Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~~~ 3
Backq~ound of the Invention
The present invention relates to a heat-resistant sintered
hard alloy, composed of a hard phase consisting mainly of a WCoB
type complex boride, and a cobalt base alloy matrix phase binding
the hard phase which hard alloy exhibits excellent room temperature
characteristics as well as excellent high temperature
characteristics such as high temperature strength and oxidation
resistance, and as a hot extruding die for a copper rod.
Requirements for wear-resistant sintered hard materials have
become increasingly severe, and the industry has sought improved
materials having wear-resistance as well as heat-resistance and
corrosion resistance or the like.
As sintered hard materials, carbides, nitrides and
carbonitrides such as WC base hard alloys and TiCN type cermets are
well known. ~s substitute materials for t~le aforementioned hard
materials, hard alloys and cermets including metallic borides such
as WB and TiBz, and metallic complex borides such as MozFeBz and
Mo2NiB2 have been recently proposed, noting excellent properties of
borides such as extreme hardness, high melting point and high
electric conductivity. Further, stellites are utilized as cobalt
base wear-resistant materials.
A hard alloy formed by binding WB with a nickel base alloy
such a--. disclosed i.n Japanese Patent Pu~licat.ions No. Sho
CA 02060723 1998-03-17
56-45985, No. Sho 56-45986 and No. Sho 56-45987 is a
paramagnetic wear-resistant material to be used
especially in watch cases and ornaments, and is not
intended for structural materials to be used at high
temperature.
Ceramics comprising metallic borides such as TiB2
as disclosed in Japanese Patent Publications No. Sho 61-
50909 and No. Sho 63-5353 exhibit extreme hardness and
pronounced heat resistance, but impart poor thermal
shock resistance due to there being no metallic binding
matrix phase.
Generally, hard materials formed by adding metals
to metallic borides suffer from the disadvantage in that
they tend to form a brittle third phase, and it is
difficult to obtain high strength or toughness.
Hard alloys comprising metallic complex borides
such as Mo2FeB2 and Mo2NiB2 formed by reaction during
sintering have been developed to eliminate the above
disadvantage.
A Mo2FeB2 type hard alloy disclosed in Japanese
Patent Publication No. Sho 60-57499 has excellent
mechanical properties, wear-resistance and corrosion
resistance at room temperature but unsatisfactory high
temperature strength and oxidation resistance due to its
iron base binding matrix phase.
A Mo2NiB2 type hard alloy disclosed in Laid Open
Japanese Patent application No. Sho 62-196353 has
excellent high temperature properties and corrosion
resistance, but poor wear-resistance and anti-
CA 02060723 1998-03-17
adhesion property, since the complex boride Mo2NiB2 is
about 15 GPa at micro-Vickers hardness and is not so
hard, and its binding phase consists of nickel base
alloy. Stellites exhibit excellent high temperature
properties, but their hardness is too low to be used for
wear-resistant materials.
It is an object of the present invention to provide
a sintered hard alloy having excellent room temperature
properties as well as pronounced high temperature
properties such as high temperature strength and
oxidation resistance.
Summary of the Invention
According to the present invention, there is
provided a heat-resistant sintered hard alloy comprising
35 to 95% by weight of a WCoB type complex boride and a
cobalt base alloy matrix phase. The hard alloy may
consist of boron of 1.5 to 4.1% by weight, tungsten of
19.1 to 69.7% by weight with the balance being cobalt
and unavoidable impurities. In addition to the above
elements, the hard alloy may contain chromium of 1 to
25% by weight for the improvement of mechanical
properties and corrosion resistance. Further, the hard
alloy may comprise boron of 1.5 to 4.1% by weight,
tungsten of 19.1 to 69.7% by weight, chromium of 1 to
25% by weight, and at least one of nickel, iron and
copper. Nickel, when present, substitutes for cobalt in
the ramge of 0.2 to 30% by weight of cobalt content.
Iron when present, substitutes for cobalt in the range
CA 02060723 1998-03-17
of 0.2 to 15% by weight of cobalt content. Copper, when
present, substitutes for cobalt in the range of 0.1 to
7.5% by weight of cobalt content. The balance of this
alloy consists of cobalt and unavoidable impurities.
Detailed Description of the Invention
In this description, WCoB and a complex boride
identified as WCoB by means of x-ray diffraction
comprising tungsten and cobalt, in which part of
tungsten may be replaced by chromium and part of the
cobalt may be replaced by chromium, nickel, iron or
copper, will be referred to as a WCoB type complex
boride.
The WCoB type complex boride offers the following
advantages. The formation of a brittle third phase,
which tends to be formed in a boride base hard alloy,
can be suppressed by forming the WCoB type complex
boride by reaction during sintering. The micro-Vickers
hardness of the WCoB type boride is larger than 30 GPa,
and higher than those of other metallic complex borides
such as Mo2FeB2 and Mo2NiB2, and the same as or higher
than those of carbides and nitrides which are currently
used for hard materials. Further, the WCoB type complex
boride has excellent oxidation resistance.
In the case where the content of the WCoB type
complex boride is less than 35% by weight, the wear
resistance of the hard alloy is reduced due to the
insufficient amount of the complex boride,
and is liable to marked deformation at hi~h temperature due to
insufficient development of complex boride networks in the cobalt
base alloy matrix phase. On the other hand, in the case where the
content of the WCoB type complex boride is more than 95% by weight,
the strength of the hard alloy is remarkably decreased, though its
hardness is increased. For the above reason, it is preferable that
the content of the WCoB type complex boride be 35 to 95% by weight.
Boron is an essential element for forming the WCoB type
complex boride in the heat-resistant sintered hard alloy. With
boron less than 1.5% by weight, the complex boride is less than 35%
by weight, and with boron more than 4.1% by weight, the complex
bor~de is over 95% by weight, leading to a pronounced decrease in
the strength of the hard alloy. For the above reason, it is
preferable that the amount of boron in the hard alloy be from 1.5
to 4.1% by weight.
Tungsten is also an essential element for forming the WCoB
type complex boride. The stoichiometric ratio in the WCoB type
complex boride is such that W:Co:B = 1:1:1. The WCoB type complex
boride which is practically applicable, however, need not be a
perfectly stoichiometric compound, but may have a composition
variance of a few percent. Accordingly, the molecular ratio of W/B
(hereafter will be referred to as W/B ratio) need not be 1, but it
i~ importilnt tha~ the W/B ratio be within a specific range
including 1 as the approximate centre.
-- 6
2¢~ 3
Test results indicate that in the case where the W/B ratio is
far smaller than l, cobalt borides such as Co2B is ormed, and in
the case where the W/B ratio is far larger than 1, intermetallic
compounds of tungsten and cobalt such as W6Co7 are formed, leading
to a decrease in the strength of the hard alloy in both cases.
When the W/B ratio is within the range of 0.75 to 0.135 x
(11.5-X), where X indicates the content of boron by weight percent,
even if the above third phase is formed, the third phase will
little affect the strength of the hard alloy; i.e., there would
be an allowable decrease in the strength.
In the case where the W/B ratio is larger than 1, part of
excess tungsten will be solid solute into the cobalt base alloy
matrix phase, which will strengthen the matrix phase, thus
improving the mechanical properties of the heat-resistant sintered
hard alloy. ~owever, since the amount of the cobalt base alloy
matrix phase decreases with the increase of the amount of the WCoB
type complex boride, it is necessary to decrease the amount of said
excess tungsten in the matrix phase accompanied by the above
increase, so as to maintain the strength of the hard alloy.
For the above reason, it is preferable that the upper limit
of the amount of tungsten be 1.35 in terms of the W/B ratio in the
case where the amount of boron is lowest (1.5% by weight), and 1
in terms of the W/B ratio in the case where the amount of boron is
highest (~.1% by weight). This range is represented by the formula
-- 7
,, ,
2~
0.135 x (11.5-X), in which X is the weight percent of boron.
Accordingly, it is desirable that the amount of tungsten in
the hard alloy be in the range of from 0.75 to 0.1~5 x (11.5-X),
preferably in the range of 0.~ to 0.135 x (11.5-X) in terms of the
W/B ratio; that is, from 19.1 to 69.7% by weight, preferably from
20.4 to 69.7% by weight, in said hard alloy.
In the case of a sintered hard alloy containing chromium, it
is presumed that chromium will be solid solute into the WCoB type
complex boride, and form a (wxcoycrz)s multiple boride of the WCoB
type complex boride, in which cobalt rather than tungsten is
replaced partially by chromium and x + y + z is equal to 2, and
further chromium will be solid solute into the cobalt base alloy
matrix also, so that the resistances to corrosion, heat and
oxidation of the sintered hard alloy will be improved.
Furthermore, chromium refines the (WxCoyCrz)8 multiple boride
pha6e and improves the mechanical properties of the sintered hard
alloy. With a content of chromium below 1% by weight, the above-
mentioned improvement can not be attained, and with the content of
chromium above 25% by weight, the mechanical properties of the
6intered hard alloy are remarkably decreased due to the generation
of a brittle phase such as a CoCr sigma (a) phase. Accordingly,
it is preferable that the content of chromium be from 1 to 25% by
weight.
2~2~3
,........................................................................ .
.
In the case of a sintered hard alloy containing nickel, it is
presumed that nickel will substitute for cobalt and be solid solute
into the co~alt base alloy matrix phase, and improve the mechanical
properties, corrosion resistance and heat-resistance of the hard
alloy. With the substitution of nickel below 0.2% by weight of
cobalt content, the aforementioned improvements of mechanical
~'
properties and the like can not be attained, and with the
substitution of nickel above 30% by weight of cobalt, abrasion
resistance is reduced due to the decrease of hardness.
Accordingly, it i5 preferable that nickel substitute for cobalt in
the range of 0.2 to 30 % by weight of cobalt content.
Iron substitutes mainly for cobalt in the WCoB type complex
boride and the cobalt base alloy matrix phase, and improves the
strength at low temperature. With the substitution of iron below
0.2% by weight of cobalt content, the aforementioned improvement
is not attained, and with the substitution of iron more than 15%
by weight of cobalt content, the hard alloy becomes less resistant
to corrosion, heat and oxidation. Accordingly, in the case of the
sintered hard alloy containing iron, it is preferable that iron
substitute for cobalt in the range of 0.2 to 15% by weight of
cobalt content.
Copper substitutes for cobalt and is solid solute into the
cobalt base alloy matrix phase, and improves the corrosion
~esistance and heat c~nductivity of the sintered hard alloy. With
the substitution of copper below 0.1% by weight of cobalt content,
:, _ g _
Z~7.~3
the above improvements are not attained, and with the substitution
of copper more than 7.5% by weight o~ cobalt content, the
mechanical properties and heat-resistance are degraded.
Accordingly, it is preferable that copper substitute for cobalt in
the range of 0.1 to 7.5% by weight of cobalt content, when copper
is added to the sintered hard alloy.
The unavoidable impurities contained in the sintered hard
alloy are mainly silicon, aluminum, manganese, magnesium,
phosphorus, sulfur, nitrogen, oxygen, carbon or the like, and it
is desirable that the content of these impurity elements be as
little as possible. However, in the case where the total amount
of these impurity elements is less than 1.0% by weight, the
detrimental effects thereof to the properties of the sintered hard
alloy are relatively small. Accordingly, it is preferable that the
total content of the unavoidable impurities be less than 1.0% by
weight, more preferably less than 0.5% by weight.
In the case where the sintered hard alloy is employed for a
wear-resistant coating in which the strength is not of critical
importance, and silicon and aluminum or the like are added
intentionally so as to improve the oxidation resistance of the
coating, the total content of the aforementioned elements may be
over 1.0% by weight.
The sintered hard alloy is made by mixing boride powders of
tungstel~, cobalt, chromium, nickel and iron; alloy powders of
boron, with at least one of tungsten, cobalt, chromium, nickel,
-- 10 --
iron and copper; or boron powder and metal powders of tungsten,
cobalt, chromium, nicXel, iron and copper, or alloy powders
containing at least two of these metallic elements, thereafter wet
milling the mixture with an organic solvent by means of a vibrating
ball mill or the like, drying, granulating, and forming, followed
by liquid phase sintering of the green compact in a non-oxidizing
atmosphere such as in vacuum, a reducing gas, or an inert gas.
The hard phase, that is the WCoB type complex boride of the
sintered hard alloy, is formed by the reaction during sintering.
A powder mixture obtained by blending metal powders such as cobalt,
chromium and nickel to form the Co base alloy matrix phase, with
the WCoB type complex boride such as WCoB and (WxCoyCrz)B which are
prepared by reacting tungsten boride, cobalt boride, boron powder
with metal powders such as tungsten, cobalt and chromium etc. in
a furnace in advance, may be employed as the raw material powders
also.
The liquid phase sintering is usually carried out at the
temperature range of 1100 to 1400~C and for 5 to 90 minutes
depending on the composition of the hard alloy. A hot press
method, a hot isostatic pressing method, and an electric resistance
sintering method or the like ma~ be also employed.
-- 11 --
CA 02060723 1998-03-17
EXAMPLES
The compound powders listed in Table 1 and metal
powders listed in Table 2 were blended in the
compositions shown in Table 3 with the blending ratios
shown in Table 5. The blended powders were wet milled
with acetone by means of a vibrating ball mill for 28
hours and then dried and granulated. The powders thus
obtained were pressed into a predetermined shape. The
green compacts were sintered at the temperature of 1150
to 1300~C for 30 minutes in vacuum.
The transverse rupture strength and Rockwell A.
scale hardness (RA) at room temperature, the transverse
rupture strength at 900~C, and the weight gain by
oxidation after holding at the temperature of 900~C for
1 hour in still air of the samples of the hard alloys
thus obtained are shown in Table 7.
Sample Nos. 1 to 10 all show extreme hardness ~and
high transverse rupture strength at room temperature as
well as high transverse rupture strength and excellent
oxidation resistance at the high temperature. A hot
extruding die was prepared using the hard alloy of
sample No. 6, and a pure copper rod was extruded through
the die. It was possible to extrude the rod 50 to 100
times satisfactorily. A similar die formed with a WC-Co
type hard alloy could not be used practically for the
pure copper rod hot extrusion.
- 12 -
COMPARATIVE EXAMPLES
The compound powders listed in Table 1 and the metal powders
listed Table 2 were blended in the composition shown in Table 4
with the blending ratios shown in Table 6.
The hard alloys were prepared by the same method as shown in
the EXAMPLES, and the properties thereof are shown in Table 8.
Sample No. 11 has a W/B ratio less than 0.75, and exhibits low
transverse rupture strength at room temperature as well as the high
temperature. Sample No. 12 exhibits low transverse rupture
strength at the high temperature and poor oxidation resistance due
to the content of iron being higher than 10% by weight, though it
shows high transverse rupture strength at room temperature. Sample
No. 13, containing a MoCoB type complex boride instead of the WCoB
type complex boride, exhibits low transverse rupture strength at
room temperature as well as the high temperature, compared with the
6amples of EXAMPLES ha~ing approximately the same hardness. Sample
No. 14 containing a Mo2FeB2 type complex boride exhibits low
transverse rupture strength at high temperature and po~r oxidation
resistance.
A similar hot extruding die as described in the EXAMPLES was
prepared using the hard alloy of Sample No. 14, and a copper rod
was extruded in the same manner as in the case of the EXAMPLES.
~nly 5 to 10 times extruding was possible with the die.
- 13 -
T~ble 1
Compound ~3 C N ~ W Fe . Cr Mo
powder wt% wt% wt% wt96 wt% wt% w t% wt~
WB 5.5 O.Q3 0.1 0~07g4.3 - - -
C~ 17.4 0.20 0.04 0.16 - - ~2.2
MoE~ 10.0 O.05 0.02 0.2 - O.03 - 8g.7
'rable 2
Met~l Purlt~ Mat~l Purity
powder wt96 powder wt%
W 99 . 95 Fe 99 . 69
Cr 99 . 75 Cu 99 . 9
Nl 9g . 75 CO 99 . 87
t
-- 14 --
~a~ 3
Table 3
Sampl~ Composltlon ( wt96 ) W~ ~unt of
No. ratio compl~x
B W Cr N1 Fe C~l Co borlde ( wt% )
3.0 51.4 ~ bsl. l.0 71
2 1.9 35.5 15.0 - - - bal. l.1 44
3 1.9 42.0 10.0 - - - ~al. 1.3 44
4 2.2 29.9 15.0 - - - bBl. 0~8 41
3 . 053 . ~15, O - _ - bal . 1. 05 ~0
6 2.0 34.3 21.0 5.0 - - bal. 1.0 46
7 3.8 S8~2 5.0 1.0 - - bal. 0.9 80
a 1.7 29.1 21.0 S.0 5.0 - bal. l.0 39
9 2.5 46.8 10.0 10.0 û.2 - bal. 1.1 58
1.9 33.5 10.0 3.0 - 2.0 bsl l.0 44
/
~able 4
Ssmple Compo~itlon (wt96 ) W/E3 Amount of
No, ratlo co~pl~x
B W Cr N~ Fe Mo Co bor~d~ ( wt~ )
11 2.4 2a.6 7.0 - - - bal. 0.7 39
12 3.0 51.4 5.0 - 15.0 - bal. ~.0 70
13 3.0 - 21.0 5.0 26.9 bal. - MoCoB 45
lg 4.0 - 17.1 10.0 bsl.33.7 - - Mo2FeB2 5
-- 15 --
~r~~~7 ~3
Table S
Sample WB W Cr tJi F~ Cu CrE3 Cc~
No. wt96 wt96 wt% wt% wt% wt~ wt% wt%
54.5 ~ 45.5
234.53.(~15.0 ~ 7~5
:~34.5 g.510.0 - ~ - -46.0
431.7 - 12.g - - - 2.652.8
554.5 2.05.0 - - - -38.5
636.4 - 21.0 5.0 - - -37.6
761~7 - ~.0 1.0 - - 2.431.9
830.9 - 21.0 ~ 0 5.0 - -3g.1
945.5 3.910.0 10.0 0.2 - -30.~
1034.5 - 10.0 3.0 - 2.0 -5~.5
Tabls 6
Sample WB W Cr Ni Fe MoB C:r~3 CO
No. wt% wt~ wt~ wt% wt~6 wt% wt% w~g6
1130.3 - 3.5 - - - 4.262.0
1254.5 - 5.0 - 15.0 - -25.5
13 - - 21.0 5.0 - 30.0 -44.0
14 - - 16.0 10.0 35.137.6 1.3
-- 16 --
CA 02060723 1998-03-17
Table 7
Sample Transverse Hardness Transverse Oxidation
No. rupture rupture weight
strength strength gain
(RT, GPa) (RA) (soooc~ GPa) (mg/mm2/h)
1 1.95 82.7 1.79 9.76
2 3.08 79.2 1.90 0.84
3 2.67 79.2 1.94 1.27
4 2.24 78.3 1.95 0.42
2.29 84.5 1.97 4.24
6 2.01 77.9 1.80 0.84
7 1.85 89.5 1.71 3.18
8 2.56 76.2 1.83 0.84
9 2.46 80.8 2.03 1.15
2.70 78.0 1.81 1.39
Table 8
Sample Transverse Hardness Transverse Oxidation
No. rupture rupture weight
strength strength gain
(RT, GPa) (RA) (goo~c, GPa) (mg/mm2/h)
11 1.63 81.6 1.42 6.37
12 2.31 85.5 1.63 13.9
13 1.81 78.7 1.28 1.63
14 1.93 79.1 1.39 20.4
