Gas nitriding and nitrocarburizing can be processed in horizontal or vertical chamber furnaces with refractory insulation. However, to allow a quick change of the atmosphere and to avoid influences resulting from previous processes in the same furnace, retort furnaces are preferred.
To enable an acceptable production capacity, large vertical retort furnaces are used. The figure is a schematic sketch of a vertical retort furnace with a compensator for sealing against the atmosphere. Nowadays large horizontal retort furnaces are also used. Horizontal retort furnaces of different size and execution are preferred for nitrocarburizing processes. Similar to the vertical execution, a good circulation of the process gas is needed to get uniform results, which can be reached by the use of a muffle and a high circulation rate. Indirect cooling is performed with an external cooling fan, blowing cold air along the retort. Other designs allow direct cooling of the process gas with an external heat exchanger.
For more information, click on the link below (subscription required). Then scroll to Figure 23
R Schneider; R. Mesquita; H. Altena; T. Müller; P. Seemann, Processes and Furnace Equipment for Heat Treating of Tool Steels, ASM International, 2014, https://doi.org/10.31399/asm.hb.v04d.a0005958
