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Experimental study and modeling of IF steel oxidation process in air Ren, Xuzhan

Abstract

This thesis examines the oxidation of Interstitial-free steels in air by means of an experimental investigation and process modeling. The ultimate objective is to develop a computer model which could be employed to predict scale growth under industrial conditions. A purely theoretical approach to developing a model for industrial use is not appropriate because there are too many factors involved in the oxidation process and it would be difficult to make realistic predictions. The experiments were conducted under conditions as close as possible to industrial practice. With IF steels, two different scale growth mechanisms were found. Based on those different mechanisms, an innovative model for simulating heat transfer and predicting scale growth has been formulated and implemented on a personal computer. The model gives the best possible results in comparison to previous studies of predicting scale growth for the whole process and has the potential of being applied in the steel industry. For oxidation experiments, samples bigger than those used in the past were employed to reduce geometrical effects. Samples that are too small tend to give oxidation data that deviates greatly from the industrial situation due to geometric effects. Below 950 °C, adherent and dense scale is usually formed while above 950 °C, detached, laminated and porous scale accounts for most of the scale formed. The gap formed due to detachment reduces the scaling rate. So after gap formation, the scaling rates at 1000 °C and 1050 °C are lower than that at 950 °C. However the porosity and fissures of the scale increase the scaling rate. Temperature plays an even bigger role in oxidation. At temperatures higher than 1050 °C, the scaling rates are higher than that at 950 °C in spite of the detachment of the scale formed. These three factors compete with each other. The primary isothermal oxidation data is very different from the secondary isothermal oxidation data. This difference is very important in developing a practical model. The microstructural aspects of the scale formed were also comprehensively examined. For modeling, the basic idea is to use a whole set of isothermal oxidation data to simulate the oxidation process for any given temperature history. However if a sample goes through two different oxidation mechanisms, the primary isothermal oxidation data cannot be used for the secondary mechanism. With other details taken into account, the scale growth model gives results in good agreement with the experimental measurements.

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