The effect of copper additions to the mechanical properties of austempered ductile iron (ADI)

Previous unpublished studies into the wear behaviour of austempered ductile irons have shown that under sliding conditions, the mechanism is one of the initial delaminations followed by so generated third body wear. Microstructural examination indicated that the sub-surface crack propagation (which lead to the delamination) was initiating at graphite spheroids in the ADI. This is similar to that type of behaviour experienced in high-speed steels which when tested in bending, exhibit sub-critical carbide fracture prior to cracks linking up before catastrophic failure. This paper describes the effect on the mechanical properties of elemental copper additions (above the levels of solid solubility), to a commercial ADI composition. Strength, toughness and hardness are all measured for a range of heat-treatment parameters. Microstructural studies are used to determine the distribution of the copper. It is proposed that the precipitation of elemental copper at the levels described does not occur at the surface of the graphite particles (as observed elsewhere). In this respect the copper cannot act to relieve any stress-induced micro-cracks in the graphite nodules, with consequent inhibition of crack growth at lower stress levels.