Manufacture of a stress relieved length of steel having an oxidised surface layer

An apparatus for manufacturing a stress relieved length of steel having an oxidised surface layer, the apparatus comprising:

• a heating chamber;
• a reaction chamber, wherein an input aperture of the reaction chamber is sealably coupled to an output aperture of the heating chamber to form a sealed unit;
• and
• a conveying mechanism arranged to convey the length of steel along a path through the heating chamber and reaction chamber,
• wherein the heating chamber comprises a heating apparatus arranged to heat the length of steel in a heating portion of the path, and
• wherein the apparatus further comprises a control means comprising the sealed unit defined by the heating chamber and the reaction chamber, the control means further comprising:
• a first atmosphere controller arranged to generate a non-reactive atmosphere in a portion of the path through the heating chamber and reaction chamber, the non-reactive atmosphere comprising an inert gas arranged to prevent or reduce oxidisation;
• and
• a second atmosphere controller arranged to generate an oxidising atmosphere in an oxidisation portion of the path through the heating chamber and reaction chamber, the oxidising atmosphere comprising a source of oxygen, and wherein
• the first atmosphere controller is arranged to generate the non-reactive atmosphere in a portion of the path preceding the oxidisation portion, and
• the control means is arranged to control the inert gas and source of oxygen provided by the first and second atmosphere controllers to adjust the position of the oxidation portion along the length of the path whereby both the temperature of the length of steel and the atmosphere to which the length of steel is exposed in the oxidisation portion is controlled.