bdGas carburizing of steel with furnace atmospheres formed in situ from methane and air and from butane and air
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INTRODUCTION
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I N previous papers, 1.2.3 we have presented the results of gas carburizing experiments in which the furnace atmosphere was formed from propane and air introduced directly into the carburizing furnace. When gas flow rates are low, sufficient time is available for gas phase reactions within the furnace to approach equilibrium. Thus, the composition of the furnace atmosphere which results is similar to that which is produced when propane and air are first reacted in an endothermic gas generator at 1100 to 1200 ~ and then introduced into the furnace at the higher flow rates typical of commercial practice. Gohring and Luiten 4 have briefly reported on a similar process. While propane is readily available for commercial operations, methane, in the form of natural gas, is even more widely available in this country. As a source of carbon and hydrogen for furnace atmospheres, propane and methane are, at present, similar in cost. It is, therefore, of interest to determine whether or not methane can be used in place of propane for this type of carburizing. As a hydrocarbon source for furnace atmospheres, butane is similar in most respects to propane. It is readily stored as a liquid, yet is easily converted to gas (boiling point -0.5 ~ It contains about 2 0 p c t more carbon per unit volume of liquid than does propane. Butane is more expensive and not as widely distributed as propane in this country, but it is frequently used elsewhere, e.g., Japan, to produce heat treating atmospheres.
C.A. STICKELS and C. M. MACK are members of the Engineering and Research Staff, Ford Motor Company, Dearborn, MI 48121. J.A. PIEPRZAK, formerly a member of the Engineering and Research Staff, is now with the Transmission and Chassis Division, Ford Motor Company, Livonia, MI. Manuscript submitted November 13, 1981.
METALLURGICAL TRANSACTIONS B
EXPERIMENTAL PROCEDURE
A small Lindberg sealed-quench carburizing furnace was used for the experiments as before.l Test specimens for carburizing were small stampings of 1010 steel. ~ Fifteen specimens were placed in the furnace basket for each trial; the amount of carburizing occurring during a trial was found by determining the average weight gain of five specimens located at the comers and in the center of the furnace basket. The uniformity of carburizing can be estimated from the standard deviation of the weight gains. For several of these trials, the center specimen was sectioned and the carbon concentration gradient was measured by electron microprobe analysis. The gas flow controls used for these experiments were simpler than the controls previously used. 3 The new system, shown schematically in Figure 1, consists of a needle valve and a 25-cm rotameter for manually setting the air flow and equipment for automatically regulating the methane or butane flow. The key components of the latter system include (1) a zirconia oxygen sensor inserted in the furnace chamber, (2) a Barber-Colman Model 560 process controller, and (3) a Hoke motorized needle valve. The controller and
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