Development of two-step softening heat treatment for manufacturing 12Cr-ODS ferritic steel tubes

The 12Cr-oxide dispersion strengthened (ODS) ferritic steel is being developed as prospective cladding materials for attaining higher burnup and higher coolant outlet temperature in advanced fast reactor. Ductility and high temperature strength in the hoop direction can be attained by recrystallization in the manufactured tube of 12Cr-ODS ferritic steels. In this study, it was found that once recrystallization has taken place during manufacturing process with four passes cold rolling and intermediate heat treatment, the recrystallized condition can not be repeated at the final heat treatment. Thus, the unique two-step softening heat treatment was developed as the intermediate heat treatment; the first step heat treatment relieves the strain energy accumulated by cold rolling to raise the recrystallization temperature, and the subsequent second step heat treatment at the temperature, which is higher than the recrystallization temperature before relieving the work strain energy, accelerates the hardness reduction without recrystallization. This process leads to easy cold rolling and complete recrystallization at the final heat treatment.