hm 7
Post on 01-Oct-2015
215 Views
Preview:
TRANSCRIPT
-
18th
Plansee Seminar HM 7/1
Precipitation of M7C3 Carbides During Sintering of TiCN-WC-Ni-Co-Cr
Alloys Used in Hot Rolling Applications
I. Iparraguirre*, N. Rodriguez*, F. Ibarreta**, R. Martinez**,
J.M. Sanchez*
* CEIT and TECNUN, Paseo Manuel de Lardizbal 15, 20018, San Sebastin, Gipuzkoa, Basque
Country, Spain.
** FMD CARBIDE S.A.L., Zorrozaurre 35, 48014, Bilbao, Spain.
Abstract
The densification of TiCN-WC-Cr-Ni cermets has been analysed by means of dilatometry and
calorimetry. Shrinkage phenomena, both in solid and liquid phase, are enhanced by the Cr additions.
Carbothermal reduction of oxides is observed to occur at much lower temperatures for high Cr contents.
The dissolution of the different additives used in the powder mixtures and the rim formation also depend
on the amount of Cr added. Thus, TiC and WC dissolution occurs at lower temperatures as the Cr
content increases. In all cases the rim formation requires the presence of a liquid phase.
Keywords
WC
Introduction
Recently, the interest in Ti(C,N) based cermets is increasing due to the dramatic rise of tungsten prices
[1]. These alloys are candidates for substitution of cemented carbides in a variety of tribological
applications due to their excellent combination of high hot hardness, fracture strength, oxidation
resistance and thermal conductivity [2-6]. Among these cermets, those with higher metallic contents (>
25 vol.%) are the less studied. These compositions are less dense than steel and, for this reason, are
typically used for the fabrication of components working under high inertial loads (i.e. roller guides in hot
rolling mills). Apart from Ferro-TiC type materials [7,8], TiMoCN-Ni and TiWCN-Ni cermets are typical
examples of these alloys [6,9-12], which are obtained by liquid phase sintering of a variety of powder
mixtures. In some cases, the carbonitride powders are homogeneous solid solutions within the Ti-Mo-C-
N and Ti-W-C-N systems but, in general, consist of different mixtures of binary carbides, nitrides and
carbonitrides [3,6,9]. The microstructures of these materials are quite complex including several variants
of the so-called core-rim structures. This term applies to the ceramic phase, in which grains have a
core ( phase) whose composition is very different from the surrounding shell ( phase). The last works
published on TiWCN-Ni alloys are focused on the behaviour of ultrafine Ti(C,N) powders [10-13],
-
18th
Plansee Seminar HM 7/2
particularly, on the dissolution kinetics of WC additions and their effect on the composition of inner and
outer rims. Apart from the properties described above, corrosion resistance is also critical for certain
applications, especially when refrigerating fluids are used [14,15]. In such cases, a higher amount of
chromium is added to the composition of the powder mixtures. The present work is aimed at analysing
the effect of these Cr additions on the shrinkage, the liquid formation and the microstructural evolution of
TiWCN-Ni cermets during sintering, since, so far, no reliable data have been published on this matter.
Experimental
The compositions of the powder mixtures, given in Table 1, were designed to have a constant Ni content
(35 wt.%). Cr and W were added as Cr3C2 and WC carbides respectively. Nitrogen was introduced in the
mixtures as Ti(C0.7N0.3) carbonitride powder and the C/N ratios were selected to be between 3.3 and
3.8 to ensure a good sintering behaviour. This was adjusted by using TiC powder additions.
Table 1: Composition of powder mixtures
Elem.
Ref.
C1 C2 C3 C4 C5
Cr3C
2 0,0 2,2 4,6 9,0 17,3
Ni 35,5 35,5 35,4 35,0 35,3
WC 12,1 12,1 12,1 11,9 10,8
Ti(C0,7
N0,3
) 38,4 36,5 37,2 37,2 31,7
TiC 14,0 13,8 10,7 6,9 5,0
Mixing/milling was carried out in planetary equipment for 7 hours in hexane with 3.5 wt% addition of
paraffin as organic binder. Afterwards, the powders were dried for 1 h. at atmospheric pressure in a
thermostatic bath (90+/-2C).Green compacts were obtained by double action pressing at 160 MPa.
Presintering was carried out in pure hydrogen using a tubular metallic furnace made from an AISI 314
stainless steel (containing 25.0 wt % Cr) on alumina coated graphite trays. Two presintering
temperatures where used for fine tuning of the C content: 450C and 650C. A constant heating rate of
5C/min and a dwelling time of 35 min was used in all cases. The sinterability and mass losses of the
different powder mixtures was analysed by combining dilatometry (Netszch TA) and thermogravimetry
/calorimetry tests (TGA/DSC Setaram Setsys Evolution 16/18). These experiments were carried out on 5
mm high and 5 mm in diameter cylinders using a heating rate of 10C/min up to 1450C (10-1 mbar).
The onset of DSC and shrinkage rate peaks was determined by using the SETSOFT software [16].
Sintering was carried out in a conventional graphite furnace with a heating ramp of 10C/min up to
1300C. The vacuum level used during this step was 10-4 mbar [17]. Above this temperature, the
pressure was increased up to 100 mbar by including argon in the furnace chamber and maintained
during the sintering plateau for 1 h. Three sintering temperatures were investigated: 1100C, 1300C and
1425C in order to analyse the dissolution process of each additive used in the mixtures. The C and N
contents were measured by means of non dispersive infrared spectrometry and thermal conductivity
methods respectively. Oxygen was measured by following DIN ISO 4491, placing the sample in a
graphite crucible and heating in argon up to 1900C. The amount of oxygen is determined by measuring
-
18th
Plansee Seminar HM 7/3
the CO content with an appropriate sensor. Standard ISO 3369 was used for density measurements
using ethylic alcohol instead of distilled water. The accuracy of this method is about 0.5%. The sintered
specimens were ground and polished down to 1 m diamond paste for microstructural analysis, which
was carried out by optical and scanning electron microscopy (FEG-SEM) and energy dispersive X-ray
spectroscopy (EDS). Phase identification was carried out by X-Ray diffraction (XRD) (with Ni-filtered
CuK radiation).
Results and Discussion
The samples sintered at 1450C-1 h are fully dense. Differences in the absolute values are due to their
different composition (theoretical values range from 6.6 g/cm3 for composition C1 to 6.9 g/cm3 for
composition C5) (Fig. 1).
(a) (b)
Figure 1: ensification vs. sintering temperatures of samples presintered at (a) 450C and (b) 650C.
The compositions with chromium contents ranging from 0 wt.% to 4 wt.% exhibit a similar behaviour with
very limited densification after 1 hour at 1300C and strong shrinkage at 1425C. The materials with
higher Cr content show much higher densities at 1300C reaching almost full density in the case of
composition C5 (15 wt.%Cr). The densification of materials presintered at 450C (i.e. with higher C
activity) is slightly improved at temperatures of 1300C and below. However, this effect is lost when the
Cr content or the sintering temperature increase.
Dilatometric experiments confirm that shrinkage in TiCN-WC-Ni-Cr alloys starts at temperatures 20C to
50C lower as the C activity increases (i.e. for samples presintered at lower temperatures). In addition,
shrinkage rate peaks are wider for materials presinter at lower temperature (Fig. 2).
3.5
4
4.5
5
5.5
6
6.5
7
1150 1200 1250 1300 1350 1400 1450
Ge
om
. D
en
s.
(g/c
m3
)
Temperature (C)
C1
C2
C3
C4
C5
450C
3.5
4
4.5
5
5.5
6
6.5
7
1150 1200 1250 1300 1350 1400 1450
Ge
om
. D
en
s.
(g/c
m3
)
Temperature (C)
C1
C2
C3
C4
C5
650C
-
18th
Plansee Seminar HM 7/4
(a) (b)
Figure 2: Dilatometric graphs of samples presintered at (a) 450C and (b) 650C.
Melting temperatures obtained by DSC are clearly correlated with shrinkage events (Table. 2).
Table 2: Temperatures corresponding to the melting phenomena found in DSC experiments
2nd melting event 1st melting event
Onset Point Peak Onset Point Peak
Pre 450 Pre 650 Pre 450 Pre 650 Pre 450 Pre 650 Pre 450 Pre 650
C1 1352 1360 1377 1383
C2 1336 1346 1366 1371
C3 1321 1334 1351 1362
C4 1285 1307 1314 1334 1230 1245 1244 1254
C5
1235 1236 1264 1265
Compostion C1 to C3 (0 wt.% to 4 wt.% Cr) exhibit only one endothermic peak, which moves to lower
temperatures as the Cr and/or C activities increase. Compositions C4 presents two melting events: one
following the trend explained above and another one at lower temperature. As said before, in general,
the eutectic temperatures are reduced as the C activity increases, except for composition C5 which
shows no significant change.
Data published for the Ti-W-C-Ni system [18] are consistent with the second melting event in Table 2.
However the 1st peak has not been described before. A similar behaviour is described for the W-C-Co-Cr
system [19], in which Cr additions also depress the melting point of these alloys. However, the best
correspondance is found with the eutectic described in the 15 wt.% isopleth section of the Cr-Ni-Cr
system which occurs at 1249C. These results suggest that the high solubility of WC and Cr3C2 additions
in the Ni phase are the key for explaining first melting phenomena in these alloys.
A summary of XRD analyses of materials sintered at different temperatures is included in Fig. 3.
-5000
-4000
-3000
-2000
-1000
0
1100 1200 1300 1400
d(D
L/L
0)d
t (x
10
-3)
Temperature (C)
C1C2C3C4C5
450C
-5000
-4000
-3000
-2000
-1000
0
1100 1200 1300 1400
d(D
L/L
0)d
t (x
10
-3)
Temperature (C)
C1C2C3C4C5
650C
-
18th
Plansee Seminar HM 7/5
Figure 3: Phases detected by XRD in samples of compositions C1, C3 and C5 sintered at different temperatures.
TiC additions are fully dissolved in the three compositions at 1300C but not at 1150C, whereas WC
and Cr3C2 are already dissolved at 1150C. M7C3 carbides are detected by XRD only in composition C5
with (with 17.3 wt.% Cr3C2 additions).
SEM images confirm that M7C3 carbides are also present in composition C4 (9.0 wt.% Cr3C2), although
its precipitation is incipient yet (Fig. 4). Therefore, its volume fraction is below the resolution limit of XRD
analyses.
(a) (b)
Figure 4: FEG-SEM images of samples presintered at 450C and sintered at 1425C-1 h (a) Alloy C4 and (b) Alloy C5. Blue
arrows point to M7C3 crystals in each case
Mass losses during sintering increase with the Cr content of the alloys and are higher for materials with
higher C content (Fig. 5). This result indicates that both Cr and C play a significant role on the reduction
of powder oxides.
-
18th
Plansee Seminar HM 7/6
Figure 5: Mass change (%) after sintering for two different presintering conditions: 450C and 650C.
These results are interesting if compared with mass loss rates (Fig. 6).
(a) (b)
Figure 6: Mass loss rates of samples presintered at (a) 450C and (b) 650C. (Calculated from TGA data)
These data show that mass losses stabilises at lower temperatures as the Cr content increases,
confirming that carbothermal reduction of oxides is strongly enhanced by increasing Cr3C2 additions.
This is clearly observed by measuring C losses as a function of the Cr content of the alloys (Fig. 7a). Up
to 4wt.%Cr, C losses clearly increase with the Cr content of the alloys. For higher Cr contents losses
tend to stabilise. Analysing oxygen contents, it is clear that deoxydation is more effective under high
carbon activities. This can be adjusted with high precision by controlling the presintering temperature
(Fig. 7b).
-1.3
-1.2
-1.1
-1
-0.9
-0.8
0 5 10 15 20
Ma
ss
va
ria
tio
n
( %
)
Cr content (wt %)
Pre 450C
Pre 650C
-0.005
-0.004
-0.003
-0.002
-0.001
0.000
600 800 1000 1200 1400
d (
m/m
0)/
dT
Temperature (C)
C1
C2
C3
C4
C5
-0.005
-0.004
-0.003
-0.002
-0.001
0.000
600 800 1000 1200 1400
d (
m/m
0)/
dT
Temperature (C)
C1
C2
C3
C4
C5
-
18th
Plansee Seminar HM 7/7
(a) (b)
Figure 7: C and O losses after sintering for different presintering conditions
(a) (b) (c)
Figure 8: FEG-SEM images of samples of composition C4 presintered at 650C and sintered at: (a) 1150C-1h, (b) 1300C-1h
and (c) 1425C-1h. Blue arrows point to M7C3 carbides.
(a) (b) (c)
Figure 9: FEG-SEM images of samples of composition C5 presintered at 650C and sintered at: (a) 1150C-1h, (b) 1300C-1h
and (c) 1425C-1h. Blue arrows point to M7C3 carbides.
SEM images (Figs. 8 and 9) show that dissolution reprecipitation phenomena are quite complex. At
1150C, significant differences are found between compositions C4 and C5. Densification is clearly
enhanced by Cr additions, although at this low temperature dissolution of WC and Cr3C2 particles is not
completed (Figs. 8a and 9a). It has to be remembered that these rests are not detected by XRD (Fig. 3).
At 1300C, high densification is found in both specimens. Cr rich M7C3 carbides are already detected in
composition C5 but not in C4 (compare Fig.8b and 9b). Another difference between these two alloys is
that in C4 grain growth has been less active with a large fraction of W rich nanometric grains (in light
10
12
14
16
18
0 5 10 15
-C
/C (%
)
Cr content (wt. %)
Pre 650
Pre 450
60
70
80
90
0 5 10 15
-O
/O (%
)
Cr content (wt. %)
Pre 650C
Pre 450C
-
18th
Plansee Seminar HM 7/8
contrast in Fig. 8b) already undissolved. In C5 these phases have dissappeared and ceramic grains are
coarser. On the other hand, at 1300C, incipient rim formation is observed in some carbonitride grains in
sample C4, but not in C5. Therefore, it seems that Ti-W-Cr-C rim formation and M7C3 precipitation are
interacting with each other. This is clearly observed by comparing the microstructures obtained at
1425C-1 h. In alloy C4, many carbonitride grains have an inner rim surrounded by the M7C3 phase,
whereas in alloy C5, such configuration is rarely found.
Conclusion
Melting in TiCN-W-Ni-Cr alloys occurs at lower temperatures as the Cr content increases. Melting
temperatures are compatible with those found in (Ti,W)C-Ni and Cr-C-Ni systems. In these cermets,
shrinkage always precedes melting. In some cases, densification occurs mainly by solid state sintering,
whereas in others a combination of solid state sintering and liquid phase sintering is observed. The
dissolution of these carbides increases the C activity in the binder phase promoting the reduction of Ni
oxides. Carbothermal reduction of oxides leads to higher mass losses and gas emission stop at lower
temperatures as the Cr content increases
M7C3 precipitation in TiCN-W-Ni-Cr alloys is detected at and above 8wt.% Cr contents. This
phenomenon is activated by the presence of a liquid phase suggesting that is a dissolution
reprecipitation process. Therefore, there is a complex interaction with the rim formation in these alloys
that needs to be analysed in deeper detail.
Acknowledgements
The Gobierno Vasco via GAITEK programme is gratefully acknowledged for the finantial support of this
work.
References
1. Critical raw materials for the EU, Raw Material Supply Group, European Commission, Entreprise
and Industry, July (2010).
2. Rudy E. US Patent 3,971,656; (1976).
3. Doi H. In: Almond E A, Brookes C A, Warren R, editors. Proc of Int Conference of Science of Hard
Materials. Inst Phys Conf Ser No 75, Bristol and Boston: Adam Hilger Ltd; 1986, 489-523.
4. Suzuki H, Matsubara H. J Japan Soc Powder Powder Metall (1983); 30:257.
5. Pastor H. Mater Sci Eng A105/106, (1988); 401-9.
6. Exner H E. In: Viswanadham RK, Rowcliffe DJ, Gurland J, editors. Proc of the Int Conf on the Science
of Hard Materials, New York and London: Plenum Press; (1983), 233-62.
7. Das K, Bandyopadhyay TK, Das S, J. Mater. Sci., 37, (2002), 388192.
8. Degnan CC, Shipway PH, Wear, 252, (2002), 83241.
9. Ettmayer P, Kolaska H, K. Dreyer, pmi vol. 23, no.4, (1991), 224-30.
-
18th
Plansee Seminar HM 7/9
10. Jung J, Kang S, Acta Mat., 52, (2004), 1379-86.
11. Ahn SY, Kang S, J. Am. Ceram. Soc., 83, 6, (2000),1489-94.
12. Park S, Kang S, Scripta Mat., 52, (2005), 129-33.
13. Demoly A, Lengauer W, Veitsch C, Rabitsch K, Int. J. Refract. Met. and Hard Mat., 29 (2011) 716
723
14. Oakes JJ, Met. Powd. Rep.,42(7-8), 492-499, (1987)
15. Wan W, Xiong J, Yang M, Guo Z, Dong G, Yi C, Int. J. Refract. Met. and Hard Mat., 31, 179186,
(2012)
16. SETARAM Instrumentation. Setsys evolution 16/18. SETSOFT user manual, 39 (2002)
17. I. Iparraguirre, M. Aristizabal, N. Rodriguez, F. Ibarreta, R. Martinez and J.M. Sanchez, Proc. of
Euro PM2011, Vol.1 PM Tool Materials, Session 17: 160, Barcelona (2011)
18. Chen L, Lengauer W, Ettmayer P, Dreyer K, Daub HW, Kassel D, Int. J. Refract. Met. and Hard
Mat., 18, 307-22, (2000)
19. Frisk K. 17th Plansee Seminar, Int. Conf. on High Performance P/M materials; Proc. Vol. 2, HM 1: 1-
10. (2009)
top related