Magnetic Properties

8 M a g n e t i c Properties

Cobalt, in WC-Co, is the phase which binds refractory WC powders, on sintering into very hard metals. The cobalt content can be varied from 3-25 mass% and WC grain sizes can range from 0.2-5 Bm. The ferromagnetism of cobalt makes it possible to develop non destructive quality control tests for such types of composites.

1.0

COERCIVE FORCE

The measurement of coercive force is of utmost practical importance for the purpose of quality control during hard metal production. The purely empirical fact that coercive force allows an evaluation of the structure of a cobalt rich solid solution is utilized for this purpose. Since cobalt serves as a binder for the hard phase, interesting conclusions can be drawn concurrently as to the degree of sintering, cobalt distribution, and grain size of the hard metal. The coercive force of technically pure polycrystals of ferromagnetic metals or single phase alloys, such as cobalt, is also composed of an internal stress component and a grain size component. The internal 227

228

Cemented

Tungsten

Carbides

stresses of the ferromagnetic base structure are just strong enough to achieve an increase in coercive force of about 40 Oe.Ell The grain size component of the coercive force lick is related as"

1

Eq. (1)

Hcr, - -~

where d is the crystallite size of the base structure. An analogous relationship can be observed for the dependence of the coercive force of thin films on their thickness, t2J Walter and Grewe,t31 while investigating technical WC-Co alloys, suggested that an estimate of the coercive force can be made from the following relationship with up to 90 vol% of a nonmagnetic foreign body which results from a derivation of the "Foreign body" theory: E41 3

--~176176 Eq. (2)

nc = 4 - 2

I.~

t~ 9 - 9 3~f-~

S

where,

Zs

=

tri b K S S

= = = = =

a

=

I~

=

magnetostrictive saturation internal stress in immediate vicinity of walls numerical factor, 1 (for sphere and cube) crystal energy width of Bloch wall distance of foreign bodies volume fraction of foreign bodies magnetic saturation

While discussing the grain growth of a particular alloy according to the "foreign body" theory, K, I s, ~, and ot are to be regarded as constant. Only S increases when small crystallites disappear and grow onto larger ones. Thus, the coercive force decreases because the Bloch wall shift may occur between obstacles which are further apart from each other, and thus less wall energy is derived from the potential energy of the magnetic field. LiteraturetSl-t71 is available describing the variation of coercive force with composition, grain size, etc. All of it reports that coercive force decreases as the cobalt percentage increases and also when grain size increasestSl (see Fig. 1).

Magnetic Properties 229

ut

9

7%Co

o

15% Co

150-

E 0

u 100-L

9 0 U

509

0 0

I 0'5

.

l 1.0

! 1.5

! 2.0

I ...... 2.5

3.0

Specific Grain Surfacej pm -1

Figure 1. Effect of specific grain surface on the coercive force of WC-Co cemented carbides.[51

Fischmeister and Exnert6] explained that a linear relationship exists between coercive force and specific cobalt surface. They also found the relation H c - 160 S# for WC-Co hard metals, where S#- specific cobalt surface. Various authorstS]-[111 established that coercive force of the cobalt phase increases with the amount of tungsten in it due to solution and precipitation hardening. Freytag et a1.[12] investigated the effect of the carbon content of cemented carbides on coercive force. According to them, coercive force increases with increasing carbon but they could not explain the decreasing coercivity at high carbon level. After short time liquid phase sintering, coercive force is constant in the two phase region but increases pronouncedly in the three phase region due to the hardening effect of the finely dispersed WaCo2C(rl) phase. Due to improper milling, ferromagnetic particles tend to agglomerate. This results in Co rich and Co lean regions, which leads to a large variation in coercive force. It is, therefore, a quick test to find out the proper milling time for the cemented carbide production. Fang and Easan tl6] confirmed the increase in H c with decreasing cobalt content or with decreasing carbon content of WC (see Fig. 2). It was shown that H c increases linearly with decreasing grain size. The scatter of

230 Cemented Tungsten Carbides the data points was attributed to the variation in the original powder particle sizes. The authors also emphasized that the estimates of hardness using H c are often incorrect, because the responses of hardness to the changes in H c vary, depending on effective factors that result in the changes of H c. Figure 3 illustrates the dependence of hardness on H c under different circumstances. The different values of slope indicate the different sensitivities of hardness to an identical change in H o depending on whether the change in H c is caused by a deviation in cobalt or carbon content.

Deviotion of Cobott Percent

662.0

-1.2

-0.4

I

. . . .

i

0.4 .....

t

1.2 ....

2.0

r

O Hc vs O'C O Hc vs O'Co

50 _o

40

-O

tJ

~

O

Corbon content

3O .2 20 o

lO _

o

o

0

Cobolt content l. -0.~0

1

o

0

decreasing

-10 -0.25

o

i -0.15

1

[] l -0.10

!

J -0.05

1 O

Deviotion of Corbon Percent

Figure 2. Variation in H c vs. changes in cobalt and carbon contents in WC based cemented carbides.t161

While studying the effect of heat treatment of WC-Co cemented carbide on coercivity, it was noticed that H c decreases after quenching and then rises slightly after tempering. A higher value in as sintered carbide is related to the presence of e-cobalt (hcp), which is highly anisotropic. The presence of (x-cobalt (fcc) in the quenched condition lowers the I-Ic values. [171

Magnetic Properties 231

89.0 88.6 ntent

a/

0

Q: 88.2

g c

6

decreasing

Q

o 87.8

2:

87.

87.1

60

70

80

90 100 Measured Hc

110

120

130

Figure 3. Variation of Rockwell A hardness on coercivity and its relationship to cobalt and carbon content of WC-Co cemented carbides. 1161

Tillwick and Joffe, [19J while studying the effect of aging at 700~ for different periods of sintered, solution treated (1075~ 10 h), and quenched WC-25 Co cemented carbide, on coercivity, noticed a maximum after 1000 minutes (see Fig. 4). This was consistent with the fact that intergranular precipitate particles impeded the domain wall motion.

2.0

MAGNETIC

SATURATION

The measurement of magnetic saturation is used as an indirect method of measuring the carbon content of sintered hard metal. Magnetic saturation measurements enable carbon content to be estimated to an accuracy of 0.01% if the test pieces are prepared within closely comparable conditions. Its greatest advantages are speed, simplicity, and sensitivity. In addition, magnetic saturation measurement is less dependent on the use of a skilled operator than chemical analysis.

232 Cemented Tungsten Carbides

. (o)

~~"-~~~C77K

o/

110-

j-,,,

~I 9o'-

i . -,

. -,.,~~

lo'

"

i0'

~0 3

"

lO ~

lo ~

A =,mr o<],,0vu~2g~tor~~r~geing Time, min then

woter

quenched

r

"o- ~D O

77K

N

:~ 180.C

~

300K

O

c 170 .2 2

'

~~O 1 6

m

o, I ~ "

-%

Solution t r 9 oted

1

101

t

I

I0 z

I

I

10 ~

I

10 4

Ageing Time, min Figure 4. Coercivity (a) and saturation magnetisation (b) variation of WC-25 Co cemented carbide as a function of ageing time.[ 191

Figure 5 shows the relation between the carbon content of the alloy and the saturation magnetization.t81 The magnetic saturation drops sharply in three phase region rl-WC-Co due to reduction in the amount of the magnetic phase when cobalt is incorporated in the nonmagnetic rl phase.[~2] Various authors[S][~31-[~s]reported that tungsten remaining in solid solution in the binder phase depends upon the carbon content, with higher amounts

Magnetic Properties 233 of tungsten in solution in alloys low in carbon and vice versa. The magnetic saturation of the sintered hard metal increases as the amount of tungsten as solid solution in cobalt decreases and vice versa. Magnetic saturation in the two phase region is, therefore, an indication of the amount of tungsten dissolved in the binder phase. Freytag and Exner [12] observed that magnetic saturation is sensitive to r I phase formation not only if the system is deficient in carbon but also when high carbon alloys are quickly cooled from the sintering temperature. Thus, magnetic saturation is not a safe indication for the amount of tungsten dissolved in the binder phase.

210 I/1 Vl

s

- 100

190u

-92

o ._.

0

.,2

t; 17(:}

-

g

IE

- 84

"~.

c

o

c

.9 150 76 9,- ~ ' / + ~ + w c - - l * ~ T+wc ~

131 _.

I

5.9

6.0

I..

6.1

C+u I

6.2

.

-----

I

6.3

6,4

Carbon Content (Converted Value to WC),%

Figure 5. Effect of carbon content of WC-Co cemented carbide on saturation magnetization and Curie point.[81 Fang and Easantl6] related the specific magnetic saturation of straight WC-Co composite, asc, with the theoretical density, p, of the alloy as Eq. (3)

p=

0064+ 0 048( /

This assumed that WC is stoichiometric. However, the magnetic saturation and density of the alloy change significantly when the carbon content of the WC Co alloy changes. Figure 6 shows the relationship in

234 Cemented Tungsten Carbides straight WC-Co alloy graphically. For carbon deficient alloys, the relationship between density, Gsc and carbon content are given by the lines drawn as carbon axes corresponding to different cobalt levels. It can be seen that each particular carbon axis, for a given cobalt level, has a different scale, which is governed by the equation,

Eq. (4)

W/o C = 6.13- 0.053 fco 1 6 0 - fco

where W/o C is the carbon content in the WC phase of the alloy. The dash line defines the boundary of the 1] phase region. It was shown that when the specific magnetic saturation of the binder (O;b,) was 70% or less than that of pure cobalt, the beginning of T1phase transition occurred. The authors found a very good tally in the composition of alloy carried out by chemical analysis and magnetic saturation measurement. The study of Maritzen et al [~81on the effect of carbon in homogeneous Co-W-C alloy showed a negligible fall in magnetic saturation polarization, although the lattice parameter was strongly affected.

16.0

15.5 -

15.0 E O

"r~,...',,,J0.0

., 1 4 . 5 -

,

>,~

s.e~31"5 .s0 : ~ ~

C: Q9

14.0-

q - Phase

Region

s'~e s.9~~~

13.5 %

13.0

I

0

8

1

I

.~t

16

I

24

t

,

32

O'sc , e m u / g

Figure 6. Density-specific magnetic saturation diagram" interrelationship of cobalt and carbon content in cemented carbide. [161

Magnetic Properties

235

Tillwick and joffe,t~91 while studying the effect of aging at 700~ for different periods of sintered, solution treated (1075~ 10h), and quenched WC-25Co cemented carbide on saturation magnatisation, found that it did not rise steadily to an equilibrium value, as was expected for simple precipitation and growth of CoaW. The existence of a distinct peak at 35 min (Fig. 4) was interpreted with the complex precipitation process, possibly involving rapid initial precipitation of tungsten for the supersaturated solid solution, followed by a slower ordering reaction in the particles so formed to yield stoichiometric Co3W. A similar two stage process, involving the initial rapid formation of an intermediate phase (ct'), which ordered on further aging to yield Co3W, was observed in cobalt rich Co-W-C alloys by Jonnson and Aronnson,E201 using transmission electron microscopy, Le Roux and McLachlan,E211 while studying WC-Co cemented carbides, observed that the remanence ratios of the ferromagnetic constituents depend on the largest cobalt lakes and, if a sufficient number of these is present for a constant grain size, then, although the cobalt content may vary through a wider range, the remanence ratio is constant. Variation of the grain size at constant cobalt content varies the size of the lakes and, hence, the remanence ratio.

REFERENCES 1. Hondremont, E., Handbuch der Sonderstahlkunde, Verlag Stahleisen m.b.H., Dusseldorf, p. 86 (1956) 2. Lavin, P. A., Cobalt, 1969, No. 4375/81 3. Walter, P., and, Grewe, H., Powd. Met. Int., Vol. 3, No. 2, p. 88 (1971) 4. Kersten, M., Grundlagen einer theorie der ferromagnetischen Hysterese und Koerzitivkraft, 2nd edition, S. Hirzel, Leipzig (1944) 5. Stjernberg, K. G., Powd. Met., Vol. 13, No. 25, p. 1 (1970) 6. Fischmeister, H., and Exner, H. E., Arch. Eisen., Vol. 37, p. 499 (1966) 7. Roebuck, B., and Almond, E. A., Recent Advances in Hardmetal Production, Loughborough, 17-19 Sept., MPR Pub., Shrewsbury, U.K., p. 28 (1979 8. Suzuki, H., and Kubota, H., Planseeber Pulvermet., Vol. 14, p. 96 (1966) 9. Suzuki, H., Yamamoto, T., and Hayashi, K., J. Jap. Soc. Powd. and Powd. Met., Vol. 13, p. 304 (1966) 10. Jung, O. et al., Cobalt, Vol. 53, p. 1 (1971) 11. Tillwick, D. L., and Joffe, I., Script. Met., Vol. 7, p. 479 (1973)

236 Cemented Tungsten Carbides 12. Freytag, J., and Exner, H. E., Mod. Dev. in Powd. Met. (H. H. Hausner and P. W. Taubenblat, eds.), Metal Powder Industries Federation, Princeton, Vol. 10, p. 511 (I 977) 13. Gurland. J., Trans. AIME, Vol. 200, p. 285 (1954) 14. Rudiger, O., et al., Techn. Mitt. Krupp Froschungs Ber., Vol. 29, p. 1 (1971) 15. Nighiyama, A., and Ishida, R., Trans. Jap. Inst. Met., Vol. 3, p. 185 (1962) 16. Fang, Z., and Easan, J. W., Int. J of Powder Met., Vol. 29, p. 259 (1993) 17. Ho Yi, L., and Jinhui, Y., Proc. 1 l th Int. Plansee Seminar, Vol. 2, (H. Bildstein and H. M. Ortner, eds.), Metallwerk Plansee, Reutte, p. 679 (1985) 18. Maritzen, W., Ettmayer, P., and Kny, E., Powder Metallurgy Int., Vol. 17, p. 68 (1985) 19. Tillwick, D. L., and Joffe, I., J. Phys. D., Applied Physics, Vol. 6, p. 1585 (1973) 20. Jonsson, H., and Aronsson, B., J. Int. ofMetals, Vol. 97, p. 281 (1969) 21. LeRoux, H., and McLachlan, D. C., J. of Magnetism and Magnetic Materials, Vol. 43, p. 143 (1984)