Stationary residual NH3 analysis for Carbonitriding

Carbonitriding is typically used for the heat treatment of unalloyed and low-alloy components. The aim is to improve the surface layer properties by introducing carbon and nitrogen. The carbon input can be controlled by an oxygen sensor (C-level probe). The reaction behavior of the NH3 on the component surface is a measure of the nitrogen input
reaction behavior of the NH3 on the component surface.

In practice, it can be observed that despite an unchanging temperature, C-level, NH3 quantity, batch size, etc., differences in the heat treatment result can arise. Therefore, it would be important to use a suitable sensor, in addition to the C-level probe, to monitor the atmosphere in the furnace systems.

One measure of the reaction behavior of the NH3 is the residual NH3 in the exhaust gas of the furnace system. The aim is a controlled carbonitriding. A control of the process is only possible if you know the residual NH3 content. The atmosphere consists of about 20% CO, 40% H2, 1% CO2, rest N2.

To perform a carbonitriding process, a maximum of 5% NH3 is added in relation to the fresh gas volume (endogas + greasing gas). This splits into H2 and atomic nitrogen, which is to diffuse into the iron structure. Residual NH3, which has not reacted with the batch surface, is the measure of N diffusion. This is to be measured and controlled.

This can only be realized by means of an NH3 measuring cell with a control range of 0... 5000 ppm and a good stability of the measuring cell at higher concentrations. The measuring range of the measuring cell should be 0... 1% NH3 in order to obtain reliable measured values even in the initial phase of the process.