Influence of manganese on mechanical properties, irradiation susceptibility and microstructure of ferritic steels, alloys and welds

The ferritic steels used world wide for nuclear pressure vessels and irradiated structures contain manganese but knowledge of the role of manganese is mainly confined to the unirradiated condition. Information from the mid 1950s to 2009 on the influence of manganese on unirradiated and irradiated properties is analysed. In unirradiated material manganese raises ambient strength, improves notched impact toughness, benefits high temperature strength, promotes dynamic strain aging and enhances creep resistance. However, by combining with carbon and by increasing phosphorus diffusion, manganese increases intergranular embrittlement after thermal aging. Neutron irradiated steels are deleteriously affected by manganese. First, the dose coefficients of irradiation hardening and embrittlement increase linearly with manganese, particularly in steels with uncombined nitrogen, and the morphology of irradiation induced interstitial loops is altered. Second, manganese occurs in irradiation induced Cu rich precipitates, altering their morphology and the associated hardening. Third, manganese becomes incorporated into irradiation enhanced Cu–Ni–Mn precipitates and Mn–Ni–Si nanoclusters that increase hardening and embrittlement at high doses. Finally, manganese segregates at grain boundaries under irradiation. Overall, the unirradiated benefits are outweighed by deleterious irradiation effects. These factors are important in supporting continued reactor operations, extending lifetimes of existing nuclear vessels and structures, improving steel specifications for advanced reactors and developing steels for nuclear fusion applications.