Etude par Sonde Atomique Tomographique de la formation de nano ...

tel-00751814, version 1 - 14 Nov 2012
II.4. Partial conclusions
Chapter 4. Nitride Dispersion Strengthened Steels
Nitrided NDS steel produced by plasma nitriding of Fe-18Cr-1W-0.75 Ti powder in
Bodycote-Nitruvid Company. The microstructure is studied in as-nitrided, annealed at 600
and 700°C during 1hour and after hot extrusion (800°C). The main conclusions of the work
are:
� Significant amount of nitrogen (~10at.%) is measured in all studied powders
(5-10µm). Measurements are done at 2 µm depth from the surface.
� No nanoparticles are observed in as-nitrided state whereas platelike Cr-N-rich
particles are observed in powder subjected to annealing treatment at 600 and 700°C
during 1hour.
� Smaller amount of nitrogen (less than 1at.%) is measured in all analyses
performed on consolidated material. Homogenization treatment of the powder before
extrusion seems to result in a more uniform distribution of nitrogen.
� However, the microstructure after consolidation remains heterogeneous.
Different types of particles are observed in each experiment: Ti-N rich, Cr-rich.
Further optimisation of nitriding process is necessary. As a possible solution, it is
proposed to use more homogeneous population of powder particles (smaller ones).
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