continues for about 3 to 7 hours thereafter the cyanide formation rate drops off significantly it is believed that this initial heavy cyanide formation is due in part to the reaction of ammonia with carbon available at the surface of the steel being case hardened it follows therefore that as the surface carbon is depleted the cyanide formation is reduced accordingly to overcome this effect the initial nitriding gas flow rate is moderately high i e about an order of magnitude greater than that required when a scrubber is used or more specifically about 50 to 200 cu ft per hour per 100 sq ft of steel surface area being nitrided after this initial period of from 3 to 7 hours the nitriding gas flow rate is reduced significantly to about 5 to 20 cu ft per hour per 100 sq ft of steel surface area being nitrided at both flow rates it is necessary to maintain the required 1 to 3 water content in the gas nitriding should continue at the reduced gas flow rate for a length of time necessary to achieve the degree of hardness desired at specified depths nitriding times may vary from several hours to one week we have learned that when the nitriding inhibitor contamination is kept low as in this process the nitriding rate approaches a diffusion controlled process which is the maximum rate theoretically possible at such a nitriding rate there exists for any given alloy being nitrided a nitrogen activity for any given temperature below which no white layer iron nitride can be formed regardless of nitriding.