Sinterizazio-atmosferaren eragina M graduko (ASP 30 ... - Euskara

Chemical composition of primary carbides and carbonitrides_
The chemical composition of MC and M 6C type carbides found in the
present work following vacuum sintering are very close to those found by other
authors for similar materials (17,18,29-31) . M 6C carbide composition can be ajusted
to a chemical composition (Fe,Co) 3(W,Mo) 2 .5 (V,Cr) 0 .5 C independentely of the steel
chemical composition or the sintering atmosphere . On the other hand, in MC
carbides, present only for vacuum sintering, the amounts of W plus Mo in atomic
percent is constant and about 22-25%, being also nearly constant, around 67% the
atomic percent of V . Carbides with similar composition have been found by other
authors (17,18,29-31) . During atmosphere sintering, as pointed before, the MC
carbides are transformed in MX carbonitrides, which contain a much higher
concentration of Vanadium, around 78% at, and decreasing the concentration of
W+Mo to values around 7% at . When an important amount of MC is transformed to
MX, for instance in Px30 steel, the rejection of W and/or Mo allows the precipitation
of additional M 6C carbides, as shown clearly in Table IV . These results agrees with
those found in a T42 (17) and in a T15 (18) steel sintered in a gas atmosphere rich in
nitrogen, reported recentely .
Eutectic carbides .
The eutectic carbides type M 6C and MC found in these materials
present chemical compositions close to those observed in the primary carbides of
the same type, but it is worth to emphasize that MC carbides have not been
observed in steels sintered in the industrial atmosphere . This seems to be due to the
fact that in the industrial atmosphere rich in nitrogen the MC carbides are
transformed to MX carbonitrides with a much lower solubility and they seem to be
present in form of primary particles even at high sintering temperatures and
therefore if they are not dissolved cannot precipitate in eutectic form . On the other
hand the presence of M 6C eutectic carbide both under vacuum and industrial
atmosphere seems to define the upper limit of temperature for a sintering with an
appropriated microstructure .
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