Comparison of low cycle fatigue of ductile cast irons with different matrix alloyed with nickel

Abstract

Total strain controlled tests have been performed on cylindrical specimens of austempered ductile cast iron (ADI) and ductile cast iron with the ferritic matrix, both alloyed with 2.75% Ni, with the aim to compare cyclic plasticity and the fatigue life at temperatures 23 °C and–45 °C. Microstructure and its features are documented using light microscope (image analysis) and neutron diffraction. In ADI the measurement of retained austenite by neutron diffraction before and after fatigue tests is used to discuss the cyclic stress-strain response. Cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves were obtained for both temperatures and materials. Cyclic stress-strain curve obtained at–45 °C is shifted to higher stress amplitudes relative to room temperature curve. The fatigue life curves at both temperatures can be well approximated by the Manson-Coffin and Basquin laws. The Manson-Coffin curves are identical for both materials and temperatures while the Basquin curve is shifted to longer fatigue life for ADI. The surface relief developed during fatigue was studied using SEM. Fatigue cracks initiated preferably from persistent slip markings at the interface of graphite nodule.