U.S. patent application number 11/248180 was filed with the patent office on 2006-02-09 for method of making a fine grained cemented carbide.
This patent application is currently assigned to Sandvik AB. Invention is credited to Per Gustafson, Susanne Norgren, Mats Waldenstrom.
Application Number | 20060029511 11/248180 |
Document ID | / |
Family ID | 29552453 |
Filed Date | 2006-02-09 |
United States Patent
Application |
20060029511 |
Kind Code |
A1 |
Gustafson; Per ; et
al. |
February 9, 2006 |
Method of making a fine grained cemented carbide
Abstract
According to the present invention there is provided a method of
making a finegrained tungsten carbide--cobalt cemented carbide
comprising mixing, milling according to standard practice followed
by sintering. By introducing nitrogen at a pressure of more than
0.5 atm into the sintering atmosphere after dewaxing but before
pore closure a grain refinement including reduced grain size and
less abnormal grains can be obtained.
Inventors: |
Gustafson; Per; (Huddinge,
SE) ; Norgren; Susanne; (Huddinge, SE) ;
Waldenstrom; Mats; (Bromma, SE) |
Correspondence
Address: |
DRINKER BIDDLE & REATH (DC)
1500 K STREET, N.W.
SUITE 1100
WASHINGTON
DC
20005-1209
US
|
Assignee: |
Sandvik AB
|
Family ID: |
29552453 |
Appl. No.: |
11/248180 |
Filed: |
October 13, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10873234 |
Jun 23, 2004 |
|
|
|
11248180 |
Oct 13, 2005 |
|
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|
Current U.S.
Class: |
419/18 ;
419/57 |
Current CPC
Class: |
B22F 2998/10 20130101;
C22C 29/08 20130101; B22F 3/1007 20130101; B22F 3/1007 20130101;
B22F 2201/02 20130101; B22F 2998/10 20130101; C22C 1/051 20130101;
B22F 2999/00 20130101; B22F 2999/00 20130101; B22F 9/04 20130101;
B22F 2005/001 20130101 |
Class at
Publication: |
419/018 ;
419/057 |
International
Class: |
B22F 3/10 20060101
B22F003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 25, 2003 |
SE |
0302131-8 |
Oct 28, 2003 |
SE |
0302835-4 |
Claims
1. In a method of making a finegrained tungsten carbide--cobalt
cemented carbide comprising mixing, milling and sintering, said
carbide containing from about 4 to about 15 weight percent cobalt
and minor amounts of grain growth inhibitors, the improvement
comprising introducing nitrogen at a pressure of more than about
0.5 atm into the sintering atmosphere after dewaxing but before
pore closure.
2. In the method of claim 1 wherein nitrogen is introduced at a
pressure more than about 0.75 atm.
3. In the method of claim 1 wherein the nitrogen is introduced into
the sintering process before the sintering temperature reaches
about 1000.degree. C.
4. In the method of claim 1 wherein the entire sintering process is
performed in nitrogen.
5. In the method of claim 1 wherein the nitrogen after pore closure
is replaced by a protective atmosphere.
6. In the method of claim 5 wherein the protective atmosphere is
argon or a vacuum.
7. (canceled)
8. In the method of claim 1 wherein the grain growth inhibitors are
metals or compounds of Cr, V and/or Ta.
9. In the method of claim 8 wherein the grain growth inhibitors are
metals or compounds of Cr and/or Ta.
10. In the method of claim 1 wherein said compounds are free of
nitrogen.
11. In the method of claim 1 wherein said compounds are carbides.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method of making a fine
grained cemented carbide. By performing the sintering at least
partly in a nitrogen-containing atmosphere, a grain refined
cemented carbide structure has been obtained.
[0002] Cemented carbide inserts with a grain refined structure are
today used to a great extent for machining of steel, stainless
steels and heat resistant alloys in applications with high demands
on both toughness and wear resistance. Another important
application is in microdrills for the machining of printed circuit
board so called PCB-drills.
[0003] Common grain growth inhibitors include vanadium, chromium,
tantalum, niobium and/or titanium or compounds involving these.
When added, generally as carbides, they limit grain growth during
sintering, but they also have undesirable side effects such as
unfavorably affecting the toughness behavior. Additions of vanadium
or chromium are particularly detrimental and have to be kept on a
very low level in order to limit their negative influence on the
sintering behavior. Both vanadium and chromium reduce the sintering
activity often resulting in an uneven binder phase distribution and
toughness reducing defects in the sintered structure. Large
additions are also known to result in precipitation of embrittling
phases in the WC/Co grain boundaries. According to WO 99/13120, the
amount of grain growth inhibitors can be reduced if a carbon
content of the cemented carbide close to eta-phase formation is
chosen.
[0004] In order to maintain a fine grain size, sintering is
generally performed at a relatively low temperature of 1360.degree.
C. followed by sinterHIP in order to obtain a dense structure. Such
production route, of course, increases the production cost.
[0005] It is known that tungsten carbonitride can be produced by
high pressure nitrogen treatment of a mixture of tungsten and
graphite powder. The process is described in JP-A-03-208811 and
JP-A-11-35327 and it is claimed that the resulting tungsten
carbonitride powder can be used as a raw material for manufacturing
of super hard alloys. JP-A-11-152535 discloses a process to
manufacture fine grained tungsten carbonitride--cobalt hard alloys
using tungsten carbonitride as a raw material.
[0006] JP-A-10-324942 and JP-A-10-324943 disclose methods to
produce ultra-fine grained cemented carbide by adding the grain
growth inhibitors as nitrides. In order to avoid pore formation by
denitrification of the nitrides sintering is performed in a
nitrogen atmosphere.
OBJECTS AND SUMMARY OF THE INVENTION
[0007] It is an object of the present invention to avoid or
alleviate the problems of the prior art. It is further an object of
the present invention to provide a cemented carbide insert with a
combination of high toughness and high deformation resistance along
with a method for making the same.
[0008] There is provided in a method of making a finegrained
tungsten carbide--cobalt cemented carbide comprising mixing,
milling and sintering, the improvement comprising introducing
nitrogen at a pressure of more than about 0.5 atm into the
sintering atmosphere after dewaxing but before pore closure.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0009] FIG. 1 shows in about 1500.times. a typical example of the
structure of a "pure" WC--Co grade, alloyed with nitrogen by
sintering according to the invention.
[0010] FIG. 2 shows in about 1500.times. a typical example of the
structure of the same grade sintered according to prior art.
[0011] FIG. 3 shows in about 1500.times. a typical example of the
structure of the same grade, alloyed with nitrogen by sintering
according to the invention, after sintering at reduced
temperature.
[0012] FIG. 4 shows in about 1500.times. a typical example of the
structure after conventional sintering at reduced temperature.
[0013] FIG. 5 shows in about 1200.times. a typical example of the
structure of the Cr.sub.3C.sub.2 containing WC--Co grade, alloyed
with nitrogen by sintering according to the invention, after
sintering at reduced temperature.
[0014] FIG. 6 shows in about 1200.times. a typical example of the
structure of the same grade after conventional sintering at reduced
temperature.
[0015] FIG. 7 shows in about 1200.times. a typical example of the
structure of a "pure" submicron (0.25 .mu.m) WC--Co grade, alloyed
with nitrogen by sintering according to the invention.
[0016] FIG. 8 shows in about 1200.times. a typical example of the
structure of the same grade sintered according to prior art.
[0017] FIG. 9 shows in about 1200.times. a typical example of the
structure of a Cr.sub.3C.sub.2 containing submicron 0.25 .mu.m
WC--Co grade, alloyed with nitrogen by sintering according to the
invention.
[0018] FIG. 10 shows in about 1200.times. a typical example of the
structure of the same grade after conventional sintering.
[0019] FIG. 11 shows in about 1200.times. a typical example of the
structure of a Cr.sub.3C.sub.2 containing submicron 0.6 .mu.m
WC--Co grade, alloyed with nitrogen by sintering according to the
invention.
[0020] FIG. 12 shows in about 1200.times. a typical example of the
structure of the same grade after conventional sintering.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE
INVENTION
[0021] It has now surprisingly been found that a pronounced grain
refining effect in combination with an improved binder phase
distribution can be obtained by introduction of nitrogen as a
process gas in sintering furnace prior to pore closure.
[0022] The method according to the present invention comprises
mixing, milling and pressing of tungsten carbide--cobalt bodies
according to conventional powder metallurgical methods followed by
sintering in a process characterized by the introduction of
nitrogen at a pressure of more than 0.5 atm, preferably more than
0.75 atm, into the sintering atmosphere after dewaxing but before
pore closure, preferably before 1000.degree. C.
[0023] In one embodiment, the whole sintering process is performed
in nitrogen.
[0024] In an alternative embodiment, the nitrogen is after pore
closure replaced by a protective atmosphere of e.g. argon or
vacuum.
[0025] The resulting sintered body is characterized by a grain
refined structure, reduced grain size and less abnormal grains, in
combination with an improved binder phase distribution compared to
sintering according to normal practices, and with a nitrogen
content of more than 0.03 weight-%, preferably more than 0.05
weight-%.
[0026] The cobalt content for these alloys should be in the range 4
to 15 weight-%, preferably 5 to 12 weight-%.
[0027] The average number of abnormal grains can be determined
using inserts etched for 2 minutes at room temperature in Murakamis
regent, examining the etched surface with optical microscope at
1500.times., counting the number of abnormal grains on ten
micrographs, taken randomly from the surface, and calculating the
average number of abnormal grains per micrograph. Each micrograph
corresponds to a surface area of 8360 .mu.m.sup.2.
[0028] Using the process of the invention with a pure WC--Co alloy,
the average number of abnormal grains per micrograph, having a
maximum length in any direction >15 .mu.m, is <1.0,
preferably <0.7. The average number of abnormal grains per
micrograph, having a maximum length in any direction >20 .mu.m,
is <0.5. The average number of abnormal grains per micrograph,
having a maximum length in any direction >25 .mu.m, is
<0.1.
[0029] Using the process of the invention with a WC--Co alloy
containing grain growth inhibitors the average number of abnormal
grains per micrograph, having a maximum length in any direction
>5 .mu.m, is <0.15.
[0030] For a WC grain size below 0.5 .mu.m, the beneficial effect
of nitrogen alloying has to be combined with an addition of
conventional grain growth inhibitors from groups IVb, Vb and/or VIb
of the periodic table, preferably Cr, V and/or Ta, most preferably
Cr and/or Ta, either as pure metals or compounds thereof except the
nitrides thereof, preferably compounds free of nitrogen, most
preferably carbides.
[0031] The process of the invention works on pure WC--Co alloys as
well as on WC--Co alloys containing grain growth inhibitors. But
the most significant improvement regarding grain growth control has
been seen for straight WC--Co alloys with a sintered average grain
size of <1.5 .mu.m, preferably <1 .mu.m but larger than 0.5
.mu.m where no further grain growth inhibitors are necessary.
[0032] It has thus been found that the introduction of nitrogen
into the sintering furnace after dewaxing but before pore closure
results in a significant nitrogen pickup even for nominally pure
WC--Co alloys. It has further surprisingly been found that the
introduced nitrogen acts as a grain growth inhibitor at the same
time as it improves the sintering activity and thus the resulting
binder phase distribution. It has also been found that the nitrogen
content achieved before pore closure becomes entrapped as soon as
the temperature becomes high enough for pore closure. Extended
sintering time in vacuum after pore closure has been found to have
only a minor effect on the resulting nitrogen content in the as
sintered samples.
[0033] The invention is additionally illustrated in connection with
the following Examples, which are to be considered as illustrative
of the present invention. It should be understood, however, that
the invention is not limited to the specific details of the
Examples.
EXAMPLE 1
[0034] From a powder mixture consisting of 6.0 weight-% Co, and
balance WC with an average grain size of about 1 .mu.m with 0.01
weight-% overstoichiometric carbon content, turning the inserts
CNMG120408 were pressed. The inserts were sintered in H.sub.2 up to
450.degree. C. for dewaxing. At 450.degree. C., the furnace was
evacuated and refilled with nitrogen up to a pressure of 0.8 atm.
The temperature was kept constant at 450.degree. C. during the
nitrogen filling procedure. After completed filling, the
temperature was increased to 1370.degree. C. with a speed of
15.degree. C./min, keeping the nitrogen pressure constant. At
1370.degree. C., the furnace was evacuated and refilled with a
protective atmosphere of 10 mbar Argon and kept at 1370.degree. C.
for 30 minutes followed by an increased Ar pressure of 40 mbar and
a temperature increase up to the final sintering temperature of
1410.degree. C. where the temperature was kept for an additional
hour before cooling and opening of the furnace.
[0035] The structure in the cutting inserts consisted of comparably
fine and uniform tungsten carbide grain size in combination with a
good binder phase distribution, FIG. 1.
EXAMPLE 2
Reference Example to Example 1
[0036] Pressed inserts from Example 1 were sintered in H.sub.2 up
to 450.degree. C. for dewaxing, further in vacuum to 1370.degree.
C., then filled with a protective gas of 10 mbar of Ar and kept at
1370.degree. C. for 30 minutes followed by an increased Ar pressure
of 40 mbar and a temperature increase up to the final sintering
temperature of 1410.degree. C. where the temperature was kept for
an additional hour before cooling and opening of the furnace.
[0037] The structure in the cutting inserts consisted of a
comparably less fine and uniform tungsten carbide grain size in
combination with a acceptable binder phase distribution, FIG.
2.
EXAMPLE 3
[0038] Pressed inserts from Example 1 were sintered in H.sub.2 up
to 450.degree. C. for dewaxing. At 450.degree. C., the furnace was
evacuated and refilled with nitrogen up to a pressure of 0.8 atm.
The temperature was kept constant at 450.degree. C. during the
nitrogen filling procedure. After completed filling, the
temperature was increased to 1370.degree. C. with a speed of
15.degree. C./min, keeping the nitrogen pressure constant. At
1370.degree. C. the furnace was evacuated and refilled with a
protective atmosphere of 10 mbar Argon. The actual sintering was
limited to a 30 min hold at 1370.degree. C. followed by cooling and
opening of the furnace.
[0039] The structure in the cutting inserts consisted of comparably
fine and uniform tungsten carbide grain size in combination with an
acceptable binder phase distribution, FIG. 3.
EXAMPLE 4
Reference Example to Example 3
[0040] Pressed inserts from Example 1 were sintered in H.sub.2 up
to 450.degree. C. for dewaxing and further in vacuum to
1370.degree. C. At 1370.degree. C., the furnace was filled with a
protective atmosphere of 10 mbar Argon. The actual sintering was
limited to a 30 min hold at 1370.degree. C. followed by cooling and
opening of the furnace.
[0041] The structure in the cutting inserts consisted of a
comparably less fine and uniform tungsten carbide grain size in
combination with an unacceptable binder phase distribution, FIG.
4.
EXAMPLE 5
[0042] From a powder mixture consisting of 5.2 weight-% Co, 0.6
weight-% Cr.sub.3C.sub.2 and balance WC with an average grain size
of about 1 .mu.m with 0.05 weight-% overstoichiometric carbon
content, turning inserts CNMG120408 were pressed. The inserts were
sintered in H.sub.2 up to 450.degree. C. for dewaxing. At
450.degree. C., the furnace was evacuated and refilled with
nitrogen up to a pressure of 0.8 atm. The temperature was kept
constant at 450.degree. C. during the nitrogen filling procedure.
After completed filling, the temperature was increased to
1370.degree. C. with a speed of 15.degree. C./min, keeping the
nitrogen pressure constant. At 1370.degree. C. the furnace was
evacuated and refilled with a protective atmosphere of 10 mbar
Argon. The actual sintering was limited to a 30 min hold at
1370.degree. C. followed by cooling and opening of the furnace.
[0043] The structure in the cutting inserts consisted of comparably
fine and uniform tungsten carbide grain size in combination with a
good binder phase distribution, FIG. 5.
EXAMPLE 6
Reference Example to Example 5
[0044] Pressed inserts from Example 5 were sintered in H.sub.2 up
to 450.degree. C. for dewaxing, further in vacuum to 1370.degree.
C. At 1370 the furnace was filled with a protective atmosphere of
10 mbar Argon. The actual sintering was limited to a 30 min hold at
1370.degree. C. followed by cooling and opening of the furnace.
[0045] The structure in the cutting inserts consisted of a
comparably less fine and uniform tungsten carbide grain size in
combination with an unacceptable binder phase distribution, FIG.
6.
EXAMPLE 7
[0046] From a powder mixture consisting of 10.0 weight-% Co, and
balance WC with an average grain size of about 0.25 .mu.m with 0.01
weight-% overstoichiometric carbon content, turning inserts
CNMG120408 were pressed. The inserts were sintered in H.sub.2 up to
450.degree. C. for dewaxing. At 450.degree. C., the furnace was
evacuated and refilled with nitrogen up to a pressure of 0.8 atm.
The temperature was kept constant at 450.degree. C. during the
nitrogen filling procedure. After completed filling, the
temperature was increased to 1370.degree. C. with a speed of
15.degree. C./min, keeping the nitrogen pressure constant. At
1370.degree. C., the furnace was evacuated and refilled with a
protective atmosphere of 10 mbar Argon and kept at 1370.degree. C.
for 30 minutes followed by an increased Ar pressure of 40 mbar and
a temperature increase up to the final sintering temperature of
1410.degree. C. where the temperature was kept for an additional
hour before cooling and opening of the furnace.
[0047] The structure in the cutting inserts consisted of compared
to the reference in example 8 finer large tungsten carbide grains
in combination with a good binder phase distribution, FIG. 7.
EXAMPLE 8
Reference Example to Example 7
[0048] Pressed inserts from Example 7 were sintered in H.sub.2 up
to 450.degree. C. for dewaxing, further in vacuum to 1370.degree.
C., then filled with an protective gas of 10 mbar of Ar and kept at
1370.degree. C. for 30 minutes followed by an increased Ar pressure
of 40 mbar and a temperature increase up to the final sintering
temperature of 1410.degree. C. where the temperature was kept for
an additional hour before cooling and opening of the furnace.
[0049] The structure in the cutting inserts consisted of large
grains and a non-uniform tungsten carbide grain size in combination
with an acceptable binder phase distribution, FIG. 8.
EXAMPLE 9
[0050] Inserts from Example 7 and 8 were etched for 2 minutes at
room temperature in Murakamis regent and examined under optical
microscope at 1500.times.. Ten micrographs were taken. In all ten
micrographs, WC grains having a length in any direction >15
.mu.m were detected and the maximum length for each such grain was
measured. An average number of abnormal grains per micrograph,
corresponding to a surface area of 8360 .mu.m.sup.2, was calculated
by dividing the number of grains by 10. TABLE-US-00001 Average
number of grains with max. length >15 .mu.m >20 .mu.m >25
.mu.m Example 7 (invention) 0.33 0 0 Example 8 (reference) 1.4 0.6
0.2
EXAMPLE 10
[0051] From a powder mixture consisting of 10.0 weight-% Co, 0.5
weight-% Cr.sub.3C.sub.2 and balance WC with an average grain size
of about 0.25 .mu.m with 0.05 weight-% overstoichiometric carbon
content, turning inserts SNUN were pressed. The inserts were
sintered in H.sub.2 up to 450.degree. C. for dewaxing. At
450.degree. C. the furnace was evacuated and refilled with nitrogen
up to a pressure of 0.8 atm. The temperature was kept constant at
450.degree. C. during the nitrogen filling procedure. After
completed filling, the temperature was increased to 1370.degree. C.
with a speed of 15.degree. C./min, keeping the nitrogen pressure
constant. At 1370.degree. C., the furnace was evacuated and
refilled with a protective atmosphere of 10 mbar Argon and kept at
1370.degree. C. for 30 minutes followed by an increased Ar pressure
of 40 mbar and a temperature increase up to the final sintering
temperature of 1410.degree. C. where the temperature was kept for
an additional hour before cooling and opening of the furnace.
[0052] The structure of the cutting inserts consisted of a uniform
submicron tungsten carbide grain size in combination with an almost
absence of large grains and a uniform Co distribution, FIG. 9.
EXAMPLE 11
Reference Example to Example 10
[0053] Pressed inserts from Example 10 were sintered in H2 up to
450.degree. C. for dewaxing, further in vacuum to 1370.degree. C.,
then filled with a protective gas of 10 mbar of Ar and kept at
1370.degree. C. for 30 minutes followed by an increased Ar pressure
of 40 mbar and a temperature increase up to the final sintering
temperature 1410.degree. C. where the temperature was kept for an
additional hour before cooling and opening of the furnace.
[0054] The structure in the cutting inserts consisted of a less
uniform submicron tungsten carbide grain size and in combination
with some large WC grains, FIG. 10.
EXAMPLE 12
[0055] Inserts from Example 10 and 11 were etched for 2 minutes at
room temperature in Murakamis regent and examined under optical
microscope at 1500.times.. Ten micrographs were taken. In all ten
micrographs, WC grains having a length in any direction >5 .mu.m
were detected and the maximum length for each such grain was
measured. An average number of abnormal grains per micrograph,
corresponding to a surface area of 8360 .mu.m.sup.2, was calculated
by dividing the number of grains by 10.
[0056] Result: TABLE-US-00002 Average number of grains with max.
length >5 .mu.m Example 10 (invention) 0-0.1 Example 11
(reference) 0.25-0.4
EXAMPLE 13
[0057] From a powder mixture consisting of 10.0 weight-% Co, 0.5
weight-% Cr.sub.3C.sub.2 and balance WC with an average grain size
of about 0.6 .mu.m with 0.05 weight-% overstoichiometric carbon
content, turning inserts SNUN were pressed. The inserts were
sintered in H.sub.2 up to 450.degree. C. for dewaxing. At
450.degree. C., the furnace was evacuated and refilled with
nitrogen up to a pressure of 0.8 atm. The temperature was kept
constant at 450.degree. C. during the nitrogen filling procedure.
After completed filling, the temperature was increased to
1370.degree. C. with a speed of 15.degree. C./min, keeping the
nitrogen pressure constant. At 1370.degree. C., the furnace was
evacuated and refilled with a protective atmosphere of 10 mbar
Argon and kept at 1370.degree. C. for 30 minutes followed by an
increased Ar pressure of 40 mbar and a temperature increase up to
the final sintering temperature of 1410.degree. C. where the
temperature was kept for an additional hour before cooling and
opening of the furnace.
[0058] The structure in the cutting inserts consisted of a uniform
submicron tungsten carbide grain size and in combination with an
almost absence of large grains and a uniform Co distribution, FIG.
11.
EXAMPLE 14
Reference Example to Example 13
[0059] Pressed inserts from Example 13 were sintered in H.sub.2 up
to 450.degree. C. for dewaxing, further in vacuum to 1370.degree.
C., then filled with a protective gas of 10 mbar of Ar and kept at
1370.degree. C. for 30 minutes followed by an increased Ar pressure
of 40 mbar and a temperature increase up to the final sintering
temperature 1410.degree. C. where the temperature was kept for an
additional hour before cooling and opening of the furnace.
[0060] The structure in the cutting inserts consisted of a less
uniform submicron tungsten carbide grain size and in combination
with some large WC grains, FIG. 12.
EXAMPLE 15
[0061] Inserts from Example 13 and 14 were etched for 2 minutes at
room temperature in Murakamis regent and examined under optical
microscope at 1500.times.. Ten micrographs were taken. In all ten
micrographs, WC grains having a length in any direction >5 .mu.m
were detected and the maximum length for each such grain was
measured. An average per micrograph was calculated by dividing the
number of grains by 10. An average number of abnormal grains per
micrograph, corresponding to a surface area of 8360 .mu.m.sup.2,
was calculated by dividing the number of grains by 10.
[0062] Result: TABLE-US-00003 Average number of grains with max.
length >5 .mu.m Example 13 (invention) 0-0.1 Example 14
(reference) 0.2-0.4
[0063] The principles, preferred embodiments, and modes of
operation of the present invention have been described in the
foregoing specification. The invention, which is intended to be
protected herein, however, is not to be construed as limited to the
particular forms disclosed, since these are to be regarded as
illustrative rather than restrictive. Variations and changes may be
made by those skilled in the art without departing from the spirit
of the invention.
* * * * *