In-situ investigation of surface layers during gaseous nitriding by means of XRD and photothermal radiometry

The aim of most applications of nitriding treatments at steel components is to obtain a compact compound layer and/or a deep diffusion layer. The possibility of a survey of the nitriding treatment by analyzing directly the component's surface state during the nitriding process is particularly interesting, since it allows a process monitoring and control based on the actual nitriding result. In the present study, two measurement methods were developed and combined with the aim of direct surface state analysis during a nitriding treatment: the in-situ X-ray diffraction method and the photothermal radiometry. An experimental setup including a miniature nitriding furnace was developed in order to allow the combined application of both methods during a nitriding process under controlled atmosphere. In the present work, results of combined in-situ measurements on the steel AISI 4140 regarding the nitride layer formation during nitriding process as well as the nitride layer change during the following denitriding of the layer in nitrogen gas are presented and discussed. The investigations show that the photothermal radiometry is sensitive to the changing surface properties due to growing compound layers and when porous layers are generated. This method has a high potential for implementation in industrial nitriding furnaces, but for this, further development for quantitative evaluations of the measurements will be required. © Carl Hanser Verlag GmbH & Co. KG.