High-Cycle Fatigue of Austempered Ductile Irons in Various-Sized Y-Block Castings

Abstract

The relationship between low-cycle fatigue (LCF) strength of austempered ductile iron (ADI) and cast section size and location was investigated. Uniaxial LCF tests under strain-control were conducted on a number of different grades of ADIs. These ADIs were selected from four positions in Y-block castings with three section sizes (25, 50, and 100 mm in thickness). LCF specimens were cut from specific locations within the castings, austenitized at 1173 K, and then austempered at 573 and 633 K, respectively. Results indicated that LCF strength of ADI degraded with increasing section size due to deteriorated graphite nodule morphology and presence of more microshrinkage pores as a result of the slower solidification rate. The effects of section size, location within the heavy-section casting, and austempering temperatures on the LCF strength of ADI are discussed in terms of the graphite nodule morphology and microshrinkage pores. Fractography with scanning electron microscopy (SEM) was applied to determine the LCF failure mechanisms and fatigue crack propagation modes.