Model-based surface layer characterization of gas-nitrided steels using photothermal radiometry
Gas nitriding is one of the most important thermochemical processes for surface treatment of steels to improve surface hardness and wear resistance. The resulting nitriding layer consists of the outer compound layer and the underlying diffusion zone. Gas nitriding can often lead to incorrect nitriding results, e.g. soft spots or thin or highly porous nitriding layers. Although X-ray diffractometry allows in-situ characterization of the boundary layer composition, there is currently no direct in-process measurement method for observing boundary layer development in an industrial nitriding furnace. As a solution, an alternative measuring technique called photothermal radiometry is used, which enables non-contact and non-destructive in-process boundary layer testing. The boundary layer modifications in the diffusion or compound layer or near-surface porous zones can thus be observed during formation and growth, since the photothermal signals depend on the thermal properties of the surface.
The general topic of the project is the in-process application of the photothermal measurement method in an industrial nitriding furnace. The thesis can be focused on several possible areas of this task. This includes, among others, the investigation of how the measuring system has to be modified in order to enable a reliable characterization of the surface layer under the process conditions of a gas nitriding furnace. On the other hand, the extension of the evaluation by a modelling of the photothermal signal can be the main part of the thesis.
Possible contents of the work can be:
- Development of a concept and construction of a modified detector head for use under the conditions of an industrial nitriding furnace.
- Development of a robust evaluation strategy for the photothermal signal
- Boundary layer simulations for modelling the photothermal signal