Attaining the Steady State of the Nitriding Potential after a Change of NH_3 Atmosphere Flow Rate during the Gas Nitriding Process

摘要

In single and multi-stage nitriding processes, each stage is characterized by the following parameters: temperature and time, type and composition of incoming atmosphere, as well as the set value of the nitriding potential. In the case of an atmosphere composed of raw ammonia and dissociated ammonia (NH_3 - NH_(3diss)), the set value of the nitriding potential can be achieved by a change of the incoming atmosphere make-up, while in the case of atmospheres comprising NH_3 and NH_3 - N_2 - by a change of the atmosphere flow rate. The time needed to reach a stabilized state, after the initiation of a change in the atmosphere gas mix can be assessed relatively easily. The problem is much more complex if we want to predict the time of reaching a new stabilized state following a change in atmosphere flow rate. The time of reaching stabilized state is, in this case, a complex function of the flow rate of the atmosphere which forces the potential change, and of temperature. This problem, in the case of the NH_3 type atmosphere, is the subject of investigation in this work. Factors are discussed, affecting the rate at which stabilized state is reached by the system after the introduction of a disturbance, necessary to attain the required nitriding potential.