Abstract
The main problems on the oxidation of cast iron are the behavior of carbon and its influence to metal oxidation, and yet, in many cases, the decarburization accompanied by the oxidation has been ignored or not to be quantitatively measured. Therefore, one has used to measure the oxidation resistance by the weight gain in the oxidation procedure, but strictly it is not true.
For a complete understanding of this subject, it is necessary to measure simultaneously the weight of oxygen that reacts to metal constituents and that of carbon which escapes out from a system by decarburization. One can not evaluate the former before he corrects the weight gain with the latter. The purpose of the present work is to measure the both values and estimate qulitatively the influence of decarburization to the metal oxidation.
The metal oxidation and decarburization rates of cast iron (3%C, 2%Si) have been measured over 700∼1,000°C for 5 hours in (O2+Ar) mixtures at a total pressure of 1.0 atm. The rates are not given by any simple rate law, because many oxidation phenomena proceed at the same time, and presumably the main reaction changes with exposure time.
In order to clarify the behavior of the oxidation, the ratio “Weight of decarburized carbon/Weight of oxygen consumed for metal oxidation” has been introduced. This means either preferential oxidation of carbon or metal constituents, and at the critical value the oxidation proceeds holding its initial chemical composition ratio. They are illustrated as functions of oxidation temperature in Fig. 1. In the case of flaky graphite cast iron in pure oxygen atmosphere, these ratios are twice or more times as large as the critical value, and these ratios increase very rapidly with lowering the oxidation temperature. In (0.2O2+0.8Ar) atmospsphere at 900°C this about 0.5 and far larger than in pure oxygen atmosphere. On the contrary in the case of spheroidal graphite cast iron, these ratios are scarcely larger than the critical value, and its temperature dependancy is small and different from the former one.
These results are mainly explained by the different shape and distribution of graphite and the differences of the temperature and oxygen partial pressure dependancies of oxidation between graphite and metal constituents. Further, the influence of decarburization are discussed in view of the two effects cancelled each other : (1) the decrease of practical diffusion area or Fe++ ion and (2) the breakdown of protective oxide scale.
