HM 7

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],

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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

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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

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(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

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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.

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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

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(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

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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.

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