Automatic control of the carbon potential of furnace atmospheres without adding enriched gas
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INTRODUCTION
THE carbon potential control of furnace atmosphere is rather important in the heat treatment of steel, because the carbon potential of atmosphere has intimate relationships with carbon distribution, surface hardness, and hardened depth of the steel treated under the atmosphere. Three methods are currently used in industry for controlling the carbon potential of furnace atmospheres: t~-4~ (1) controlling carbon potential by controlling the atmosphere's 02 partial pressure, which is monitored by using an oxygen sensor; (2) controlling carbon potential through controlling the atmosphere's CO2 content, which is measured by using an infrared analyzer; and (3) controlling carbon potential by controlling the atmosphere's H20 content, which is detected by using a dew cell. The controlling principle for any of these three methods is the addition of an enriched gas (CH4 o r C3H8) to the atmosphere for the sake of regulating the level of 02 partial pressure, CO2 content, or H20 content of the atmosphere. The carbon potential of the atmosphere is basically controlled by controlling the amount of enriched gas being added. This principle is applied to control the carbon potential of atmospheres generally being used in industry, such as endothermic gas, exothermic gas, methonal dissociated gas, nitrogen-based atmosphere, etc. Water-gas atmosphere is one of the furnace atmospheres developed by National Taiwan University in recent years, t~-~~ The components of water-gas atmosphere are similar to those of an endothermic gas, namely, N2, CO, H2, CO2, H20, CH4, and minute amounts of 02. The carbon potential of water-gas atmosphere can indeed be controlled by adding proper amounts of enriched gas. However, adding enriched gas to water-gas atmosphere will raise the cost of the atmosphere. Solving this problem with a different method has therefore been the attempt of this study. The carbon YONG-CHWANG CHEN, Associate Professor, is with the Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan. Manuscript submitted November 10, 1992. METALLURGICAL TRANSACTIONS B
potential of water-gas atmosphere has been controlled by controlling the generation condition of the atmosphere instead of adding enriched gas to the atmosphere. Can the carbon potential be effectively controlled by using this method? No investigation on this point has ever been reported. Water-gas atmosphere is generated by reacting an N2 + H20 mixture with hot charcoal. The temperature of charcoal will consequently affect the chemical composition and carbon potential of the atmosphere. According to a previous investigation, t6J the higher the charcoal temperature, the higher the carbon potential of the atmosphere. However, the carbon potential would not remain at a steady level if the charcoal temperature was controlled at a specified value by using a temperature controller. This is because charcoal will be gradually consumed during the generation of water-gas atmosphere. Moreover, the gas velocity through the charcoal layer will al
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