Analytical models for the gas carburizing process
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I. A STEADY-STATE MODEL FOR CONTINUOUS FURNACES C A R B U R I Z I N G heat treatments for automotive gears, as well as other components of automotive powertrains and steering systems, are generally carried out in continuous "pusher" furnaces (Figure 1). Parts to be carburized are placed on fixtures, or loaded loosely in baskets, which are mounted on trays. To charge a tray of parts from the entrance vestibule into the furnace, a ram pushes the tray into the furnace, displacing each of the trays already in the furnace and discharging one tray to the exit vestibule at the opposite end of the furnace. Controls are provided so that the total operation of the furnace, including the timing of vestibule and furnace door openings, pushing, and quenching, is fully automatic once the time interval between pushes (the "cycle" time) is set by the furnace operator. Furnaces may contain 15 to 30 trays; usually, the longer the carburizing treatment, the longer the furnace. Cycle times m a y vary from 12 to 90 minutes, depending on the case depth desired, the necessary production rate, and the length of the furnace available. To provide the flexibility needed to optimize the carburizing process, pusher furnaces are divided into three to five zones. Each zone is provided with a temperature control and has an inlet for the carrier gas and enriching gas;* the carburizing zones are usually equipped with *It is assumed that the carrier gas is endogas made from natural gas (primarily methane) and that the enriching gases are natural gas and air.
fans to circulate the atmosphere. The zones are often separated from one another by arches which support the roof, reducing the cross-sectional area of the furnace at that point. C.A. STICKELS is with the Manufacturing Development Center, Ford Motor Company, 24500 Glendale Avenue, Detroit, MI 48239. Manuscript submitted January 15, 1988.
METALLURGICAL TRANSACTIONS B
The usual operating strategy is to (1) use the first zone for heating parts to the carburizing temperature; (2) use the middle zones for carburizing; and (3) use the final zone for adjusting the maximum case carbon content. The temperature in the final zone is often lower than that in the other zones in order to reduce the distortion of parts during quenching. A relatively low (0.3 to 0.5 pct) atmosphere carbon potential is desired in the first zone to minimize sooting on cold parts while they are being heated to the furnace temperature. The atmosphere carbon potential desired in the middle zones is relatively high (1 to 1.2 pct C) but not so high a~ to produce sooting. The carbon potential of the atmosphere in the final zone is lower (0.7 to 1.0 pct), depending on the maximum case carbon content desired. The furnace operator setting up his equipment to produce a specified case depth on a given volume of parts per hour has difficulty anticipating the additions of natural gas (or air) required for each zone, because it is difficult to predict the pattern of internal gas flow within the furnace. Figure 2 shows that endogas is added
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