Nondestructive characterization of materials (ultrasonic and micromagnetic techniques) for strength and toughness prediction and the detection of early creep damage

In recent years, nondestructive testing techniques for materials characterization have been developed in Germany under the sponsorship of the Ministry of Research and Development, as part of the Reactor Safety Research Programme, in order to provide techniques for PSI and ISI that are sensitive and reliable, in particular with respect to the prediction of strength and toughness. As ferritic steels (pressure vessels and pipelines in the primary circuit) are of special interest, R&D was concentrated on micromagnetic techniques which are sensitive to the microstructure and its changes under service and/or repair conditions. In order to characterize microstructural states superimposed by residual stresses in an unambiguous way, numerical modelling was applied using advanded tools of mathematical approximation theory, i.e. multiregression algorithms and neural networks. For the detection of early creep damage in fossil power plant applications, i.e. micropores and their subsequent development to linked pores and microcracks, besides the micromagnetic techniques an ultrasonic techniques was also applied and optimized for in situ applications on components such as pipe bends. Whereas the ultrasonic technique is sensitive to pore concentrations as small as about 0.2%, the parameters of the micromagnetic techniques are mainly influenced by temperature- and load-induced microstructural changes occurring in service, dependent on the steel quality. The techniques are applied at two pipe bends (steel grades 14 MoV 6 3 and X 20 CrMoV 12 1) loaded under near practical conditions during seven inspection intervals between 2048 h and 21 000 h to evaluate the progress of damage.