Mechanism and Kinetics of Plasma Nitriding of the Nb-Alloyed PM Tool Steel |
| Journal |
Defect and Diffusion Forum (Volume 263) |
| Volume |
Diffusion and Thermodynamics of Materials |
| Edited by |
J. Čermák and I. Stloukal |
| Pages |
87-92 |
| DOI |
10.4028/www.scientific.net/DDF.263.87 |
| Online since |
March, 2007 |
| Authors |
Pavel Novák,
Dalibor Vojtěch,
Jan Šerák,
M. Novák,
Barbora Bártová
|
| Keywords |
Kinetic, Mechanism, Niobium, Plasma Nitriding, Tool Steel |
| Abstract |
The aim of this work was to describe the mechanism and kinetics of plasma nitriding of a
Nb-containing PM (powder metallurgy) tool steel. Material containing 2.5 wt.% C, 3.3% Si, 6.2%
Cr, 2.2% Mo, 2.6% V, 2.6% Nb and 1.0% W was prepared by nitrogen melt atomization and hot
isostatic pressing. Heat-treated steel (quenching from 1100 °C, triple tempering at 550 °C for 1h)
was plasma nitrided at temperatures ranging from 470 °C to 530 °C / 30 - 180 min. Light
microscopy, TEM, SEM and WDS were used to study the nitrided steel. It has been shown, that
nitriding at 470°C leads to the formation of thin layers composed only of a diffusion zone
containing nitrogen-rich martensite and fine nitride precipitates, no layer of nitrides is formed on
the surface. Nitriding is probably controlled by the nitrogen diffusion in martensite to the material
or by the processes in the nitriding atmosphere at this temperature. Nitriding at the temperature of
500°C and more leads to the formation of a continuous layer of nitrides and carbonitrides on the
surface that limits further nitrogen diffusion. Niobium, as a prospective element in tool steels, was
not found to play a role in the formation of the nitrided layer directly. Niobium replaces vanadium
in very thermodynamically stable primary MC carbides. This results in higher vanadium content in
others less stable carbides and in the matrix. Due to this effect, higher portion of vanadium can
precipitate as VC carbides and VN nitrides during heat treatment and nitriding, respectively. |
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