CONTROLLED GRAPHITIZATION AS A POTENTIAL OPTION FOR IMPROVING WEAR-RESISTANCE OF UNALLOYED WHITE IRONS

Effect of heat treatment on the microstructure and resistance to abras ive wear has been studied in an unalloyed white iron used for manufact uring cylindrical pebbles used as grinding media by the cement and oth er industries. Heat treatment comprised holding at 800 degrees C, 850 degrees C, 900 degrees C, and 950 degrees C for 30, 60, 90, 120, and 1 80 minutes followed by oil quenching. Heat treatment in general improv ed the wear resistance over that in the as-cast las-received) state. T he extent of maximum improvement differed with temperature and in the decreasing order occurred at(1) 180 minutes, 800 degrees C, OQ; (2) 30 minutes, 950 degrees C, OQ; (3) 90 minutes, 900 degrees C, OQ; and (4 ) 180 minutes, 850 degrees C, OQ. From the point of view of commercial application, the heat treatment at (2) is most favored. Microstructur al changes occurring during heat treating comprised (1) changes in mat rix microstructure; (2) a reduction in volume fraction of massive carb ides due to its part graphitization/destabilization; and (3) changes i n graphite morphology, size, and distribution. Amongst the aforesaid c hanges, graphitization has emerged as the key parameter in improving w ear resistance. Graphite morphology in a near-nodular form of optimum size and distribution was found to be most effective. Upon increasing the heat-treating temperature, the tendency of nodules to develop spik es increased. Similarly, interlinking of graphite flakes was also obse rved. These features and the possible formation of free ferrite advers ely affected wear resistance. The role of other beneficial changes in the microstructure, e.g., globularization of carbides, possible retent ion of austenite, and formation of optimum volume fraction of martensi te, have been duly considered while optimizing microstructure(s). The key feature of the present study is that, despite its fundamental sign ificance, it has a well-focused application potential.