Effect of graphite morphologies on the tribological behavior of austempered cast iron

Three different cast irons, gray, nodular and compact iron, with different chemical compositions were subjected to austempering to investigate the effects of graphite morphology, composition and austempering on tribological behavior. The austempering process was carried out at 375 °C for different periods to achieve optimum mechanical properties. The austempered cast iron specimens with optimum mechanical properties were used for wear tests. For comparison, wear tests were also carried out on a pearlitic gray iron specimen made from railway brake shoes. Sliding wear tests were performed in a block-on-ring wear tester with the test materials rubbing under dry atmospheric condition against a ring made from a railway wheel at speeds of 250 and 950 rpm with nominal stress of 2 MPa. A non-contacting capacitance transducer with a computer interface was used to measure the coefficient of friction of the specimens. The impact test results show that austempered nodular iron with 105 J/cm2 has the highest impact energy while austempered gray iron with 7–9 J/cm2 shows the lowest. The optimum impact energy of the austempered compact cast iron was 14–22 J/cm2 as a function of chemical composition. The wear test results indicate that austempered cast iron shows superior wear resistance than pearlitic gray iron, particularly at the lower speed. For austempered cast iron specimens, compact iron shows the highest wear resistance while gray and nodular cast iron have lower wear resistance.