EVALUATION OF THREE-DIMENSIONAL MICROSTRUCTURAL EFFECTS ON DAMAGE EVOLUTION DURING VHCF LOADING OF A DUPLEX STAINLESS STEEL - MECHANISMS OF DAMAGE EVOLUTION AND CORRESPONDING FATIGUE LIFE

摘要

The present paper is an overview of recent investigations dealing with the fatigue damage in the VHCF regime of duplex stainless steel (DSS, 1.4462, German designation) and super duplex steel (SDSS, 1.4410). The fatigue damage mechanisms have been investigated under vacuum, air and corrosive atmosphere. For the cyclic loading of the fatigue specimens an ultrasonic fatigue testing machine with a testing frequency of f = 20000Hz and a stress ratio of R = -1 have been used. The application of light microscopy, thermography and scanning electron microscopy as in-situ techniques, allows the observation of the damage evolution on the electrolytically polished surface of the specimens during the fatigue tests. The investigations show a strong slip band formation within the austenitic phase for the DSS, which leads to crack initiation at the austenitic/ferritic phase boundaries. In comparison to that, crack initiation in SDSS occurs at slip bands within the ferritic phase. The fatigue life of the duplex stainless steels is strongly dominated by the barrier effect of the microstructure surrounding the initially plastified grains.