Ferrous metal alloys become hardened and build up stresses when subjected to working such as drawing, stamping, and bending, stresses that make further working difficult. In order to remove these stresses the alloys are bright annealed by heating to an elevated temperature. Annealing must be carried out using an atmosphere which does not chemically react with the alloy either to produce discoloration by formation of oxide scale on the surface or to sensitize the alloy by forming precipitates such as nitrides in the bulk of the alloy. Such reactions are particularly troublesome in the annealing of chromium containing alloys.
Pure hydrogen or dissociated ammonia have previously been used as atmospheres for bright annealing. Pure hydrogen works well, but it is more expensive than many other gas compositions. Dissociated ammonia is cheaper, but subject to quality control problems when the catalyst used to generate the atmosphere by decomposition of the ammonia operates at less than maximum efficiency, since it is believed that residual ammonia can cause undesirable nitriding of some steels being annealed.
More recently, cheaper hydrogen/nitrogen blends containing high levels of nitrogen have been investigated. The high levels of nitrogen in the gas atmosphere lead to unacceptably high levels of nitriding of some steel alloys which results in precipitation of metal nitrides and subsequent loss in corrosion resistance of the part. As reported by N. K. Koebel ("Iron and Steel Engineer", July 1964, p. 81, and "Heat Treating", December 1977, p. 14) and by others, the addition of water vapor to the atmoxphere in small amounts can partially inhibit the nitrogen uptake to keep nitriding levels below unacceptable limits. A more practical approach to the addition of oxygen-containing inhibitors to the gas atmosphere is the addition of a gas, such as nitrous oxide or carbon dioxide, the level of which may be controlled, e.g., as described in U.S. Pat. No. 4,334,938, issued June 15, 1982.
Although oxygen-containing compounds have been used to inhibit nitrogen uptake, care is required to prevent discoloration of stainless steel parts through excessive oxidation of chromium. The equilibrium oxidation-reduction boundaries for chromium in any alloy may be utilized as a guide to prevent excessive oxidation, as described by Ellison, et al. in "Metal Progress", June 1983, p. 39. However, as noted in that article the optimum atmosphere for each stainless steel type must still be determined empirically to find the oxidant/hydrogen ratio which inhibits nitrogen uptake and gives acceptable surface appearance. For those alloys containing strong nitride formers such as Ti, only hydrogen without nitrogen has been used because of the fear of sensitizing the alloy by forming nitrides in the bulk.
U.S. Pat. No. 4,334,938 describes a process for limiting the absorption of nitrogen by ferrous metal containing chromium as an alloying additive (e.g. stainless steel) during high temperature annealing in an atmosphere of nitrogen and hydrogen by controlled additions of an inhibitor selected from the group consisting of water vapor, oxygen, nitrous oxide, carbon dioxide or mixtures thereof to the atmosphere while controlling the dew point of the furnace atmosphere and/or the ratio of the partial pressure of the inhibitor to the partial pressure of the hydrogen.