ISSN:
1573-8973
Source:
Springer Online Journal Archives 1860-2000
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes:
Conclusions 1. The simplest and cheapest method consists in using the gases formed during nitriding for repeated nitriding, and this is possible in hermetically sealed furnaces. In this case it suffices to connect the exhaust gases from one furnace to the entering gases in a second furnace. With increasing degrees of dissociation of ammonia the nitriding capacity of the mixture becomes exhausted and for a given critical degree of dissociation such a mixture cannot form \ge-phase rich in nitrogen on the surface of iron, and this slows down the process and decreases the thickness of the nitrided layer. Apparently, under ordinary nitriding conditions (550\dgC, P=1 atm) this critical degree of dissociation is $${\text{V}}_{{\text{N}}_{\text{2}} + {\text{H}}_{\text{2}} } = 60\% $$ (see Fig. 2). This number divided by the degree of dissociation at the exit of each furnace determines the acceptable number of the successively connected furnaces. 2. The use of molecular nitrogen to dilute ammonia is advantageous because it widens the limits of the existence of nitride phases rich in nitrogen. This makes it possible to use mixtures poorer in nitrogen for nitriding. Figure 4a shows that even the mixture containing 20% ammonia forms the \ge-phase within the whole temperature range between 227 and 591\dgC. The use of molecular nitrogen as a solvent for ammonia is advantageous also because the amount of explosive hydrogen formed in the reaction zone then decreases. 3. The use of molecular hydrogen to dissolve ammonia is thermodynamically disadvantageous (Figs. 3c and 4c). 4. Nitriding at pressures different from 1 atm alters the position of static equilibrium. Nitriding in dissociated ammonia somewhat increases the ration of phases rich in nitrogen (Fig. 2a) but to a much lesser degree than dilution of ammonia with nitrogen. The increase in pressure is thermodynamically disadvantageous for nitriding (Fig. 2b). 5. Dilution of ammonia with some inert gas (argon, for example) does not change the position of equilibrium as compared to that resulting from dilution with molecular nitrogen.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00649318
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