HIGH-CYCLE FATIGUE OF AUSTEMPERED DUCTILE IRONS IN VARIOUS-SIZED Y-BLOCK CASTINGS

High-cycle fatigue (HCF) properties of a number of different grades of austempered ductile irons (ADIs) were investigated. Ductile irons wer e prepared as Y-block castings with three section sizes (25, 50, and 1 00 mm in thickness). Fatigue specimens were selected from various loca tions within the castings, austenitized at 1173 K, and then austempere d at 573 and 633 K, respectively. HCF tests were conducted by rotary b ending method to generate the S-N curves and the associated fatigue li mits. As section size increased, the HCF strength of ADI was decreased and affected by the inferior graphite nodule morphology and microshri nkage porosity resulting from the slower solidification rate. The effe cts of section size, location within the heavy-section casting, and au stempering temperatures on the HCF strength of ADI are interpreted in terms of the graphite nodule morphology and microshrinkage pores. An e mpirical equation was introduced to correlate the fatigue limit of ADI with the impact toughness value and mean nodule diameter.