Characterization of SG Iron

Abstract

Present investigation is carried out to determine the relation between microstructural aspects and wear properties of spheroidal graphite (SG) cast iron material to be used for production of automotive parts such as gears, camshafts and cylinder pistons. SG cast iron due to its better damping capacity, strength to weight ratio, elongation and tribological properties is the most preferable and widely used material among all type of cast irons. The considerable amount of ductility achieved in SG cast iron is due to the spherical form of graphite rather than flakes as present in grey cast iron which eliminates the stress concentration effect when subjected to different loading conditions and for which it is preferred in automotive parts manufacturing. In order to determine the optimum wear resistance of SG cast iron, the current investigation involves annealing, normalizing, quench & tempering and austempering heat treatments leading to transformation of as-cast matrix to ferritic, pearlitic, tempered martensitic and coarse upper bainitic matrices respectively. Specimens with varying alloying elements were austenitized to 1000°C for 90 minutes, followed by furnace cooling, air cooling, oil and salt bath quenching followed by air cooling for respective heat treatments. Vickers hardness test (at 20Kg applied load) & dry sliding wear test were performed under 20N, 40N & 60N for a distance of 7.54m using Vickers hardness tester & ball-on-plate wear tester respectively. The quench tempered specimen for alloy SG-02 has maximum hardness whereas in case of alloy SG-01 normalized specimen has maximum hardness, the first one has tempered martensitic and second one has pearlitic matrix. The lowest hardness was obtained for annealed specimen having ferritic matrix for both the alloys, whereas the hardness values of other specimen were in between these two for respective alloys. There was not much difference in hardness of ferritic, pearlitic and bainitic matrices, but marginal difference was observed in as-cast and tempered martensitic matrices. The reason for this difference is attributed to the fact that in as-cast condition alloy SG-01(higher hardness) has bull’s eye ferritic/pearlitic matrix, whereas that of alloy SG-02 is fully ferritic resulting lower hardness. For quench & tempered as well as austempered specimens alloy SG-01 has lower hardness value than alloy SG-02 because of presence of more C, Mn, Ni, Cr, Mo and Si. These elements may cause strengthening of solid solution of ferrite in ductile cast iron. When operating under 20N load weight gain was observed in every specimen due to the formation of oxides over the worn surface, whereas no such phenomenon was observed for 40N and 60N. The major wear mechanism observed in softer matrix phases were adhesive type signified by delamination layers over graphite nodules while in the harder phase wear rate decreases because of plastic deformation.