Tribological Behavior of Heat Treated Spheroidal Graphite Cast Iron

Abstract

The main aim of this project is to investigate the microstructures of different matrix samples and correlate to the wear system response by varying matrix microstructure. SG iron is used for the production of different automobile parts such as camshaft and gears. SG iron is most important member among different members of cast iron family due to its high strength, toughness together with ductility. The main reason of this high ductility is presence of nodular structure of graphite that is present in it unlike present in grey cast iron as the flakes. In grey cast iron, stress concentration is high at the corners of these flakes but has been removed in the case of spheroidal graphite cast iron with the nodular shape. As a result of this its ductility increases along with toughness. In order to obtain 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 having different alloying elements were austenitized to 1000°C for soaking time 90 min, followed by furnace cooling, air cooling, oil and salt bath quenching followed by air cooling for respective heat treatments. Vickers hardness test was conducted over the samples under 20Kg followed by wear test was carried over DUCOM TR-208 M1 ball on plate type wear monitor under 10N, 30N and 50N loads respectively in dry sliding condition for the sliding distance of 7.54m with the speed 0.063m/s. The hardness was maximum for quenched & tempered specimen in case of SG-02 samples due to presence of tempered martensitic matrix phase while, in case of SG-01, normalized specimen has maximum hardness due to presence of pearlitic matrix phase. The lowest hardness was observed to be for annealed sample with ferritic matrix phase. On the other hand hardness of ferritic, pearlitic and bainitic matrices was observe to be in between these range, also marginal difference was observed in as-cast and tempered martensitic matrices. The cause of this difference is that in as-cast specimen of iv SG-01 has bull’s eye ferritic/pearlitic matrix which may cause its hardness increases, whereas in case of as-cast sample of SG-02 fully ferritic phase is obtained as a result of which hardness decreases. For quench & tempered as well as austempered specimens of SG-01 has lower hardness value in comparison of that of in SG-02 because of more percentage of C, Mn, Ni, Cr, Mo and Si in SG-02 specimens that has tendency of strengthening solid solution of ferrite in ductile cast iron. It was observe that when sample of as-cast specimen subjected to load in between 10N- 20N it lost its weight continuously, while with increase in load to 30N subsequently weight gain was observe. This weight gain is due to the formation of oxide layer. There was no weight loss or gain observe for normalized specimen however in case of austempered specimen weight gain was observe for 20N and 30N due to formation of oxide which can be investigated by means of EDAX analysis. Finally it has been concluded that for the samples having ferritic, pearlitic, bainitic matrices phase, adhesive wear is main wear mechanism along with shallow pits are produced due to delamination and deposited over the sites where graphite nodules have already formed before.