Initial microstructure effect on surface relief and fatigue crack initiation in a high strength ferritic-martensitic steel

M. C. MARINELLI; M. N. BATISTA; I. ALVAREZ-ARMAS

Darmstadt

Congreso; Materials Science Engineering European Congress and Exhibition on Advanced Materials and Processes; 2018

DGM / German Materials Society

This work is focused on the effect of the initial tempered-lath microstructure on the surface relief and nucleation of microstructural fatigue cracks developed during cyclic training of the ferritic-martensitic AISI 410 steel. Transmission and scanning electron microscopy (TEM and SEM) as well as electron back-scattered diffraction (EBSD) were used to study the surface-damage evolution in smooth, cylindrical, notched specimens. We focus on going beyond the characterization of cyclic behavior, microstructure development and microcrack nucleation by means of analyzing EBSD results in order to broaden the understanding of the fatigue processes. Previous work showed that low-cycle fatigue (LCF) of 9-12%Cr ferritic-martensitic steels produce cycling softening behavior, due to microstructural instability over a wide range of temperatures.During in situ observations of the notched area of a specimen subjected to LCF, it is observed that the first deformation lines appear relatively soon and they lie mainly along lath boundaries orientated at about 45o with respect to the tensile axis (vertical axis). After a certain number of cycles, the lines intensified and turned into bands. Finally, cracks nucleated at these slip marks. To our knowledge, there are very few reported studies dealing with the application of the EBSD technique to describe the initiation of microstructurally short fatigue cracks in the initial microstructure of ferritic-martensitic stainless steels. The present study introduces an EBSD analysis that gives an additional insight into the initiation behavior of microstructurally short fatigue cracks in an initial tempered-lath microstructure of martensitic AISI 410 steel. The EBSD technique has been demonstrated to be a very powerful tool for the identification of areas of concentrated strain within the microstructure. The main conclusions obtained from the present work indicate that microcracks nucleate principally at high angle boundaries such as block boundaries. Though, the formation of microcracks has been observed in zones were the {112} systems reorient to a direction more favorable for slip. It was found that different Taylor factors between adjacent microstructural subunits (laths or blocks) and a high Schmid factor produce strain incompatibility during cycling and the consequent nucleation of microcracks. Finally, the accumulated cyclic strain produces the reorientation of microstructural subunits, such as lath and blocks, thus increasing the number of high-angle boundaries.