Life-time prediction of austenitic stainless steel by applying magnetic NDT-method

The detection of material degradation prior to technical crack initiation of safety-relevant components, like piping in the primary circuit of nuclear power plants, is an ongoing research topic. Since in some metastable austenitic stainless steels (paramagnetic), fatigue is accompanied by a transformation of the face centred cubic phase (γ-fcc) into the body centred cubic martensitic (ferromagnetic) phase (α'-bcc) with a slight distortion to a tetragonal shape, the corresponding changes of magnetic properties can be used as an indication for the remaining life time of in-service components. However, the affinity for such phase transitions depends on several parameters e.g. temperature, chemical composition of the material, heat treatment, state of cold working and loading conditions. These influencing parameters have to be analysed carefully in order to allow a reliable prediction of the remaining operation-time of safety relevant components. In this paper we compare different methods for the detection of martensitic contents concerning their performance and potential for the application in laboratory and plant environment. These are: quantitative evaluation of martensite by neutron diffraction as a calibration method, measurement of the eddy current impedance by means of giant magneto-resistance sensors (GMR), magnetic remanence field measurement by fluxgate sensors and the determination of magnetic permeability using a Ferromaster. Furthermore, the influence of the chemical composition, heat treatment, operation temperature and applied load on the affinity for martensitic transformations is discussed. It is shown, that all the investigated magnetic measuring methods are able to detect the amount of martensite in fatigue specimens. However, their application requires the knowledge of material specific calibration curves and is limited by the low affinity for martensitic transformation of some austenitic stainless steels. The results presented in this paper contribute to failure analyses of austenitic stainless steel components. For fatigue relevant components, an increase of the martensitic phase content is an indication for a certain accumulated plastic deformation. A screening criterion for the volume fraction of martensite for the given material and loading conditions can be used to avoid technical cracks caused by thermo-mechanical fatigue. (authors)