Cold Bending of Boiler Tubes in New Grades

New high creep resistant steels have to be bent according to specific procedures in order to preserve parent tube properties. Cold bending of tubing can lead to an important level of deformation in extrados and intrados areas depending on the bending radius and the process used. Fabrication codes allow bending without any heat treatment in case of small deformation. However in case of larger deformation they may require a stress relieving treatment (SRT) in order to eliminate the brittleness of severely cold worked areas. In addition the bends often undergo a post weld heat treatment (PWHT) due to welds in boiler components. A softening effect can occur on these cold worked areas during heat treatment (SRT or PWHT) or service depending on the operation conditions. This softening effect resulting from recrystallization or recovery phenomena can be detrimental to the creep properties. This conference paper focuses on the creep behavior of several new steel grades such as T23, T24, T92 and VM12 after cold bending followed by different heat treatments. The results are compared with the well-known T91 grade, taken as a reference. Recommendations for cold bending are proposed to define the best fabrication routes for these new steels and compared with fabrication codes, mainly with the recent ASME proposal for T91 steel.