In this study, thermal analysis has been used to characterize thermal behavior and oxidation resistance of some nodular cast iron. One has studied samples of nodular cast iron, in several stages of elaboration, with different chemical compositions. The samples have been heated in air, in the temperature range: 18-1000 °C, and the thermal (TG, DTG, and DTA) curves have been recorded. A group of samples with low silicon content shows similar behavior: a continuous increase of weight, some peaks, and exothermal effect up to 850 °C. The thermal peaks correspond to iron oxides (Fe304, FeO, Fe203) formation. At high temperatures (T>850 °C), one can see a decrease of weight and an endothermal effect. A superficial decrease of carbon content by combustion ("decarburization" effect) takes place in the range of high temperatures. The two effects: oxidation-decarburization, depend on the structural changes of cast iron, wich takes place at high temperatures. The decarburization process has been modeled, and the kinetical parameters have been determined (reaction order n=0.76; activation energy E=l41.5 kJ). Kinetical study of the oxidation process has been achieved by nonisothermal methods using two mechanisms: bidimensional transport -for low temperatures and three-dimensional transport through a sphere for high temperatures. The activation energies have been calculated: 67.7 kJ for low temperatures and 122.5 kJ for high temperatures.
CHARACTERIZATION OF SOME NODULAR CAST IRONS BY THERMAL ANALYSIS
