Cold bending of advanced ferritic steels: ASTM grades T23, T91, T92

The metallurgical background of advanced ferritic steels must be considered during every stage of fabrication, including forming operations, such as cold bending, because they can negatively affect the high-temperature properties of the material when not performed properly.In this work, the results of a broad cold bending program carried out on ASTM Grades T23, T91 and T92 tubes in the range 1.0 ≤ R/OD ≤ 4.5 are presented; the industrial window for a safe and repeatable cold bending of 9%Cr tubes was defined: ovalisation and thickness variation were measured after the bending; the microstructure, the hardness values and the mechanical properties at extrados and intrados positions were investigated and compared with properties of unbent materials. Optimum post bending heat treatment of most severe bends was defined.This paper aims to integrate the International Codes for cold bending with recommendations for proper post bending heat treatments, in order to obtain a final component with the desired microstructure, cold workability and mechanical properties. Recommendations for the post bending heat treatment were formulated and compared with the prescriptions given by the relevant International Codes and Standards.Creep properties of large cold bends were also assessed by means of specimens directly machined from extrados and intrados portions of real bends. Both extrados and intrados areas of T23, T91 and T92 bends exhibited a creep resistance within the lower scatter band of base material isothermal curve. The creep-rupture ductility of the intrados was measured higher than that of the extrados.