METHOD FOR IMPROVING SERVICE LIFE OF MANDREL BAR FOR MANUFACTURING HOT SEAMLESS STEEL TUBE

摘要

PROBLEM TO BE SOLVED: To prevent generation of surface cracks in a heat treatment, to ensure the application effect of MgCl₂ solution when rust scales are formed, and to improve the burning resistance by heat-treating a mandrel bar after use at the temperature below the tempering temperature after the initial hardening in new production, grinding or cutting it by not less than the prescribed dimension, adhering rust scales thereto after grinding, and achieving the heat treatment at similar temperature. ;SOLUTION: The surface hardness of a mandrel bar after use is unified, and the mandrel bar is re-heated at the temperature below the initial tempering temperature in newly manufacturing the mandrel bar in order to unify the reduction in the outside diameter by grinding and cutting. A heat affecting zone layer of a surface layer of the mandrel bar is completely removed, and the outside diameter is reduced by not less than 0.06 mm in order to ensure that the surface hardness of the mandrel bar in re-use is similar to that in new manufacture. After MgCl₂ aqueous solution is applied, the mandrel bar which is dried and on which red rust is formed, is heat-treated at the temperature below the initial tempering temperature to form a burning resistant scale film on its surface, and a lubricant containing oxalic acid is applied thereto.;COPYRIGHT: (C)1999,JPO