Gas carburizing of steel with furnace atmospheres formed In Situ from propane and air: Part III. Control of furnace atmo
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IN a previous paper t it was concluded that automatic control of the air-hydrocarbon ratio would be necessary for commercial carburizing operations using air and propane introduced directly into a carburizing furnace. The reason is that very low flow rates of propane and air are needed to achieve a thorough reaction of the gases at relatively low temperatures (about 850 ~ as a consequence, air entering the furnace when the load is charged or when negative vestibule pressures are produced during quenching, cannot be purged from the furnace in any reasonable length of time. The only practical approach is to offset the effect of entrained air by supplying a rich gas mixture to the furnace without significantly increasing the total flow rate of atmosphere gases. Control of both air and propane flow rates into the furnace, adjusting the flows to maintain a constant flow of reacted gas but a variable air-propane ratio, is desirable. However, if just the propane flow is controlled while the air flow is constant, the variation in reacted flow rate for a considerable range of airpropane ratios is small, Table I. The reacted flow rates in Table I are realistic for "steady-state" control conditions, e.g., maintaining a set-point in a thoroughly-purged furnace when carbon demand varies with time. Under transient conditions, when the control system is attempting to offset a considerable quantity of air present in the furnace, the reacted gas flow rates might be as much as twice as great as those listed in Table I for the same inlet air flow. It is difficult to make a realistic estimate of the transient reacted gas flow under these conditions. In our earlier work ~it was shown, that provided that gas flow rates were low, the composition of the furnace gases approached the equilibrium composition. In particular, the CO 2 content of the furnace gases varied systematically with air-propane ratio (A/P), suggesting that CO 2 content could be monitored for control purposes, just as it is with endothermic gas-base carburizing atmospheres. Subsequent experimentation with a C. A. STICKELS, C. M. MACK, and J. A. PIEPRZAK are with the Engineering and Research Staff, Ford Motor Co., Dearborn, MI 48121. Manuscript submitted January 31, 1980.
zirconia oxygen sensor showed that the atmospheric oxygen potential also varied systematically with A / P . In order to avoid the time lag inherent in gas sampling when infrared CO2 analysis is used, a zirconia oxygen sensor inserted directly into the carburizing furnace was chosen for the atmosphere control in these experiments. EXPERIMENTAL PROCEDURE The same furnace was used in these experiments as in previous work. 1,2The control system, shown schematically in Fig. 1, consisted of three main elements: 1) A Zirconia Oxygen Sensor, manufactured by Kent Instruments, Ltd., designed for use in reducing atmospheres. Ambient air is used as a reference gas. 2) A Honeywell Electr-o-volt Proportioning Controller 3) A motorized valve powered by a stepping motor, its power supply and a mass flowmeter supplied b
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