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Binder mean-free-path determination in cemented carbide by coercive force and material composition
Materials Science and Engineering, A 105/106 (1988) 289-292
289
Binder Mean-free-path Determination in Cemented Carbide by Coercive Force and Material Composition* R. PORAT and J. M A L E K
ISCAR Ltd, Nahariya (Israel) (Received November 9, 1987)
Abstract In the present investigation, cemented carbide with different cobalt contents varying from 9 to 37 vol.% were prepared by conventional methods, and their mechanical and magnetic properties measured. Using optical microscopy and scanning electron microscopy analysis, the average WC grain size and binder mean free path were determined. These values were found to relate to the coercive force and cobalt volume percentage. K~c measurements using the "short-rod" test method, as a function of the binder mean free path calculated from the equation, were found to agree very well with K~c values for cemented carbide published by other investigators in thefield, 1. Introduction In metallurgical systems, the mean grain size and distribution of grain size have an important effect on material properties and performance. In cemented-carbide systems (WC-Co), properties such as hardness, toughness and strength depend strongly on the microstructure, and in particular on the mean free path of the cobalt binder, The cobalt binder phase is ferromagnetic in hard metals and therefore the cemented carbide can be characterized by the parameters, the coercive force H~ and the specific magnetic saturation (SMS). The coercive force measures the ease with which the ferromagnetic domain walls are aligned by an induced magnetic field. The immobilization of the domain walls is caused by structural defects, particularly non-magnetic inclusions and material imperfections. Domain walls are attracted to these imperfections because the wall *Paper presented at the 3rd International Conference on the Science of Hard Materials, Nassau, The Bahamas,
November9-13.1987.
energy and the magnetostatic energy are thereby reduced. In cemented carbide, the total surface area of the WC grains (the WC being non-magnetic) is considered to be the main imperfection. Any process parameter which affects the microstructure of cemented carbide, such as the initial grain size of raw material, the milling time, the carbon balance and the sintering temperature, will directly influence the coercive force. Many articles have been published [1-6] which show the existence of a strong relationship between the binder mean free path and fracture toughness of the material. In all the abovementioned work the average carbide grain size was measured using the well-known method [7, 8] of line intersection on the sample cross-section. This method is tedious and requires many measurements to obtain statistically valid results. The aim of our work was to find a better method of determining the mean free path by establishing a mathematical relationship between the coercive force (an easily measured parameter) and the cobalt mean free path. The validity of our mathematical model was examined by measuring the K~c values for known compositions using the short-rod method [9], by relating these values to the calculated mean free path and by comparing the results with the data published by other researchers in the field of fracture toughness measurement. 2. Experimental details 2.1. Sample preparation Samples of various materials were prepared according to the following compositional groups. 2.1.1. Group A: WC + Co The only difference in the material in each group here is the WC grain size.
290
2.1.2. G r o u p B . ' W C + T a C - N b C + C o This group has a coarse grain size. 2.1.3. Groups C and D: WC + TiC + TaCNbC + Co
Group C differs from group D in the cobalt volume percentage. Other differences are in the grain size. 2.1.4. Groups E and F: WC + TiC + TaCN b C + Co
Groups E and F differ from groups C and D in the percentages of TiC and TaC. Group E differs from group F in the cobalt volume percentage. In group F, various grain sizes were prepared. All the materials were pressed as transverse rupture strength (TRS) specimens and vacuum sintered in the following manner: groups A and B at 1420 °C for 80 min; groups C - F at 1450 °C for 80 min. Table 1 is a summary of the compositions of the various groups, 2.2. Tests
All samples were tested for hardness, TRS, density, shrinkage, coercive force and specific TABLE 1
Material A B C D E F I. TABLE 2
Material
magnetic saturation. The microstructure was examined according to ASTM specifications. Table 2 summarizes some of the mechanical and magnetic results. For the Klc measurements, samples of cylinder rods were made from the same composition groups as mentioned above. They were sintered together with the above samples. The cylinder rod samples were then centreless ground to the exact dimension as specified by the fractometer producer. The samples were then prepared for TABLE 3
Material A1 A2 A3 A4 B1
Cl c2 c3
Summary
of results on Kzo Hc and 3:
Hc
f~
Kk
(kA m ~)
(/zm)
(MPa m~/2)
18.2 14.2 13.7 10.0 9.2 17.6
0.204 0.279
10.97 13.06
14.7 13.9
DI D2 E1 F1 F2
0.291
13.39
0.430 0.514 0.208
14.90 15.67 10.29
0.260
11.27 11.50
0.279 0.214 0.389 0.295 0.364 0.45
18.3 11.0 14.7 13.1 10.7
11.97 13.72 11.52 12.21 14.49
Composition of the cemented carbide
Density
Amount of £b
Amount of TiC
Amount of T a C
Amount of NbC
A m o u n t of WC
(g cm- 3)
(vol.%)
(vol.%)
(vol.%)
(vol.%)
(vol.%)
15.01 14.74 14.20 14.10 14.09 13.94
10.1 13.3 9.6 11.1 12.7 14.90
--6.5 6.4 5.7 5.6
-1.8 5.5 5.4 4.4 4.8
-0.4 1.1 1.1 1.0 1.0
89.9 84.5 77.3 76.0 76.2 73.7
Mechanical properties of the cemented-carbide materials Hc
Rockwell A
(kA m i)
hardness
(HRA)
Vickers hardness
TRS
TRS
(lbfin -2)
(N mm 2)
320000 330000 350000 330000 400000 255000 270000 290000 270000 290000 320000 340000 330000
2250 2300 2450 2300 2800 1800 1900 2050 1900 2050 2250 2400 2300
(load, 30 k~ (HV30)
A1 A2 A3 A4 B1 C1 C2 C3 D1 D2 E1 F1 F2
18.3 14.2 13.7 10.0 9.2 17.6 14.7 13.9 18.3 11.0 14.7 13.1 10.7
92.4 91.6 91.5 90.8 89.0 92.5 92.1 92.0 92.2 90.9 91.5 91.2 90.8
1750 1600 1570 1460 1250 1650 1570 1550 1600 1420 1500 1450 1400
291
the test according to the short rod method [9]. Table 3 summarizes Kit results, The TRS samples were prepared for metallographic examination. A most representative sample area was finely examined with an optical microscope and a photograph was taken (Figs. 1 and 2). The mean particle intercept distance was measured on each photograph by a microscope with a coordinate table, and a measurement fixture device capable of movement in the x-y direction with an accuracy of 1 mm. The distance length for measurement was 30 ram, and in each picture measurements were made at five cross-sections, This procedure was followed on 25 different compositions and various grain sizes. The coercive force for each sample was measured. Figure
3 shows the results of the d and H c measurements. 3. Results anddiscussion Figure 3 shows the mean intercept length of d, referred to as the mean WC grain size, as a function of coercive force H c (kA m -t) for various cobalt volume percentages on a log-log scale. It is observed that for a given d (ram) the coercive force decreases as the cobalt volume percentage increases. For a given cobalt volume percentage, the coercive force H c increases as the grain size decreases. From this graph, the following equation is deduced: L.~~:.... (8()/°s7' = 0.3 ( ~ ] (#m) (1) From the work of Underwood [7], the relationship between the mean intercept length and the mean free path of binder is
Vv (#m)
~.=[t
(2)
1-V~
Equation (1) can be transformed into a more practical and meaningful relation, as follows:
....
il
§
p
=o.3 l
....
[ i---------I~,,~i
Fig. 1. Scanning electron micrograph (H,,=14.2kAm 1; c)= 2.5 #m).
of
sample
A2
(#m)
- Vv \ H ~ ]
(3)
By introducing the weight percentage of cobalt instead of Vf using Xco Pth VF -
(4)
8.9 100
~0.0
~
6.0 ~
°c2 ° 0 ~ \ \ \
24% co-~C/~_'~"-.'%. \ ~ s . o % ,oo o co ~ T 13.3 % ' ~
.o 3.0
2"0" d(#m no.
iO.O
k
co " ~'
"--
I
L. . . . . . . . . . .
-I4
.
~
/
ij
~
i
]
o37-
~i
f
.... ; o u c ~ ~. Theoretical
.
.
.
.
.
o 30
.
.
.
.
2° t ~.o
0.3
0,0
0.0
Fig. 2. Microstructure of sample F2 (Hc=10.7 kA m-I; d = 2.6 #m).
IO0
200 300
Iooo
Fig. 3. Average grain size vs. coercive force for different cobalt volume percentages.
292
z5
i . [
size for cemented-carbide materials which differ in composition, grain size and cobalt content. (2) Equation (5) gives the mean free path of cobalt, using the cobalt weight percentage, the coercive force and the theoretical specific weight. (3) Kic results for the calculated mean free path are in agreement with other published data. (4) Our method is a valuable, useful and nondestructive way of determining material toughness without requiring actual fracture toughness testing.
~ z0
J
_ 15 'e *no 0
D~, 2"e~'" ,B/.. ,.,~;.~¢ ~.~," " • • m eF . A c~ '
.
I0 ~ , r C •
References
5
0 0.0
0.5
1.0
1.5
(lO"-6*m) Fig. 4. Fracture toughness as a function of the mean free path of cobalt. w e obtain the following final expression for ,~: 0"3Xpth )~
(801(164.822/XcoP~h),/3
890--XcoPt h \Hcc]
(/~m)
(5)
The validity of this equation was tested by measuring the fracture toughnesses Kjc of known compositions, using the short-rod method. Table 3 shows the coercive force, the calculated ~ (from eqn. (5)) and the measured Kic for the various materials. Figure 4 is a graph which shows K~c as a function of 2 using published data [1-6]. Our measured K~ values, labelled by letters indicating the compositions, are seen to fit the distribution averages very well. 4. Conclusions (1) Coercive force measurement makes it possible to calculate the average carbide grain
1 R. C. Leuth, Ph.D. Thesis, Michigan State University, 1972. 2 J. L. Chermant and F. Osterstock, J. Mater. Sci., 11 (1976). 3 L. Lindau, in D. M. R. Taplin (ed.), Fracture 1977, Proc. 4th Int. Conf. on Fracture, Waterloo, Ontario, June 19-24, 1977, University of Waterloo Press, Waterloo, Ontario, 1977. 4 J. R. Pickens and J. Gurland, Mater. Sci. Eng., 33 (1978) 135. 5 R. Porat and A. Ber, £TRPAnn., 32 ( 1 ) (1983). 6 Z. Goffer and R. Porat, Rep. EURO CVD 4, 1983 (Eindhoven). 7 Wesley,E' E. Underwood,Reading, MA,Quantitative1970. Stereology, Addison8 J. Gurland, Trans. Metall. Soc. AIME, 236 (1966). 9 L. M. Barker, Rep. TR77-91R, 1977 (Salt Lake City,
UT). 10 M.T. Laugier, Powder Metall. Int., 5 (1986).
Appendix A: N o m e n c l a t u r e d H~ Vv Xco
mean intercept length (mean grain size) of WC coercive force (kA m-~), volume fraction of cobalt weight percentage of cobalt
Greek symbols )~ Pth
mean free pass of cobalt binder theoretical density of the composition
289
Binder Mean-free-path Determination in Cemented Carbide by Coercive Force and Material Composition* R. PORAT and J. M A L E K
ISCAR Ltd, Nahariya (Israel) (Received November 9, 1987)
Abstract In the present investigation, cemented carbide with different cobalt contents varying from 9 to 37 vol.% were prepared by conventional methods, and their mechanical and magnetic properties measured. Using optical microscopy and scanning electron microscopy analysis, the average WC grain size and binder mean free path were determined. These values were found to relate to the coercive force and cobalt volume percentage. K~c measurements using the "short-rod" test method, as a function of the binder mean free path calculated from the equation, were found to agree very well with K~c values for cemented carbide published by other investigators in thefield, 1. Introduction In metallurgical systems, the mean grain size and distribution of grain size have an important effect on material properties and performance. In cemented-carbide systems (WC-Co), properties such as hardness, toughness and strength depend strongly on the microstructure, and in particular on the mean free path of the cobalt binder, The cobalt binder phase is ferromagnetic in hard metals and therefore the cemented carbide can be characterized by the parameters, the coercive force H~ and the specific magnetic saturation (SMS). The coercive force measures the ease with which the ferromagnetic domain walls are aligned by an induced magnetic field. The immobilization of the domain walls is caused by structural defects, particularly non-magnetic inclusions and material imperfections. Domain walls are attracted to these imperfections because the wall *Paper presented at the 3rd International Conference on the Science of Hard Materials, Nassau, The Bahamas,
November9-13.1987.
energy and the magnetostatic energy are thereby reduced. In cemented carbide, the total surface area of the WC grains (the WC being non-magnetic) is considered to be the main imperfection. Any process parameter which affects the microstructure of cemented carbide, such as the initial grain size of raw material, the milling time, the carbon balance and the sintering temperature, will directly influence the coercive force. Many articles have been published [1-6] which show the existence of a strong relationship between the binder mean free path and fracture toughness of the material. In all the abovementioned work the average carbide grain size was measured using the well-known method [7, 8] of line intersection on the sample cross-section. This method is tedious and requires many measurements to obtain statistically valid results. The aim of our work was to find a better method of determining the mean free path by establishing a mathematical relationship between the coercive force (an easily measured parameter) and the cobalt mean free path. The validity of our mathematical model was examined by measuring the K~c values for known compositions using the short-rod method [9], by relating these values to the calculated mean free path and by comparing the results with the data published by other researchers in the field of fracture toughness measurement. 2. Experimental details 2.1. Sample preparation Samples of various materials were prepared according to the following compositional groups. 2.1.1. Group A: WC + Co The only difference in the material in each group here is the WC grain size.
290
2.1.2. G r o u p B . ' W C + T a C - N b C + C o This group has a coarse grain size. 2.1.3. Groups C and D: WC + TiC + TaCNbC + Co
Group C differs from group D in the cobalt volume percentage. Other differences are in the grain size. 2.1.4. Groups E and F: WC + TiC + TaCN b C + Co
Groups E and F differ from groups C and D in the percentages of TiC and TaC. Group E differs from group F in the cobalt volume percentage. In group F, various grain sizes were prepared. All the materials were pressed as transverse rupture strength (TRS) specimens and vacuum sintered in the following manner: groups A and B at 1420 °C for 80 min; groups C - F at 1450 °C for 80 min. Table 1 is a summary of the compositions of the various groups, 2.2. Tests
All samples were tested for hardness, TRS, density, shrinkage, coercive force and specific TABLE 1
Material A B C D E F I. TABLE 2
Material
magnetic saturation. The microstructure was examined according to ASTM specifications. Table 2 summarizes some of the mechanical and magnetic results. For the Klc measurements, samples of cylinder rods were made from the same composition groups as mentioned above. They were sintered together with the above samples. The cylinder rod samples were then centreless ground to the exact dimension as specified by the fractometer producer. The samples were then prepared for TABLE 3
Material A1 A2 A3 A4 B1
Cl c2 c3
Summary
of results on Kzo Hc and 3:
Hc
f~
Kk
(kA m ~)
(/zm)
(MPa m~/2)
18.2 14.2 13.7 10.0 9.2 17.6
0.204 0.279
10.97 13.06
14.7 13.9
DI D2 E1 F1 F2
0.291
13.39
0.430 0.514 0.208
14.90 15.67 10.29
0.260
11.27 11.50
0.279 0.214 0.389 0.295 0.364 0.45
18.3 11.0 14.7 13.1 10.7
11.97 13.72 11.52 12.21 14.49
Composition of the cemented carbide
Density
Amount of £b
Amount of TiC
Amount of T a C
Amount of NbC
A m o u n t of WC
(g cm- 3)
(vol.%)
(vol.%)
(vol.%)
(vol.%)
(vol.%)
15.01 14.74 14.20 14.10 14.09 13.94
10.1 13.3 9.6 11.1 12.7 14.90
--6.5 6.4 5.7 5.6
-1.8 5.5 5.4 4.4 4.8
-0.4 1.1 1.1 1.0 1.0
89.9 84.5 77.3 76.0 76.2 73.7
Mechanical properties of the cemented-carbide materials Hc
Rockwell A
(kA m i)
hardness
(HRA)
Vickers hardness
TRS
TRS
(lbfin -2)
(N mm 2)
320000 330000 350000 330000 400000 255000 270000 290000 270000 290000 320000 340000 330000
2250 2300 2450 2300 2800 1800 1900 2050 1900 2050 2250 2400 2300
(load, 30 k~ (HV30)
A1 A2 A3 A4 B1 C1 C2 C3 D1 D2 E1 F1 F2
18.3 14.2 13.7 10.0 9.2 17.6 14.7 13.9 18.3 11.0 14.7 13.1 10.7
92.4 91.6 91.5 90.8 89.0 92.5 92.1 92.0 92.2 90.9 91.5 91.2 90.8
1750 1600 1570 1460 1250 1650 1570 1550 1600 1420 1500 1450 1400
291
the test according to the short rod method [9]. Table 3 summarizes Kit results, The TRS samples were prepared for metallographic examination. A most representative sample area was finely examined with an optical microscope and a photograph was taken (Figs. 1 and 2). The mean particle intercept distance was measured on each photograph by a microscope with a coordinate table, and a measurement fixture device capable of movement in the x-y direction with an accuracy of 1 mm. The distance length for measurement was 30 ram, and in each picture measurements were made at five cross-sections, This procedure was followed on 25 different compositions and various grain sizes. The coercive force for each sample was measured. Figure
3 shows the results of the d and H c measurements. 3. Results anddiscussion Figure 3 shows the mean intercept length of d, referred to as the mean WC grain size, as a function of coercive force H c (kA m -t) for various cobalt volume percentages on a log-log scale. It is observed that for a given d (ram) the coercive force decreases as the cobalt volume percentage increases. For a given cobalt volume percentage, the coercive force H c increases as the grain size decreases. From this graph, the following equation is deduced: L.~~:.... (8()/°s7' = 0.3 ( ~ ] (#m) (1) From the work of Underwood [7], the relationship between the mean intercept length and the mean free path of binder is
Vv (#m)
~.=[t
(2)
1-V~
Equation (1) can be transformed into a more practical and meaningful relation, as follows:
....
il
§
p
=o.3 l
....
[ i---------I~,,~i
Fig. 1. Scanning electron micrograph (H,,=14.2kAm 1; c)= 2.5 #m).
of
sample
A2
(#m)
- Vv \ H ~ ]
(3)
By introducing the weight percentage of cobalt instead of Vf using Xco Pth VF -
(4)
8.9 100
~0.0
~
6.0 ~
°c2 ° 0 ~ \ \ \
24% co-~C/~_'~"-.'%. \ ~ s . o % ,oo o co ~ T 13.3 % ' ~
.o 3.0
2"0" d(#m no.
iO.O
k
co " ~'
"--
I
L. . . . . . . . . . .
-I4
.
~
/
ij
~
i
]
o37-
~i
f
.... ; o u c ~ ~. Theoretical
.
.
.
.
.
o 30
.
.
.
.
2° t ~.o
0.3
0,0
0.0
Fig. 2. Microstructure of sample F2 (Hc=10.7 kA m-I; d = 2.6 #m).
IO0
200 300
Iooo
Fig. 3. Average grain size vs. coercive force for different cobalt volume percentages.
292
z5
i . [
size for cemented-carbide materials which differ in composition, grain size and cobalt content. (2) Equation (5) gives the mean free path of cobalt, using the cobalt weight percentage, the coercive force and the theoretical specific weight. (3) Kic results for the calculated mean free path are in agreement with other published data. (4) Our method is a valuable, useful and nondestructive way of determining material toughness without requiring actual fracture toughness testing.
~ z0
J
_ 15 'e *no 0
D~, 2"e~'" ,B/.. ,.,~;.~¢ ~.~," " • • m eF . A c~ '
.
I0 ~ , r C •
References
5
0 0.0
0.5
1.0
1.5
(lO"-6*m) Fig. 4. Fracture toughness as a function of the mean free path of cobalt. w e obtain the following final expression for ,~: 0"3Xpth )~
(801(164.822/XcoP~h),/3
890--XcoPt h \Hcc]
(/~m)
(5)
The validity of this equation was tested by measuring the fracture toughnesses Kjc of known compositions, using the short-rod method. Table 3 shows the coercive force, the calculated ~ (from eqn. (5)) and the measured Kic for the various materials. Figure 4 is a graph which shows K~c as a function of 2 using published data [1-6]. Our measured K~ values, labelled by letters indicating the compositions, are seen to fit the distribution averages very well. 4. Conclusions (1) Coercive force measurement makes it possible to calculate the average carbide grain
1 R. C. Leuth, Ph.D. Thesis, Michigan State University, 1972. 2 J. L. Chermant and F. Osterstock, J. Mater. Sci., 11 (1976). 3 L. Lindau, in D. M. R. Taplin (ed.), Fracture 1977, Proc. 4th Int. Conf. on Fracture, Waterloo, Ontario, June 19-24, 1977, University of Waterloo Press, Waterloo, Ontario, 1977. 4 J. R. Pickens and J. Gurland, Mater. Sci. Eng., 33 (1978) 135. 5 R. Porat and A. Ber, £TRPAnn., 32 ( 1 ) (1983). 6 Z. Goffer and R. Porat, Rep. EURO CVD 4, 1983 (Eindhoven). 7 Wesley,E' E. Underwood,Reading, MA,Quantitative1970. Stereology, Addison8 J. Gurland, Trans. Metall. Soc. AIME, 236 (1966). 9 L. M. Barker, Rep. TR77-91R, 1977 (Salt Lake City,
UT). 10 M.T. Laugier, Powder Metall. Int., 5 (1986).
Appendix A: N o m e n c l a t u r e d H~ Vv Xco
mean intercept length (mean grain size) of WC coercive force (kA m-~), volume fraction of cobalt weight percentage of cobalt
Greek symbols )~ Pth
mean free pass of cobalt binder theoretical density of the composition