Effect of heat treatment and precipitation state on toughness of heavy section Mn-Mo-Ni-steel for nuclear power plants components

With regard to the mechanical properties of heavy forgings, made from steel grade 508 ASTM Class 3 (20 MnMoNi 5 5) the influence of the cooling intensity during hardening was investigated and the possibilities and limits of accelerating the cooling rate were studied. The importance of the transformation heat is discussed. It can be concluded from this, that it is impossible to avoid the formation of upper bainite as a typical microstructure of heavy forgings from this steel type. To obtain high impact energy the annealing condition must be conform with this type of microstructure. The reason for impact decrease by over-annealing is related to Mo2C precipitation in the bainitic ferrite. Finally, experiences with intercritical heat treatment of this steel type are described and the special behaviour of the microstructure obtained is demonstrated. (orig.)