Effect of initial microstructure on surface relief and fatigue crack initiation in AISI 410 ferritic‐martensitic steel

Abstract This work is focused on the effect of the initial tempered‐lath microstructure on the surface relief and nucleation of microstructurally short fatigue cracks developed during low‐cycle fatigue tests of the ferritic‐martensitic AISI 410 steel. Transmission and scanning electron microscopy as well as electron back‐scattered diffraction were used to study the surface‐damage evolution in smooth, cylindrical, notched specimens. Results from the electron back‐scattered diffraction analysis broaden the understanding of the processes of nucleation of microstructurally short fatigue cracks in the initial tempered‐lath microstructure of AISI 410 ferritic‐martensitic steels. Results prove that during fatigue, microcracks nucleate mainly at high‐angle boundaries represented by block subunit interfaces formed in this tempered microstructure. Besides, the progress of fatigue cycling causes the reorientation of the {112} <111> systems to a direction more favourable to slip, giving rise to the formation of extrusions within the blocks and consequently the formation of microcracks.