Fatigue limit and microstructure in lamellar graphite iron

Demanding environmental legislation and improve performance specifications requires increasing fatigue strength for the engine components that are made of lamellar graphite iron (LGI). Components design, metallurgy and casting conditions define the microstructure formation and mechanical properties. The graphite inclusions embedded in the metallic matrix acting as defects and have a detrimental effect on the fatigue strength of LGI. The cooling conditions determine the coarseness of the microstructure and also have, a great impact on the fatigue resistance. The experimental material was an LGI alloy produced with three different solidification times. A fully reversed fatigue test was performed, and various microstructure features were quantitatively estimated by utilizing the Gumbel's statistics of extremes. The stereological parameter of Hydraulic Diameter of the Inter-dendritic Phase and the graphite Feret size found to be the most suitable microstructure parameters to be correlated with the fatigue limit. The results also indicate the sizeable effect of the solidification time on the fatigue limit. Several other microstructure features that have been reported to influence the tensile strength were also found to be related to the fatigue limit. The obtained endurance ratio ranges from 0.25 to 0.30, a value that is in line with previous investigations.