Material Selection and Optimization for Post-SNR-300 Steam Generators

Abstract

The design lifetime of the steam generators for the SNR-300 is 100 000 h. For the steam generators of the next SNR generation, this design lifetime has been increased to 300 000 h. This prevents the application of the current structural material, a niobium stabilized 2¼ Cr–1 Mo and started a materials selection process based on the primary requirement of a high creep strength. Other important criteria in this selection process have been a high yield stress at design temperature, adequate ductility, good corrosion resistance in liquid sodium, water, and steam, and acceptable manufacturing properties. This resulted in the selection of the high alloy steel X20CrMo12 1 (12% Cr, 1% Mo) as prime candidate material. This material has been subjected to an optimization program with regard to its mechanical properties and to a determination of its susceptibility to stress corrosion at high temperatures. It has been demonstrated that this type of steel shows a rather strong influence of tempering treatment and thermal aging on its strength and impact ductility, especially in the heat-affected zone of welded joints. The material shows a mixed transgranular-intergranular corrosion attack only under severe corrosive conditions in FeCl₂ solutions under high mechanical stress. High tempering temperatures and electroslag remelting have a beneficial influence on impact ductility as well as on the corrosion susceptibility.