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

tel-00751814, version 1 - 14 Nov 2012
II. Study of nitrided NDS
Chapter 4. Nitride Dispersion Strengthened Steels
Another way to elaborate NDS steel is to introduce nitrogen into ferritic matrix by
nitriding. This second part is devoted to the samples elaborated in this way. Nitrided NDS
powders were studied in their initial state, i.e. after nitriding treatment, as well as after
annealing at 600 and 700°C during 1hour. Consolidated material is also investigated. As it is
known, nitriding treatment may result in a non homogeneous distribution of nitrides through
the microstructure. To achieve more uniform distribution of nitrogen, the powder before hot
extrusion is subjected to homogenization treatment (for details see section ΙΙ.4 of the Chapter
2). This is not the case for other powders.
For characterization of powders as well as consolidated material, the LAWATAP
technique is preferably used, as it allows analyzing relatively large volumes. The summary of
performed analyses performed on nitrided NDS powder is summarized in Table 4.4.
Table 4.4. APT experiments performed on nitrided NDS steel by APT.
Material APT
Number of
analyses
As-nitrided powder LAWATAP 1
Annealed at
600°C/1h powder
Annealed
700°C/1h powder
Hot extruded sample
Number of atoms,
LAWATAP
11.10 6 at
13×10 6 at
LAWATAP 1 20×10 6 at
LAWATAP 1 20×10 6 at
TAP 1 200 000at
LAWATAP 2
22×10 6 at
10×10 6 at
The microstructures are described below. First, the information is provided on nitrided
NDS powders. Afterwards, characterization of consolidated by hot extrusion material is
discussed.
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