the flow rates of the gas flowing through the said gas locks.