OXIDATION OF LOW-CARBON STEEL IN MULTICOMPONENT GASES .2. REACTION-MECHANISMS DURING REHEATING

Oxidation behavior of low carbon steel during reheating in an industri al walking-beam steel reheat furnace was investigated. It was observed that scaling (oxidation) rates were reduced by reducing the input air /fuel ratio to the furnace, thereby lowering concentrations of free ox ygen in the combustion products from about 3 to 1.5 pct. Laboratory ex periments involving isothermal and nonisothermal oxidation were carrie d out in atmospheres consisting of oxygen, carbon dioxide, water vapor , and nitrogen. A general equation for the prediction of weight gains due to oxidation during reheating, using isothermal oxidation rate con stants, was developed. The prediction of weight gains from nonisotherm al oxidation conducted in the laboratory was poor, owing to a separati on of the scale from the metal substrate which took place at about 900 degrees C. The predicted weight gains during reheating in the industr ial reheat furnace indicated that oxidation rates during reheating wer e intermediate between linear and parabolic, especially during-reheati ng with high air/fuel ratio. However, the linear mechanism predominate d. Laboratory isothermal experiments for oxidation in atmospheres cont aining free oxygen showed that the magnitude of the linear oxidation r ates was determined by the oxygen concentration in the atmosphere. It was concluded that the observed reduction in scaling rates during rehe ating of low carbon steel in the industrial reheat furnace was a resul t of the lower free oxygen level in the furnace atmosphere.