In situ XRD Investigations for Understanding Nitrogen Mobility and Changes in the Microstructure of Advanced Stainless Steels during Nitriding

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Using in situ XRD in combination with ion implantation and ion sputter etching gives direct access to diffusion and phase transitions processes. The main advantage of this developed laboratory method is the excellent combination of a time resolution of a few minutes with a depth resolution of better than 50 nm and the possibility to analyse thick layers up to 5–20 µm (as the information depth of the X-rays is no longer a limiting factor). Furthermore, the existence of separate diffraction peaks for different phases permits a phase selective analysis of the depth resolved data. The focus of this project is to elucidate the processes occurring during nitriding of duplex stainless steels – a very attractive and important class of steels for industrial use – which is especially demanding as the mixture of austenite and ferrite phases is hard to separate in conventional diffusion experiments. Yet, these phases exhibit different nitrogen diffusivities, nitrogen solubilities, and structural transformations. The complexity of the phenomena occurring during nitriding, as indicated by a preliminary experiment, is the most probable reason for the strongly limited literature which is often contradictory. Now, combining in situ XRD with ex situ advanced materials characterization methods as STEM, EBSD, 3D-SIMS imaging and depth profiling, as well as atomic probe tomography, will allow us detailed insights into the nitrogen mobility and induced changes in the microstructure of such materials.Furthermore, nitriding of precipitation hardening steels is investigated with the same combination of in situ and ex situ methods as similar experimental obstacles are present for this other class of advanced – and equally technologically important – stainless steels with no unequivocal data available in the literature.The main aspect combining these two classes of stainless steels and conventional austenitic stainless steel is a limited stability of the Fe-Cr-Ni-N phase. This phase starts to decay into CrxN and a Fe-Ni matrix in either bcc or fcc structure (according to XRD data) with varying grain size and, thus, fluctuating nitrogen content within and nitrogen transport through this phase. All these effects strongly influence the nitriding process after this decay started already before it becomes visible in the XRD data. Thus, the understanding of this phenomenon is of significant, fundamental interest. The underlying transformation behind this process will be investigated and a deep insight into the complex interplay of the different effects will be obtained using the advanced combination of in situ XRD and ex situ characterization techniques. One anticipated result shall be a guidance for a more efficient and reproducible nitriding of advanced stainless steels, i.e. duplex and precipitation hardening grades, in industrial environments.