High-temperature oxidation tests were conducted on three commercial carbon steels in simulated combustion environments of natural gas with 99?XOtheoretical air at a total pressure of 1atm. It was found that 0.2 and 40 ppm sulfur contamination in the environments tended to accelerate the rate of scaling at 1200°C,but slowed it at 1100°C.At 11OO°C,the sulfur was thought to adsorb on the scale surface, blocking the surface mobility and hence partly blocking the access of oxidant molecules to the surface. However at 1200°C, it was apparent that the retarding effect of the adsorbed sulfur on the scale surface was lost, and instead surface nobilities were enhanced. The oxidation kinetics and mechanisms for the observed results are discussed.
INTRODUCTION
The hot-rolling of steels during secondary processing is typically preceded by reheating the steel slabs, billets or blooms in direct-fired furnaces to temperatures between 1100 and 1250°C. Large quantities of scale are formed during reheating as a result of reaction between the base iron and gaseous oxidants. Excessive scaling is undesirable because it increases production costs due to high yield losses. According to Obaro (1), yield loss during slab reheating can be up to 4%; although more typical levels are 1.5?2%.Factors affecting the extent of scaling during reheating include: heat-up and residence times in the furnace temperature and atmosphere; and steel composition.
The fuels most commonly used in reheat furnaces are coke ovens gas, blast furnace gas, and natural gas. Patient combustion would be obtained by mixing and burning a given quantity of feel with the exact amount of air required. The quantity of air consumed in a perfect combustion process is often termed stoichiometric air and is commonly presented as a percentage (i.e., 100% air). The combustion of natural gas (CH4)with 100% dry air can be represented as follows:
CH4+ 2(O2+ 3.776N2) = C02 + 2H20 + 7.552Nz [1]
The stoichiometric fuel/air mass ratio for this reaction is about 0.06. Equilibrium combustion with stoichiometric air, as well as sub-stoichiometric air (i.e.,<10OVOair), yields only a small amount of O2 resulting from the equilibrium dissociation of the C02 and H20 product gases. In practice, however, complete combustion of the fuel is often ensured by operating the reheat furnace at 103?112V0of the theoretical air level required to burn the fuel completely. Combustion with such hyper-stoichiometric air levels yields O2 in addition to the standard product gases shown in Esq. [1]. Lee et al. (2) recently showed that carbon steels oxidize according to rapid parabolic kinetics in a 112% air environment, but oxidize according to much slower linear kinetics in 99% and 95% air.
The purpose of this study was to (i) further analyze the isothermal scaling behavior of three commercial carbon steels in simulated combustion environments of natural gas with 99°/0theoretical air at a total pressure of 1atm, and (ii) determine the effect of 0.2 and 40 ppm sulfur contamination in the 99% air environments on the oxidation behavior of the steels. The oxidation temperatures studied were 1100and 1200°C.
