Nondestructive characterization of ageing phenomena in heat resistant steels by means of micromagnetic techniques

Microstructural changes during thermal ageing of ferromagnetic materials can be detected in a rapid and nondestructive way by means of micromagnetic methods. The Fraunhofer Institute for Nondestructive Testing (IZFP) proves the suitability of those techniques for the characterization of microstructural changes caused by thermal ageing (at different times and temperatures) of the different heat resistant ferritic, ferritic/martensitic steels and iron-based alloys. Differing microstructure features and subsequent thermal ageing of high chromium ferritic steels produces populations of strengthening Laves phase precipitates, largely deviating from each other in terms of particle number, size distribution and inter-particle spacing and thus differing mechanical properties. The effect of different thermal ageing procedures on the microstructure evolution was characterized by means of micromagnetic techniques. Thermal aged and creep damaged 9-12Cr ferritic/martensitic steelspecimens were investigated by means of micromagnetic techniques in order to characterize the microstructure changes due to chromium rich M23C6 carbides, MX-type precipitates, Laves phases and modified Z-phases and changes of dislocation density. In case of heat resistant Cu-containing ferritic/bainitic steels and Cu-Ni-Mn alloyed iron based materials nanoscale Cu-precipitates and the corresponding residual micro stresses play a dominant role in the ageing process. By means of micromagnetic techniques the characterization of the microstructural changes induced by the thermal ageing was successfully performed. The nucleation and growth of precipitates at different temperatures in case of the thermally aged states could be pursued by means of micromagnetic techniques. Furthermore the world-wide first experimental method for measuring residual micro stresses of III order induced by nanoscale precipitates during thermal ageing was developed, tested and patented at Fraunhofer IZFP.