New gas nitriding technique using the interior of the nitrided pressure vessel as the process chamber

The need to nitride the interior of large machine housings, which are also pressure vessels, for use in a high-temperature gas reactor resulted in the development of a suitable new nitriding technique whereby the vessel interior is used as a process chamber while being heated in a conventional top-hat heat treatment furnace. However, this new concept introduced several mechanical design challenges, such as an extended-length fan shaft, a high-temperature bearing, and sealing flanges for operation under extremely high temperatures. A prototype nitriding plant was constructed and operated to verify the design. The different tunnels machined inside the forged vessel were nitrided evenly by measuring and balancing the gas flow through each tunnel. Test specimens placed at different positions in the housing were also nitrided during the process. The nitrided specimens were subjected to microhardness and layer thickness tests. Measured gas flow rates and other operational data confirmed the inverse proportionality between ammonia supply flow rate and measured crack ratio, as well as a crack ratio temperature dependence, typical of conventional gas nitriding processes. The design and operation of the nitriding plant were successful and a nitride layer thickness of 400 m and a hardness of approximately 1100 VHN were achieved. This proves that a large pressure vessel can be successfully nitrided using the vessel's interior as a process chamber.