Process for carburizing steel

Claim 1

In a process for carburizing steel in a furnace having at least one carburizing chamber, said chamber being closed except for at least one passage through which the steel passes into and out of the chamber and having means for opening and closing the passage, said process comprising the following steps: (a) opening the passage; (b) introducing the steel through the passage into the chamber; (c) closing the passage; (d) introducing a carrier gas and a gaseous hydrocarbon into the chamber to provide a carburizing atmosphere, said carrier gas being introduced at a high flow rate when the passage is open and at a low flow rate when the passage is closed with the minimum high flow rate being sufficient to essentially prevent oxidation and decarburizing of the steel, and said atmosphere comprising: ______________________________________ component of atmosphere percent by volume ______________________________________ carbon monoxide about 4 to about 30 hydrogen about 10 to about 60 nitrogen about 10 to about 85 carbon dioxide 0 to about 4 water vapor 0 to about 5 hydrocarbon about 1 to about 10, ______________________________________ said percent by volume being based on the total volume of the atmosphere; (e) exposing the steel to the carburizing atmosphere at a temperature in the range of about 1200° F. to about 2200° F. until the steel is carburized; (f) opening the passage; (g) withdrawing the steel through the passage; and (h) closing the passage, the improvement comprising: (i) providing hydrocarbon in an amount sufficient to maintain Z a at a level about equal to (K A /100) (X 2 /Y g ) wherein: Z a is the percent by volume of carbon dioxide; X is the percent by volume of carbon monoxide; K a is the equilibrium constant for the reaction 2 CO ⃡ C + CO 2 ; Y is a predetermined percent by weight of carbon that is to be present on the surface of the steel, based on the weight of the steel; and g is the activity coefficient for carbon dissolved in the steel; (ii) providing a minimum low flow rate of carrier gas sufficient to limit the oxygen species entering the atmosphere whereby an amount of no greater than about 10 percent hydrocarbon, as a component of atmosphere, will be required to maintain the value of Z A as set forth above; and (iii) providing a maximum low flow rate of carrier gas no greater than about one half of the minimum high flow rate of the carrier gas.