Development of thick wall 2.25Cr-1MoNiNb steel forgings for steam generators of fast breeder reactors

Abstract The large and thick forgings made of 2.25Cr-1MoNiNb steel are required for vessel material of steam generator (SG) of the fast breeder reactor SNR 300. In order to study the feasibility of 2.25Cr-1MoNiNb steel heavy section forging, chemical composition, melting practice and ingot making, hot working and heat treatment conditions were investigated. The following recommendations were derived: (1) 0.04% C, 0.10% ΔNb, (2) application of electro-slag remelting process, (3) grain refining by hot working, (4) two-step austenitizing at 1020°C. Based on these recommendations, the actual products such as hollow cylinders with maximum 290 mm thickness, solid bars with 420 mm diameter and forged plates with maximum 185 mm thickness could be supplied for application in the helical coiled SG of the SNR 300. Statistical analysis of the products showed the sufficient and isotropic material properties, which fulfill the requirements of the basic safety rules.