An apparatus for manufacturing a stress relieved length of steel having an oxidised surface layer includes: a heating chamber; a reaction chamber coupled to the heating chamber; and a conveying mechanism conveying the length of steel along a path through the heating chamber and reaction chamber. The heating chamber includes a heating apparatus arranged to heat the length of steel in a heating portion of the path. The apparatus further includes a control means including a sealed unit defined by the heating chamber and the reaction chamber and arranged to control both the temperature of the length of steel and the atmosphere to which the length of steel is exposed in an oxidisation portion of the path within the reaction chamber in which the oxidised surface layer is formed. A method of manufacturing a stress relieved length of steel having an oxidised surface layer is also disclosed.