CONSIDERATIONS ON THE THERMAL-BEHAVIOR AND OXIDATION RESISTANCE OF CAST-IRON

Thermal analysis was used to characterize the thermal behaviour and ox idation resistance of some nodular cast irons. Samples of nodular cast iron in various stages of elaboration, with different chemical compos itions, were studied. The samples were heated in air, in the temperatu re range 291-1273 K, and the thermal (TG, DTG and DTA) curves were rec orded. A group of samples with low silicon content exhibit similar beh aviour: a continuous increase in mass and an exothermic effect up to 1 123 K. The thermal effects correspond to iron oxide (Fe3O4, FeO, Fe2O3 ) formation. At high temperatures (T>1123 K), there is a decrease in m ass and an endothermic effect. A decrease in the superficial carbon co ntent by combustion ('decarburization' effect) occurs in the range of high temperatures. The two effects of oxidation and decarburization de pend on the structural changes which occur in cast iron at high temper atures. The decarburization process was modelled and the kinetic param eters were determined (reaction order n=0.76; activation energy E=141 kJ mol(-1); pre-exponential factor A=2.10(2) s(-1)). The oxidation pro cess was studied by non-isothermal methods with regard to two mechanis ms: two-dimensional transport for low temperatures, and three-dimensio nal transport through a sphere for high temperatures. The activation e nergies were calculated: 68 kJ mol(-1) for low temperatures and 122 kJ mol(-1) for high temperatures.