Efficiency of grain and phase boundaries as microstructural barriers during hcf and vhcf loading of austenitic-ferritic duplex steel

U. KRUPP; A. GIERTLER; M. C. MARINELLI; H. KNOBBE; H-J CHRIST; P. KÖSTER; C-P FRITZEN; S. HEREÑÚ; I. ALVAREZ-ARMAS

Berlin

Conferencia; 5th International Conference on very high cycle Fatigue June 28-30, 2011. pp. 127-132 Berlin Germany; 2011

German Association for Materials Research and Testing (DVM)

HCF and VHCF studies on austenitic ferritic duplex steel specimens in two different heat treatment conditions have revealed that even beyond 108 cycles local plasticity causes surface modifications in form of pronounced slip bands that are extending with an increasing number of cycles. By means of scanning electron microscopy (SEM) in combination with electron back-scatter diffraction (EBSD) being applied after certain intervals during fatigue loading of the specimens, the appearance of the slip bands was correlated with the crystallographic misorientation distribution and the phase arrangement within the duplex microstructure. In general, plastic deformation of the austenite is present for all stress amplitudes of technical relevance, manifesting itself by the generation of heat. Therefore, the existence of a fatigue limit in duplex steel is believed to depend quantitatively on (i) the efficiency of the austenite-ferrite phase boundaries, and (ii) the free slip length within austenite phase patches. The experimental observations have been served as input for the development of short crack modelling concepts using both a finite-element and a boundary element approach, which are outlined and discussed in the present paper