Carbon Control

Carbon content of cemented carbides must be controlled to within very narrow limits, typically ±0.04 wt%, to prevent formation of brittle lower carbides, such as the eta phase in tungsten carbide alloy systems on the low carbon side and carbon precipitates on the high carbon side.

Because carbon content can be changed substantially during sintering by reactions with oxygen-containing phases in the powder and by carbon exchange reactions with the furnace atmosphere, control of the furnace atmosphere carbon potential and the oxygen content of the powder is essential to produce high-quality P/M sintered parts.

During hydrogen presintering, reactions between the hydrogen atmosphere and cemented carbides are relatively sluggish at the lower end of the peak temperature range (400 to 600 °C, or 750 to 1110 °F), but reaction rates are sufficiently high at temperatures >600 °C (1110 °F) to cause significant changes in the carbon content of the part if the atmosphere is not properly adjusted.

This phenomenon is illustrated in Fig. 44, which shows the effect of varying the carburizing potential of the hydrogen atmosphere by methane additions on the carbon content of a 94WC-6Co composition held 1 h at the peak presinter temperature. The carburizing potential can be described as:

* Hi where k is the equilibrium constant for the reaction 2H2 + C ■! > CH4, and CH4/ 2 is a partial pressure ratio

Lil IS

e

00 °C (1470 *F

/

/ /

700

I

f

rl

/

>

500

°C (030 eF)-—

*C-*F)

-40

rc (

750"

n

r

(750

^ * V 1

/ HP » + —

jP

00 00 J<

S(93i 3 (121

1 *F)

_ --

/ v >

f

30 *F

J -

1 -

^B00'C(1470*F)

Ac -

0.01 0.02 0.05 0.1 02 05 1-0 2.0 Carburizing potential

0.01 0.02 0.05 0.1 02 05 1-0 2.0 Carburizing potential

Fig. 44 Effect of carburizing potentials of hydrogen-methane mixtures on carbon content of WC-6Co. 1 h hold; 1 atm pressure. Ac is the thermodynamic carbon activity. Total carbon content is the sum of the compounded carbon in the carbide phase plus any graphite present. It does not include carbon present in the lubricant.

At temperatures up to 500 °C (930 °F), reactions causing carbon loss are too slow to noticeably change compositions. At 700 and 800 °C (1290 and 1470 °F), however, large carbon losses occur when the carburizing potential (relative to pure carbon) is less than ^0.05.

Thus, even pure hydrogen can cause large carbon losses above ^-600 °C (1110 °F). When the carburizing potential exceeds 1.0 (saturation value), large carbon increases occur at 500 °C (930 °F) and above, thus allowing buildup of the carbon-containing volatilized lubricant in the peak temperature portion of the furnace.

Above ■--'900 °C (1650 °F), hydrogen-base atmospheres can change the carbon content of cemented carbides substantially by:

3WC + 3Co + 2H2O ± W3CO3C + 2CO + 2H2WC + Co + CH4 ± WC + CO + C + 2H2

To prevent the formation of undesirable carbon reaction phases, the carburizing potential (carbon activity) of the atmosphere must be carefully controlled. Table 20 shows experimentally determined activities required to maintain the desired carbon contents in tungsten carbide/cobalt cemented carbides at 1000 to 1450 °C (1830 to 2640 °F), with examples of typical gas compositions required to provide such carbon activities (Ref 44). The three major constituents of cemented carbide sintering atmospheres are hydrogen, carbon monoxide, and methane.

Table 20 Atmosphere compositions required to maintain neutral carburizing potentials for tungsten carbide/cobalt as a function of temperature and total oxygen content

Temperature

Carbon

Hydrogen, %

Carbon

Water, ppm

Carbon

Methane, %

°C

°F

activity

monoxide, %

dioxide, ppm

1450

2640

0.6-0.25

99.944-99.976

None

None

None

0.056-0.023

1450

2640

0.6-0.25

99.45-99.47

0.5

3.4-8

4.7 x 10-3-1.1 x 10-2

0.055-0.023

1450

2640

0.6-0.25

89.96-89.98

10

60.0-144.0

1.7-4.0

0.045-0.019

1200

2190

0.6-0.25

99.84-99.93

None

None

None

0.24-0.10

1200

2190

0.6-0.25

99.27-99.40

0.5

12.2-29.2

3 x 10-2-7.2 x 10-2

0.24-0.10

1200

2190

0.6-0.25

89.79-89.87

10

220.0-570.0

12.2-29.2

0.19-0.08

1000

1830

0.5-0.2

99.56-99.82

None

None

None

0.44-0.18

1000

1830

0.5-0.2

99.05-99.29

0.5

114.0-290.0

0.34-0.85

0.44-0.18

1000

1830

0.5-0.2

89.43-89.34

10

2080.0-5200.0

135.0-338.0

0.36-0.14

At pressure of 1 atm

At pressure of 1 atm

Typical commercial practice for hydrogen sintering carbon control is based on partial reaction of the incoming gas (hydrogen plus impurity levels of water vapor and oxygen), which has zero carbon activity. Graphite fixturing is used to carry the carbide parts through the furnace to increase the atmosphere carbon activity. Fixturing may thus contain openings that are empirically sized to react the incoming gas to the proper degree with the graphite to obtain the desired carbon activity.

Alternately, the graphite carrier may completely enclose the parts, thus causing carbon activity to increase to near unity. Modifiers such as aluminum oxide sand are included within the carrier to react with the atmosphere to thus lower the carbon activity to the desired level. A common practice uses a packing mixture of aluminum oxide sand and a small amount of carbon that surrounds the cemented carbide parts inside graphite carriers. By adjusting the carbon addition, carbon activity can be adjusted to the desired level.

Because the rates of carbon and oxygen exchange between the atmosphere and cemented carbides are low during vacuum sintering, the final carbon content of the cemented carbide when vacuum sintered is determined primarily by the initial carbon content of the powder and the amount of carbon lost during heating due to reactions between carbon and chemisorbed oxygen in the powder. Much of the oxygen present as oxides of tungsten and cobalt reacts with carbon in the powder during the vacuum heating cycle and evolves as carbon monoxide and carbon dioxide gases, whereas in hydrogen sintering, those oxides are reduced by hydrogen and evolve as water vapor. Studies have shown that the cobalt binder phase is reduced of chemisorbed oxide between 550 to 650 °C (1020 to 1200 °F); tungsten oxides are reduced around 900 °C (1650 °F), while reduction of the oxides of the cubic carbides (Ta2O5 and TiO2) occurs at 1000 °C (1830 °F) (Ref 45). When titanium carbides and tantalum carbides are present, much of the oxygen contained in these compounds is also removed by carbon reduction. To compensate for this carbon loss, vacuum-processed powder usually contains about 0.1 to 0.3 wt% added carbon than is used for hydrogen-processed powder.

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