Gas carburizing of steel with furnace atmospheres formed In Situ from propane and air: Part I. The effect of air-propane
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+ 2H 2 + 2N 2
is 27.9 kcal/gm-mol CH 4 at 1027 ~ After leaving the generator, the reacted gases are rapidly cooled to room temperature (wasting 37.0 kcal/gm-mol CH 4 of sensible heat), then piped to a carburizing furnace and reheated to the furnace temperature. Thermal losses of at least equal magnitude arise from having to operate two furnaces rather than just one; the endothermic gas generator often must be run when its output cannot be fully utilized. For these reasons, there has been considerable interest in recent years ~5 in learning to gas carburize without a gas generator, introducing unreacted gases directly into the carburizing furnace. Even if gas flows and the furnace gas composition remained the same, this step alone would reduce hydrocarbon consumption by about 50 pct. Since the new processes also usually use C. A. STICKELS and C. M. MACK are with the Engineering and Research Staff, Ford Motor Company, Dearborn, MI. M. BRACHACZEK (deceased) was with the Engineering and Research Staff, Ford Motor Company, Dearborn, MI. Manuscript submitted January 31, 1980.
lower flow rates, and may substitute nitrogen for part of the carrier gas, the hydrocarbon consumption may actually be reduced by 75 to 90 pct. In this paper we are reporting the results of carburizing trials run with blends of propane and air introduced directly into a carburizing furnace. Over 30 years ago, Jenkins 6 reported the results of carburizing experiments with air-propane mixtures in laboratory tube furnaces. Using linear flow rates of 5 ft/min (2.5 cm/s), temperatures of 900 to 920 ~ and air-propane ratios ranging from 2 to 4, he was able to carburize alloy steels in a systematic manner, i.e., as the air-propane ratio decreased, the amount of carbon pickup increased. Carburization, however, was accompanied by heavy sooting; without the soot, there was no carburizing. For many years C. I. Hayes, Inc. of Cranston, Rhode Island marketed what they called a "Certain Curtain" furnace in which a protective atmosphere was formed by reacting propane and air (or methane and air) in a small chamber just below the furnace hearth. 7 Appropriate air-hydrocarbon ratios for "neutral" hardening of various grades of steel were determined by trial and error. The resulting atmospheres often contained free oxygen and always contained large amounts of CO2. 8 More recently Cook 9 discusses carburizing trials run with propane-air-nitrogen blends containing about 95 pct added nitrogen. Graphs of pct CO 2 in the furnace atmosphere vs surface carbon content are presented for 920 and 940 ~ No experimental details are given. Sheehy and Watson 2 ran trials in a small industrial carburizing furnace using 80 to 85 pct inlet nitrogen and sufficient air to give 4 to 6 pct CO. They state that carburizing rates at 980 ~ are the same as with an endothermic carrier gas. The flow rates used were equivalent to 90 to 95 pct of the flow rates with endothermic carrier gas, after allowing for the expansion in gas volume which occurs when air and propane react. No actual flow rat
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