Influence of graphite morphology and matrix structure on fatigue strength and wear resistance of ductile cast iron
By Khaled M. Ibrahim and Ahmed El-Sawy
Unalloyed ductile iron, fatigue strength, wear behaviour, forging, austempering heat treatment, metallographic investigations, graphite morphology, fractography, mechanical properties
The effect of graphite morphology and matrix structure on both fatigue strength and wear behaviour of the unalloyed ductile iron (DI) was studied. The graphite morphology has been changed from spheroidal as in the as-cast condition to ellipsoidal shape after applying a forging process. The matrix structure changed from pearlite-ferrite as in the as-cast and forged condi-tions to ausferritic (i.e. austenite and acicular ferrite) by applying austempering heat treatment at 400 °C for 1 h. Optimum mechanical properties were achieved for the austempered ductile iron (ADI), where it obtained an ultimate strength of 1060 MPa and hardness value of 390 HB, whereas the lowest tensile strength and hardness of 620 MPa and 236 HB, respectively, were reported for the as-cast DI. ADI also showed optimum values of fatigue strength and wear resistance compared to the forged DI that obtained the lowest fatigue strength due to the harmful effect of graphite ends that facilitate crack initiation and propagation into the matrix. Forged DI showed relatively better wear resistance than the as-cast DI due to the matrix strength-ening effect observed after forging. Hence, the mechanical properties of DI can be enhanced by applying the austempering heat treatment process rather than the forging process.
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