PROTECTIVE IRON OXIDE SCALE ON HEAT-TREATED IRONS AND STEELS

The invention provides a process for the heat treatment of iron-based alloys , including irons and steels, such as carbon steels and low alloy steels, in a controlled oxidative environment, to modify the microstructure of the metal to obtain both improved mechanical properties and a protective surface oxide scale. In a first high-temperature treatment, a layer of wüstite (FeO) is formed on the surface of the material and austenite forms in the interior of the material. Rapid cooling follows the high temperature treatment so as to preserve the wüstite scale and to obtain at the same time an internal steel microstructure comprising martensite or bainite. In a second step of lower- temperature heat treatment, the scale is transformed so that it comprises alternatively (i) a base layer composed predominantly of wüstite (FeO), with a surface of predominantly magnetite (Fe3O4); (ii) a base layer composed predominantly of wüstite, an intermediate layer composed predominantly of magnetite and a surface layer composed primarily of hematite; or (iii) a bas e layer composed predominantly of magnetite and a surface layer composed primarily of hematite. The mechanical properties of the material may be tailored for specific applications.