Why is steel annealed in an inert or reducing atmosphere?

Steel is annealed in an inert or reducing atmosphere primarily to prevent the oxidation of the steel surface during the heating and cooling process.1

When steel is subjected to the high temperatures required for annealing—often held for several hours at approximately 680° C (1,260° F) or higher—exposure to oxygen would cause the surface to react, forming scale or other unwanted surface defects. By utilizing a sealed environment filled with an inert gas or a reducing atmosphere, manufacturers ensure that the steel maintains its desired surface quality and integrity throughout the treatment. This is a critical step in both batch annealing, where coils are protected by a heat-resistant shell, and continuous-annealing lines, where the strip is processed within a controlled atmosphere to ensure uniform mechanical and surface properties.1