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

Table 4 MX carbonitride compositions, at .-%
Steel grade V W + Mo Fe + Co
M3/2 81 3 11
Px30 75 8 11
M2 81 2 11
M42 80 5 9
with high precision . Table 1 summarises the chemical
compositions of the M 6C primary carbides found in the
different steels after sintering at the optimum temperature
both in vacuum and in the industrial atmosphere .
Although there are individual differences due to the different
chemical compositions, adding W + Mo and Fe + Co
gives practically constant values . However, although the
chemical composition of the M6C carbides is very similar
after sintering in the industrial atmosphere and in vacuum,
the V concentration is lower in the industrial atmosphere
sintered material . This is due to the higher concentration
of this element found in the MX carbonitrides, as mentioned
above . The table also gives the chemical composition
of M 6 C carbides analysed by other authors in M2
and M7 vacuum sintered steels, with almost identical
values to those found in the present work for vacuum
sintering . The mean stoichiometric composition of these
carbides is (Fe,Co) 3(W,Mo)7. 5 (V,Cr)0 .5C. Exactly the
same composition of M6C carbides has recently been
found for T grade steel .'
MC primary carbides, MX carbonitrides
The size of the primary MC carbides (> 4 µm) was sufficient
to allow analysis without matrix interference in steels
with a V composition equal to or higher than 3 wt-% .
Vacuum sintered M2 and M42 steels presented MC carbides
in the form of thin films . In these conditions, the
effect of matrix interference in the chemical analysis was
very clear (Table 2) . The chemical composition of the MC
carbides for the various vacuum sintered steels is given in
Table 3 where it can be seen that although the relative
amounts of W and Mo are different, due to the different
compositions of the steels, the sum of W + Mo is almost
the same . This has also been observed in several T type
steels .' It can also be seen that the more important
metallic element in this carbide is V and that Co is not
present . Similar carbide compositions have been reported
by Karag6z et al."
When the steels are sintered in the industrial atmosphere
containing N2, the incorporation of nitrogen transforms
the MC carbides to carbonitrides, as reported by
Palma et al .' This is due to the higher stability of the
nitride than the carbide . Table 4 summarises the chemical
compositions (metallic elements) of the carbonitrides present
in the different steels analysed in the present work .
Again the compositions are very similar for the different
steels and if the results are compared with those for the
MC carbides, a significant increase in the amount of V in
Table 5 M2C carbide compositions, at.-%
Steel Mo V Cr Fe W W+Mo
M42* 39 23 11 15 11 50
ASP60t' 8 31 2 49 9 8 39
M220 21 23 14 12 26 46
M2 16 36 21 12 7 23 59
M7, primary 17
M7, primary
30 19 9 39 3 33
17
39 23 10 25 4 43
* Present work .
t Secondary carbides, atom probe .
Urrutibeaskoa et al. Metallographic changes on sintering M grade steels 311
Table 6 Eutectic type 1 carbide compositions, at .-%
Steel Mo V Cr Fe Co W W + Mo Co + Fe
M3/2* 24 28 27 17 . . . 4 28 17
M3/2+N* 28 11 27 28 . . . 7 35 28
Px30* 25 19 12 27 2 15 40 29
Px30 + N* 23 12 13 37 3 12 35 40
M2* 23 19 20 33 . . . 5 28 33
M2+N* 22 9 27 36 . . . 6 28 36
M2 M23C6 ' 4 8 50 37 . . . 2 6 37
M2 EMC + M6C 1 2 30 26 13 13 . . . 18 48 13
M42* 34 16 19 28 1 2 36 29
M42 + N* 26 6 17 46 4 2 28 50
* Present work .
the MX carbonitride is observed . Similar results have been
obtained with several T type HSS .' a
M2C carbides
The primary M2C carbide, with its typical rectangular
elongated aspect (Fig . 7), was found only in steel M42
which had a higher Mo content . It can be differentiated
from the M 6 C carbide by its chemical composition, given
in Table 5, which compares the chemical composition of
the M,C found in this work with other compositions
reported in the literature . 1B.17 .18 This carbide has also been
identified by X-ray diffraction and distinguished from M6C
by the presence of a (0001) reflection in vacuum sintered
materials, which was not found after sintering in the
nitrogen based atmosphere ." This could indicate that the
presence of nitrogen in the steel either prevents the formation
ofW carbides, or favours the decomposition of M2C
carbides to M6C and MC carbides, as has been reported by
17 .18
other authors . 16,
Eutectic carbides
The type MC and M 6C eutectic carbides found in the
present specimens show chemical compositions close to
those observed in the primary carbides of the same types .
However, MC carbides were not observed in steels sintered
in the industrial atmosphere . This could be because
in the nitrogen rich industrial atmosphere the MC carbides
are transformed to MX carbonitrides and the possible
transformation on cooling of the liquid to MX and austenite,
which happens when MC carbide is formed, disappears
or is shifted to much higher temperatures . This type
of MC eutectic carbide was not observed in M42 steel, due
to the low V content of the steel . The presence of M 6C
eutectic carbides both under vacuum and in the industrial
atmosphere seems to define the limit of adequate sintering
. The addition of carbon or the introduction of nitrogen
through the atmosphere does not greatly affect the temperature
at which the first M 6C appears in each steel .
However, because both the addition of carbon and the
introduction of nitrogen in steels M3/2 and Px30 decreases
the optimum sintering temperature, an increase in the
concentration of carbon, and occasionally nitrogen, in the
steel in the cases mentioned above could have enlarged the
sintering `gate' .
The composition and morphology of the needle type
eutectic carbide found in the M3/2, Px30, and M2 steels
after oversintering in vacuum is similar (adding W + Mo)
to that found for M2C primary carbide in M42 steel and for
a carbide identified as M,C in M2 .16 .19, 2°
Other
workers"- '7 . 15 have shown that M 2 C carbides are unstable
and decompose during cooling to M6C and MC . In the
present work the cooling rate was very high and possibly
explains why this decomposition did not take place . To
confirm this hypothesis, a M2+0 . 2%C sample was cooled
slowly in the furnace (20 K h - ') after oversintering by
Powder Metallurgy 1990 Vol . 33 No . 4