Non-destructive characterization of thermal ageing of the Cu-rich WB 36 steel 15 NiCuMoNb 5 (1.6368)

The phenomenon of precipitation hardening in steels is usually associated with carbon and nitrogen additions which precipitate out under heat treatment or specific service conditions as carbides or carbo-nitrides. In some alloys, however, for instance in Western nuclear pressure vessel steels, small additions of copper can be the driving force for embrittlement and hardening. In the case of the steel grade WB 36 (15 NiCuMoNb 5, 1.6368), which is in service in fossil power plants and German nuclear power plants, the effects of copper precipitation are even larger. The progress of precipitation can be characterized by Vickers hardness measurements which, however, cannot be applied in an area-wide manner under practical circumstances. A demand for the development of equivalent NDT techniques, therefore, arises. To this end, Fraunhofer IZFP has adapted and developed the so-called 3MA approach which combines several micro-magnetic testing quantities for the characterization of mechanical properties. This paper presents and discusses recent results obtained using the 3MA approach applied to characterizing the ageing response of steel WB36.