Note: Descriptions are shown in the official language in which they were submitted.
1178092
l ~ACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
This invention relates to a hard alloy containing
molybdenum and more particularly, it is concerned with a
composition of a hard alloy comprising a hard phase consis-
ting of at least one compound having a crystal structure of
simple hexagonal MC type (M: metal, C: carbon) and a binder
phase, which is suitable for use as a tool capable of resis-
ting a high impact for a long time.
2. DESCRIPTION 0~ THE PRIOR ART
The first report on a (Mo, W)C base alloy is seen in
British Patent No. 635,221~ This describes a process for
producing the (Mo, W)C base alloy by nitriding oxides of
molybdenum and tungsten in ammonia stream, carburising the
nitrides with release of nitrogen~ adding an auxiliary metal
in powder form to serve as a binder in the sintered alloy,
and sintering. This alloy was new at that time as an alloy
consisting of one or two carbides of (W, Mo)C and (W, Mo)2C
with a binder metal, but has not been put to practical use.
Molybdenum monocarbide (MoC) is considered as a use-
ful substitute, since this carbide only has the same crystal
structure, a simple hexagonal type, as tungsten carbide as
well as mechanical properties similar to tungsten carbide.
However, the existence of the hexagonal molybdenum monocar-
bide as a simple substance has remained in question to this
date and thus an attempt to stabilize molybdenum has exclu-
sively been carried out by forming a solid solution with
tungsten carbide. This method was firstly reported by ~l.
Dawihl in 1950, but this solid solution was no~ examined in
detail and the commercial worth was not found in those days.
~k
-1- ~
117~ 9X~
l Of late, hawever, the study to utilize the solid X
solution (Mox~Jy)C where x + y = 1 has become active with
the rise of the price of tungsten, It is very interesting
why a study on this solid solution and an attempt to use
the same has not been carried out so actively up to the
present time.
In the prior art process for the production of a
solid solution of MoC-WC, WC, Mo and C powders or W, Mo, C
and Co powders are mixed, charged in a carbon vessel and
reacted at a temperature of 1660 to 2000 C (W. Dawhil:
"Zeitschrift f. Anorganische Chemie" 262 (1950) 212). In
this case, cobalt serves to aid in forming the carbide and
to dissolve molybdenum and carbon in the tungsten carbide.
In the absence of c~balt, it is very difficult to obtain a X
solid solution of (Mo, W)C When a (Mo, W)C powder obtained
by this method is used for the production of a cemented car-
bide alloy with a binder metal of cobalt as a substitute for
WC, however, (Mo, W)C decomposes in the alloy to deposit
needle crystals of (Mo "~)2C. Deposition of even a small
amount of such a subcarbide with aggregation in the alloy
causes deterioration of the strength of the alloy. For this
reason, the use of MoC as a substitute for WC has not been
attempted positively.
In a process for the production of mixed carbides, in
general, carbides are heated in the presence of each other,
optionally using a diffusion aiding agent such as cobalt, to
give a uniform solid solution in most cases, but in the case
of a composition of solid solution containing at least 70 'h
of MoC, a uniform solid solution cannot be obtained by coun-
ter diffusion only at a high temperature. This is due tothe fact that MoC is unstable at a high temperature and is
decomposed into solid solutions such as (Mo, W)C1 x and (Mo,
W)3C2 and, consequently, a solid solution (~lo, ~)C of '~C
117~309Z
l type cannot be formed only by cooling it. As a method of
stabilizing this carbide, it has been proposed to react the
components once at a high temperature, to effect diffusion
of Mo2C and WC, and to hold the product at a low temperature
for a long time (Japanese Patent ~.pplication (OPI) No.
146306/1976). However, a considerably long diffusion time
and long recrystallization time are required for forming
~ ) rom (Mo~ W)C1-x and (Mo~ W)3~2 at a low tempera-
ture. ~or the practice of this method on a commercial scale,
the mixture should be heated for a long time in a furnace to
obtain a complete carbide. ~his means that the productivity
per furnace is lowered and a number of furnaces are thus
reguired. When using a continuous furnace, on the other
hand, a long furnace is necessary and mass production is
difficult industrially.
Under the situation, we, the inventors, have made vari-
ous efforts to provide a solid solution (Mo-W) in an economi-
cal manner based on the thought that if an alloy consisting
of a solid solution (Mo-W) can be prepared at a low cost and
a (Mo-W)C powder as a hard material can readily be produced
on a commercial scale, the use of these materials or their
cemented carbide alloys will remarkably be enlarged and con-
sequently, have reached an invention as disclosed in US Patent
No. 4,216,009 which consists in a process for the production
of an alloy powder containing molybdenum and tungsten and
having a crystal structure of simple hexagonal WC type, com-
prising mixing molybdenum and tungsten in the form of com-
pounds thereof selected from the group consisting of oxides,
hydroxides, chlorides, sulfates, nitrates, metallic acids,
salts of metallic acids and mixtures thereof, the resulting
mixture of the compounds having a particle size of at most 1
micron, reducing the mixture with at least one member selec-
ted from the group consisting of hydrogen and ammonia to form
117809~
l an alloy powder of molybdenum and tungsten, and then car--
burizing the alloy powder.
Furthermore, the inventors have proposed cemented
carbide alloy as disclosed in Japanese Patent Application
(OPI) Nos. 145,146/1980 and 148742/1980, which are suitable
for impact resisting tools. ~he former invention provides
an impact resisting cemented carbide alloy containing moly-
bdenum, characterised in that the friction coefficient is
less than 70 b of that between WC-Co type alloys and steels,
but this alloy does not have a sufficient life, in particu-
lar, in a use subject to repeated impacts because of conta-
ining a hard phase of MC type in the alloy. The latter
invention provides an impactresisting cemented carbide
alloy comprising a hard phase of mixed carbides of moly-
bdenum and tungsten of MC type and a binder phase of cobalt
and nickel, represented by (MoxW1 x)Cz-(NiyCo1 y) where
0.5 ~ xC 0.95, 0.5 ~ y ~ 1.0 and 0.90 C z C 0.98, but this
alloy does not have a long life under such a severe condi-
tion as being subjected to high impacts for a long time.
SUMMARY OF THE INVEN~ION
It is an object of the present invention to provide
a hard alloy containing molybdenum and tungsten.
It is another object of the present invention to pro-
vide a hard alloy corresponding to a cemented carbide alloy
consisting of a hard phase of tungsten carbide (WC) a part
of which is replaced by molybdenum carbide (MoC) and a bin-
der phase of an iron group metal.
It is a further object of the present invention to
provide a hard alloy having a hard phase consisting of a
carbide, carbonitride or carboxynitride of molybdenum and
tungsten with simple hexagonal crystal structure of MC type
1~7~3092
1 and a binder phase consisting of a least one of iron, cobalt
and nickel, in which the hard phase has a relatively large
mean particle size and a uniform distribution of molybdenum
and tungsten.
It is a still further object of the present invention
to provide a hard alloy capable of resisting high impacts
for a long time.
It is a still further object of the present invention
to provide a tool suitable for use where a high resistance
to cyclic loads is required.
These objects can be attained by a hard a]loy compris-
ing two phases of a hard phase consisting at least one eom-
pound having a crystal structure of simple hexagonal MC type -
(M: metal; C: earbon) seleeted from tne group consisting of
mixed earbides, earbonitrides and carboxynitrides of moly-
bdenum and tungsten, and a binder phase consisting of at
least one element seleeted from the group eonsisting of iron,
eobalt and niekel, in whieh the hard phase is one prepared
by earburizing an (Mo, W) alloy obtained by reducing oxides
of molybdenum and tungsten with a partiele size of 1 mieron
or less, is of eoarse partieules with a mean partiele size
of 3 mierons or more and has a uniform molybdenum to tung-
sten ratio in the partieles, and whieh has a gross eomposition
defined by the relationships:
0.1 ~x <0.9,
10 wt% ~y <70 wt%, and
4 wt% <~ y,
wherein y is the binder eontent in wt% and x is the atomie
ratio of W/(Mo + W).
--5--
1178D9Z
1 BRIEF DESCRIPTION OF THE DRAWING
The accompanying drawings are to illustrate the prin-
cipal and merits of the present invention in more detail:
Fig. 1 is graphical representation of the composition
of a hard alloy according to the present invention in the
relationship of W/(Mo ~ W) atomic ratio and binder content.
-5a-
r ~
~17809Z
l Fig. 2 is a graphical representation of the relation-
ship between the carbon content in alloy and the change of
the transverse rupture strength (TRS) in an (MoO 7Wo 3)C-35
wt ,~ Co alloy.
~ ig. 3 is a graph showing the results of a fatigue
test of an (MoO 7WO 3)C-35 wt % Co alloy (~: MC-~ alloys;
O: r~C-M2C-~ alloys) in which a cyclic load ~ is applied.
~ ig. 4 is a micrograph, magnified 5,OOO times, of
(MoO 5Wo 5)C according to the present invention, in which
Mo and W are uniformly distributed.
~ ig. 5 is a micrograph, magnified 5,OOO times, of
(M~ ~O 5)C according to the prior art, in which Mo/W ratio
is not uniform in each particle.
DETAI~ED DESCRIPTION OF THE INVENTION
In accordance with the present invention, there is
provided a hard alloy comprising two phases of a hard phase
consisting of at least one compound having a crystal struc-
ture of simple hexagonal rlc type (M. metal; C: carbon) sele-
cted from the group consisting of mixed carbides, carbonit-
rides and carboxynitrides of molybdenum and tungsten, and a
binder phase consisting of at least one element selected
from the group consisting of iron, cobalt and nickel, in
which the hard phase is one prepared by carburizing an (Mo,
W) alloy obtained by reducing oxides of molybdenum and tung-
sten with a particle size of 1 micron or less, is of coarse
particles with a mean particle size of 3 microns or more and
has a uniform molybdenum to tungsten ratio in the particles,
and which has a gross composition within the shaded portion
~BCDE~ in ~ig. 1.
The inventors have hitherto made various efforts to
improve (Mo W)C-iron group metal alloys and consequently,
--6--
~17~3~92
1 have found that uniform dispersion of granular or globular
(Mo, W)2C (which will hereinafter be referred to as M2C)
therein is effective for increasing the yield stress and
breaking strength (US Patent No. 4,265,662), but this alloy
is not suitable for a use where a high fatigue strength is
required upon exposure to a high impact for a long time.
This is possibly due to that the dispersed (Mo, W)2C rather
acts as a harmful element for this purpose. ~here a high
fatigue strength is required upon exposure to a high impact
for a long time, "crack propagation" is regarded as impor-
tant rather than "crack initiation" and in particular, "crack
propagation" tends to extend along the boundaries between the ha,r'd
phase and binder metal with high probability. Thus, it is
necessary to reduce the boundaries of the hard phase and bin-
der metal and this can be achieved by increasing the particle
size of the hard phase and the thickness of the binder phase.
The inventors have carried out a heading test of bolts by
utility tools made of materials as shown in Table 1 by chan-
ging the particle size of a hard phase in a considerably
binder metal rich region, thus obtaining results as shown in
Table 2:
Table
Particle Compressive TRS* VHN**
Size of Strengt~
Carbide CA) (Kg/mm ) (Kg/mm2)
(Mo, W)C-25 wt ~ Co (~) 5 327 270 760
(Mo, ~J)C-25 wt O Co ~B) 1 345 295 810
WC~22 wt ~ Co (C) 5 310 280 830
Note: * ~ransverse Rupture Strength
*~ Vickers Hardness Number
J 178092
l Table 2
Die ~ife (Number of Samples Processed/Die) x 105
0 2 4 6 8 10
_
Alloy (A) x
Alloy (B) - -x
Alloy (C) x
Note: Mark x means a broken point.
Test Conditions: workpiece: S45C Steel
forging speed: 100 samples per
minute
As is evident from these results, the strength of an alloy
and the die life are not always consistent with each other
and the low hardness and low strength alloy with a coarse
2rain size exhibits the longest life. ~hat is, the tool can
be used even after cracks or deformations occur. ~his sugg-
ests that the life of a tool does not depend on the initia-
tion of crack but depends on the propagation speed of crack
leading to the overall breakage thereof.
~ herefore, the inventors have concentrated their ener-
gies on preparation of a coarse grain carbide and consequen-
tly, have found that it is more difficult to obtain an (Mo,
W)C with a large particle size than WC since molybdenum has
an effect of retarding particle growth. However, it is fur-
ther found that when using particularly the solid solution
(Mo-W) prepared by the process of US Patent No. 4,21G,009 as
mentioned above, a carbide with a particle size o~ 3 microns
or more can readily be obtained by controlling the carburiz-
ing condition, for example, by adjusting the carburization
temperature to a temperature which is sufficiently high but
lower than the decomposition point of (Mo, W)C into ~I~,o, W)2C,
for example, to 1450 C in the case of (~loO 7W0 3)C. For
~178Q9Z
l the preparation of a carbide with a larger particle size,
e.g. 6 microns or more, the carburization is preferably
carlied out after the solid solution (Mo-W) is suhjected
to a heat treatment. ~he heat treatment is generally carrr-
ied out at a temperature of 1100 to 1400 C for 1 to 5 hours
in a stream of nitrogen or hydrogen. In the case of ~MoO 5
WO 5)C, for example, the solid solution is thermally treat-
ed at 1300 C for 3 hours in a stream of nitrogen.
When WC, Mo2C and C are used as starting materials
and subjected to carburization according to the prior art,
on the other hand, it is very difficult to form a coarse
carbide with a particle size of 3 microns or more, and even
if coarse starting materials are used, there is only formed
a carbide having a fluctuating Mo/W ratio in each particle,
because the carbon in the carbide acts as a diffusion bar-
rier. ~he use of sueh a carbide with a binder metal results
in a nonuniform alloy which mechanical strength is low.
In view of the above described faets, it will clearly
be understood that an alloy containing a coarse carbide with
a partiele size of at least 3 mierons, in partieular, at
least 5 mierons and having a desired mechanical property,
i.e. impact resistance must be prepared by way of the process
comprising reac-ting the solid solution of (Mo, W) with carbon,
which is capable of forming a uniform and large particle size
(Mo, W)C, and otherwise, preparation of such an alloy is impo-
ssible.
As a result of our studies on the sintering phenomenon
of an alloy consisting of two phases of (Mo, W)C and a binder
metal in more detail, it is found that in (Mo, W)C base alloys,
there does not take place growth of carbide particles due to
Ostwald ripening of the dissolving and precipitating type which
1~7809Z
l can be seen in the ordinary WC base alloys, but there is
found a very slow particle growth of diffusion rate-contro-
lling t~pe. In the (Mo, W)C base alloys, the particle
growth during sintering, which can be seen in the prior art
WC base alloys, is scarcely expected and therefore, a car-
bide to be used as a raw material must be of coarse parti-
cles so as to prepare an alloy containing a hard phase with
a large particle size.
Similarly, the inventors have conducted various expe-
riments and measured typical properties in order to make
clear the features of M2C precipitated alloys and M2C non-
precipitated alloys.
Fig. 2 is a graphical representation of the relation-
ship between the carbon content (~ by weight) and transverse
rupture strength (Kg/mm2) in an (MoO 7Wo 3)C-35 weight ~ Co
alloy. As is apparent therefrom, the transverse rupture
strength lowers rapidly with precipitation of free carbon,
but does not so lower even if M2C is precipitated. This is
considered to be due to that the M2C phase is dispersed uni-
formly and finely so that dispersion strengthening appears,
but it is hardly related with the lowering of the transverse
rupture strength.
Fig. 3 is a graph showing the results of a fatigue test
of an (MoO 7Wo 3)C-35 weight ~ Co alloy, in which a static
load at a certain level is applied to a sample cyclically.
It is apparent from these results that the M2C-precipitated
alloy (MC-M2C-~ alloys represented by mark 0) is inferior to
the M2C-nonprecipitated alloy (MC-~ alloys represented by
mark O in fatigue strength. This is possibly due to that
the M2C phase finely dispersed increases the boundaries
between the hard phase and binder metal phase and acts as
-10-
117809Z
l an element to promote crack propaga-tion, since cracks pro-
pagate predominantly along the boundaries between the hard
phase and binder phase. In the case of wear resisting tools,
in general, a high stress is intermittently applied for a
long time and in addition, some factors of promoting crack
propagation, such as thermal impact and corrosion embrittle-
ment are entangled, so a high fatigue strength is required.
In such a case, M2C should not be precipitated.
M2C tends to aggregate and grow larger abnormally with
the increase of the quantity thereof, which acts as a stress
concentrating source causing to lower the fatigue toughness
when high impact energy is applied.
As the same time, it is also proved by a field test
that the quantity of M2C precipitated should be held as little
as possible or reduced to substantially null for the purpose
of displaying sufficiently the performance of a tool in a case
where an alloy having a relatively large binder phase and a
structure such that the mean free path of the binder phase is
large is used.
~ he inventors have made further studies on a hard alloy
consisting of (Mo, W)C and an iron group metal by changing the
ratio of Mo and W and the amount of the iron group metal over
a wide range and it is thus found that the two phase region
{(MC + ~) zone} is about 1/3 of that of WC base alloys in the
case of (MoO 7Wo 3)C base alloys, and about 1/5 of that of WC
base alloys in the case of (MoO 9Wo 1)C base alloys. When
the binder metal is changed from cobalt to iron in order, some
shift takes place in the two phase region, but there is little
change in width. ~hese data are collected and arranged to
give results as shown in Fig. 1 wherein the boundary line of
~i2C-precipitated zone and M2C-nonprecipitated zone is drawn
-11-
i~7t309Z
l by line a.
Furthermore, the inventors have conducted a number of
exeriments by changing the ratio of Mo and W and the quan-
tity of iron group metals over a wide range and ahve thus
found that when the critical wid-th of controlling carbon
industrially is 0.07 % by weight, a zone wherein there is
(MC + ~) zone in an amount of at least 0.07 % by weight as
carbon can be represented by the relationship of q x r
~ 4.0, i.e. above line a in ~ig. 1, where the alloy compo-
sition consists of (MOp~Jq)C - r weight % binder metal~ In
other words, it is quantitatively determined that M2C tends
to be precipitated with the increase of the ratio of Mo in
the ordinary (Mo, W)C base alloys and the amount of a binder
metal should be increased so as to suppress precipitation of
M2C. For example, as can be seen from Fig. 1, the two phase
region of (MC +~ ) free from precipitation of M2C and free
carbon amounts to at most 0.07 % by weight as carbon content
used as a parameter unless the amount of a binder metal is
more than 13.5 % by weight in the case of (MoO 7Wo 3)C base
alloys, and at least 20 ~ by weight of a binder metal is
required for the similar width of carbon value in the case
of (MoO 8W0 2)C base alloys. As a matter of course, line a
is shifted above when (MC + ~) zone exceeds 0.07 ~ by weight
as carbon content, wherein the two phase region of (MC +~ )
is stable.
Referring to ~ig. 1, the ground of limiting the W/(Mo +
W) ratio to 0.1 ~- ~0 + W ~ 0.9 is that if the ratio is less
than 0.1, the carbide is so unstable that it tends to be
decomposed into M2C while if more than 0.9, there is little
effect of molybdenum as (Mo, W)C. The ground of limitin~
the amount of a binder metal to 10 to 70 % by weight is that
if less than 10 ~ by weight, the alloy itself becomes so
brittle that it cannot be used in fact, while if more than
117~9~:
l 70 ~ by weight, the sintering is so difficult that a desired
shape cannot be held.
The iron group metal as a binder phase can na-turally
dissolve Group IVa, Va and VIa met'~s and itis possible to X
add even other elements having solubility therein such as
aluminum, silicon, calcium, silver, etc. with holding the -
merits of the present invention.
The basic concept of the present invention can be held
even when a part of molybdenum and tungsten carbide is rep-
laced by a B1 type mixed carbide containing titanium, ~irconium,
hafnium , vanadium, niobium, tantalum, chromium, molybdenum
and/or tungsten in a proportion of 3O % by weight or less,
preferably 0.5 to 25 ~ by weight~ -
Furthermore, there is the similar relationship even
in the case of an alloy wherein a part of C in the carbide
is replaced by nitrogen and/or oxygen. Examples of the
preferred embodiment in this case are as follows.
The first embodiment is incorporation of N in (W, Mo)C
to give (W, Mo)(C, N) whereby a stable starting material of
hexagonal WC type can be obtained without a heat treatment
for a long time.
The second embodiment is incorporation of O in
(IJ, Mo)(C, N) to give (W, Mo)(C, N, O) which is more stable.
The third embodiment is incorporation of Cr in
(W, Mo)(C, N) or (W, Mo)(C, N, O) to give (IJ, ilo, Cr)(C, N)
or ('J, Mo, Cr)(C, M, O) whereby a starting material with a
low weight and low price can be obtained.
-13-
1178(~9;~
l The fourth embodiment is that in the production of
these starting material powders, a mixture of oxides,
metals, carbides and/or carbon is exposed to an atmosphere
having a nitrogen partial pressure of 300 Torr or more at
a temperature of 700 C or higher in a part of the carbu-
rization step to form a stable starting powder.
The fifth embodiment is that, when the above described
starting powder is combined with an iron group metal, t~Jo
or more kinds of hard phases of simple hexagonal WC type
differing in composition are caused to be present in the
finished alloy, thereby imparting a high toughness thereto.
In these five embodiments, a part of the MC type phase
can also be replaced by a B1 type solid solution containing
one or more of Group IVa, Va and VIa metals and non-metallic
elements, or the ordinary additives to cemented carbides,
such as silver, silicon, bismuth, copper, aluminum, etc. can
also be added to the iron group binder metal with holding
the merits of the present invention.
The above described embodiments will now be illust-
rated in greater detail:
In the important system of the present invention wherein
there are a simple hexagonal phase containing molybdenum and
tungsten, it is found in the sintered alloy with a binder
metal that, when A = ( N atom ~ ~ x (1 _ W atom %
(~o + W) atom ,' ~Mo + W) atom Y~
the suitable range of A is 0.005 ~ A C o.5 If ~ is less
than the lower limit, the effect of nitrogen does not appear,
while if more than the upper limit, sintering to give exce-
llent properties is difficult. The most suitable range of A
is 0.01 ~ A ~ 0.4.
Concerning the effect of oxygen, it is found that,
when B = (~ a~)tomato~m ~) x (1 ~ 0 + I~al)tom~t~m S')' t~le
suitable range of B is 0.005 ~ B ~ 0.05. If B is lcss than
-14-
117~092
l the lower limit, there is no favourable effect of o~ygen, while
if more than the upper limit, sintering is difficul-t to
give excellent properties. The most suitable range of B is
0.01 ~ B ~ 0.0~.
On the other hand, a W/~io ratio is preferably 10/90
to 90J10, since if less than 10/90, the alloy is unstable,
while if more than 90/10~ the merits of the replacement
(light weight, low price) are substantially lost. The quan-
tity of chromium used for replacing molybdenum or tungstenlO if 0.5 or less by atomic ratio of (W ~ Mo), since if more
than 0.5, the alloy is brittle although the corrosion resi-
stance is increased.
As well known in the art, it is advantageous for cut-
ting tools to form a B1 type solid solution composed of at
least one of Group IVa, Va and VIa metals such as titanium,
zirconium, hafnium, vanadium, tantalum, chromium, molybdenum
and tungsten with at least one of non-metallic components
such as carbon, nitrogen and oxygen in addition to the simple
hexagonal phase. The quantity of the B1 type solid solution
is preferably changed depending upon the cutting use.
Concerning the quantity of nitrogen in this case, it
is found as a result of our various experiments that, when
the definition of A is changed to (Group IVa Va VIa metals atom c,~)
x (1 - W atom % ), the suitable range
Group IVa,Va,VIa metals atom %
of A is also 0.005 ~ A ~ 0.5 although a part of the nitrogen
is occluded in the B1 type solid solution. The optimum range
of A is 0.01 C A c o 4 Concerning the quantity~of oxygen,
it is found as a result of our various experiments that,
when the definition of B is changed to
O atom %
Group IVa,Va,VIa metals atom ~) x
(1 - W atom % ), the suitable range of
Group IVa,Va,VIa metals atom ,a,
-15-
~17~09Z
l B is also o.oo5 C 3 ~ 0 5 The optimum range of B is
0.01 C B ~ 0.04.
As the binder metal, there is preferably used an iron
group metal in a proportion of 10 to 70 ~ by weight based
on the gross composition, since if less than 10 ~ by weight,
the alloy is brittle and if more than 70 ~ by weight, the
alloy is too soft.
For the preparation of starting materials, the reac-
tion is carried out at a high temperature in a hydrogen
lO atmosphere in the case of carburization of a (Mo, W) powder `
with carbon, reduction and carburization of oxide powders
with carbon or combination thereof. At this time, it is
found as a result of our studies on the decomposition
nitrogen pressure of (Mo, W)(C, N) that the external nitro-
gen pressure, depending on the temperature, should be 300
Torr or more at 700 C or higher at which the carbonitriza-
tion reaction takes place. The coexistence of hydrogen is
not always harmful, but it is desirable to adjust the quan-
tity of hydrogen to at most two times as much as that of
nitrogen, in particular, at most the same as that of nitro~
gen not so as to hinder the nitriding reaction. In the case
of using an ammonia decomposition gas, it is necessary to
enrich with nitrogen.
For the preparation of starting materials containing
oxygen, the coexistence of carbon monoxide and carbon dioxide
is required in an atmosphere. In this case, the quantity of
hydrogen is not limited as described above, but should not
exceed 50 ~O of the atmosphere. Heating and sintering in an
atmosphere of nitrogen or carbon oxide is effective for the
purpose of preventing an alloy sintered from denitrification
or deoxidation.
The inventors have further made studies to develop an
alloy having a higher wear resistance and toughness and
-~G-
1~780~Z
l consequently, have found that the deformation at a high
temperature can remarkably be improved by changing tungsten
carbide to a carbide composed of a solid solution of three
elements, molybdenum, tungsten and chromium. ~hat is to
say, a (Mo, ~)C-Co alloy has a higher hardness at a high
temperature than a WC - Co alloy and, when Cr is further
dissolved in this carbide, the hardness is further raised
and the high temperature hardness is also improved. Thus,
the disadvantages of the prior art I~C - Co alloy can be
overcome by one effort. It is to be noted that the carbide
phase consists of a solid solution of (Mo, W, Cr)C. It is
also found that when Cr is dissolved in a solid solution of
(Mo, W)C, the carbide particles can be made finer and sta-
bilized as a monocarbide of (Mo, W, Cr)C. On the contrary,
the known method of adding merely chromium to the binder
phase has the disadvantages that it is impossible to make
finer the carbide and the carbide phase is not stabilized
as a monocarbide of a solid solution of (Mo, W~ Cr). ~he
quantity of chromium to be added to the solid solution car-
bide (Mo, ~)C ranges preferably 0.3 to 10 %j since if lessthan 0.~ %, the carbide cannot be made finer, while if more
than 10 ,~, Cr3C2 is separated and precipitated in the alloy,
resulting in lowering of the hardness.
In a further embodiment of the present invention, a
part of the carbon in the solid solution carbide (Mo, 1~, Cr)C
is replaced by nitrogen, oxygen and/or hydrogen. ~hat is,
it is assumed that if the carbon contained in (Mo, ~], Cr)C is
added as solid and reacted with a reactivity of 100 ,~, the
crystal is stabilized, but now it is found that incorporation
of no-t only carbon but 21so nitrogen results in stabilization
of the monocarbide as (Mo, l;~ Cr)(C:N) and furthcr incorpora-
tion of o~-ygen and hydrogen stabilizes more the monocarbiae
as (l~o, 1~, Cr)(CaNbOcHd)(a + b r c + d = 1), because if thcI~c
-17-
117~3092
l are defects in the carbide, the carbide is unstable during
sintering and an M2C type mixed carbide precipitates needle-
wise to thus lower the strength.
In a still further embodiment of the present invention,
one or more of manganese, rhenium, copper, silver, zinc and
gold are incorporated in the binder phase to change the
micronstructure of the binder phase and to make non-magnetic.
~t the same time, it is found that, when these elements are
added, the binder phase is alloyed, whereby the corrosion
resistance of the alloy is improved.
In the last embodiment of the present invention, the
toughness of the alloy can be raised by using, in combina-
tion, two or more carbides having a simple hexagonal phase
but differing in the ratio of Mo/W. The detailed reason of
increasing the toughness if not clear, but it is assumed
that when (Mo, W)C is separated into two phases, the solu-
tion strain of both the phases is lowered to give a higher
toughness than in the case of a single phase. ~ince at
least an alloy consisting of a (MoxWy)C (y ~ x) phase having
the similar property to that of WC and a (MoxWy)C (x ~ y)
phase having the similar property to that of MoC has two
properties, i.e. toughness of WC and heat and deformation
resistance of MoC, this embodimen-t is advantageous more than
when using one kind of (Mo, W)C only. ~IOSt preferably, the
carbide is composed of WC or a solid solution of some MoC
dissolved in WC and a solid solution of WC dissolved in MoC
This corresponds to a case where the peak of plane (1, 0, 3)
is separated in two in X-ray diffraction. Whether there
are two or more simple hexagonal phases of (MoxW )C or not
can be confirmed by observation using an optical microscope
after etching with an alkaline solution of a hexacyanoferrate
(III) or by XM~ observation.
The application or use range of the alloy of the pre-
~7~309~
1 sent invention is as follows. For example, the alloy ofthe present invention can be used for wear resisting tools
such as guide rollers, hot wire milling rollers, etc., and
for cutting tools, because of having a toughness and hard-
ness similar to or more than those of WC-Co alloys. In
particular, when the alloy of the invention as a substrate
is coated with one or more wear resisting ceramic layers
such for example as of TiC~ TiN, A12O3, cutting tools more
excellent in toughness as well as wear resistance can be
obtained than the prior art tools having WC-Co type alloys
as a substrate. As well known in the art,at this time, a
decarburization layer called ~-phase is formed at the boun-
dary between the substrate and coating layer and this appears
similarly in the alloy of the present invention. In order
to prevent the embrittlement directly under the coating
layer due to decarburization, the presence of free carbon
(FC) in the surface layer within a range of 3OO microns is
effective without deteriorating the toughness.
) When using the alloy of the present invention as a
watch frame, it shows more excellent properties as a watch
frame than WC-Co type alloys, which are summarized below:
(1) Beautiful brightness can be given when the alloy
is specularly finished.
(2) Grinding and polishing workings are possible.
(3) Corrosion resistance is excellent, in particular,
for sweat in the case of trinkets.
(4) Mechanical strength is considerably high.
The present invention will be further illustrated in
greater detail in the following examples. It will be self-
evident to those skill in the art that the ratios, ingre-
dients in the following formulation and the order of opera-
tions can be modified within the scope of thc preser.t inven-
tion. Therefore, the present invention is not to be
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~171~)9Z
1 interpreted as being limite~ -to the fo1]owing examp]es. A11
parts, percents and the like are to be taken as those by weight
unless otherwise indicated.
Example l
54 g of Mo powder and 46 g of W powder were dissolved in
28% aqueous ammonia, neutralized with 6 N hydrochloric acid to
coprecipitate and then subjected three times to filtration with
water washing and drying. In the resulting precipitate, WO3
and MoO3 were finely mixed. The mixed oxides were fired at
800C in the air and then reduced at 1000C in a hydrogen stream.
X-ray diffraction showed that the resulting powder was of a
complete solid solution of (MoO 7Wo 3).
The resulting solid solution (MoO 7WO 3), carbon powder
and Co powder as a diffusion aid were mixed in such a proportion
that the final composition be (MoO.7WO 3)Cl O
carburization reaction at 1450C for l hour in a nitrogen stream
under a nitrogen pressure of l atm. It was found by X-ray
diffraction that the carbide had a crystal structure of simple
hexagonal WC type and measurement of the particle size using
Fisher Sub Sieve Sizer showed a mean particle size of 4.5 microns.
This powder was mixed with Co powder in such a proportion
that the~ final composition be (MoO 7Wo 3)C - 30% Co, ball
milled with alcohol medium, pressed in a desired shape and then
sintered in a vacuum of lO 2 Torr. The thus obtained alloy had
a structure consisting of two phases of MC phase and binder
metal phase, and a hardness of 880 by Vickers hardness and a
hending strength of 290 Kg/mm2.
Example 2
A solid solution of (MoO 5Wo 5) was prepared in an
analogous manner to Example l except changing the Mo/W atomic
ratio to 0.5 : 0.5. This solid solution was mixed with carbon
. .
~7809Z
l powder and Co powder as a diffusion aid in such a proportion
that the final compositin be (M0 5W0.5)C1.0 and subjected
to carburization reaction at 1500 C for 1 hour in a nitro-
gen stream under a nitrogen pressure of 1 atm. It was found
by X-ray diffraction that the resulting carbide powder had
a crystal structure of simple hexagonal WC type and by mea-
surement of the particle size thereof using Fisher Sub Sieve
Sizer that it had a mean particle size of 5.2 microns
(Cf. Fig. 4).
This powder was mixed with ~Ti powder and Co powder in
such a proportion that the final composition be (MoO 51~o 5)C
- 15 c~O Ni - 15 S Co, ball milled with alcohol medium, pres-
sed in a desired shape and then sintered in a vacuum of 10-2
~orr. The thus obtained alloy had 5.49 % of total carbon
and 0.01 % of free carbon as analytical values, a structure
consisting of two phases of MC and binder metals,and ahard-
ness of 900 by ~ickers hardness and a bending strength of
300 Kg/mm .
For comparison, ~JC powder with a particle size of 4
microns, Mo2C powder with a particle size of 3.5 microns,
carbon po~der and Co powder as a diffusion aid were mixed
in such a proportion that the final composition be
(MoO 51~lo 5)C1 0, and sintered at 1700 C in a vacuum of 10
Torr. The temperature was then lowered to 1350 C and the
mixture was held at this temperature for 12 hours. X-ray
diffraction showed that the resulting carbide was substan-
tially of (Mo, W)C having a crystal structure of simple
hexagonal WC type, but there were partly peahs of (Mo, W)2C
(Cf. Fig. 5).
This powder was mixed with Ni powder and Co powdcr in
such a proportion that the final composition be (r~1OO 5Wo 5)C
- 15 ~ Ni - 15 ~ Co and an alloy was prepared in the same
manner as described above. The thus obtaincd allo~r had
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117809Z
l 5.48 90 of total carbon and 0.02 c~ of free carbon as analy-
tical values, a structure consisting of MC phase, binder
metal phase and M2C phase grown up through aggregation, and
a hardness of 910 by Vickers hardness and a bending strength
of 230 Kg/mm2
When using carbides as starting materials as in this
comparative example, a uniform quality mixed carbide cannot
always be obtained in the formation of the mixed carbide
with a large particle size, and in an alloy obtained from
this carbide, there is locally a carbon deficiency zone
leading to formation of an aggregated M2C phase and resul-
ting in lowering of the strength thereof.
Referring to ~ig. 4 and ~ig. 5, the heights of peaks
of ~J and Mo show respectively the contents of 1J~I and Mo on
the lines drawn in the micrographs of carbide crystals. It
will be understood from comparison of ~ig. 4 and Fig. 5 that
in the case of Fig. 4 according to the present invention,
the fluctuation of peaks of 1~1 and Mo on the line crossing
the carbide crystal is smaller, i.e. the Mo~J ratio is more
constant, than in the case of ~ig. 5 according to the prior
art.
~xample 3
Various alloys each having a composition within the
range of shaded area of ~ig. 1 were prepared and subjected
to measurement of the hardness and transverse rupture strength,
thus obtaining results shown in Table 3. The sintering tem-
perature ~JaS varied from 1200 C to 1450 C every composi-
tion:
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09Z
l Table 3
Sample No. Carbide ~inder Hardness ~R~
~ur-lnvention Composition Metal (Hv) (K~mm
1 (M0.85W0.15)C 30Ni-5Co 870 310
2 (M0~85W0~15)C 40Ni 720 280
3 ( 0.85 0.15) 30Co-30Ni 500
4 (MoO,7W0.3)c 10Ni-10Co 1150 260
(Mo0~70W0~3)c 15Ni-30Co 600 250
( 0.7 0,3) 50Ni-15Co 445
7 (MoO 55W0 45)C 15Ni 1280
8 ( oO,50Wo.5o)c 20Ni-5Co 1020 265
9 (M0.3owo~7o)c 35Co 880 300
(MoO 30~J0 70)C 25Co-10~e 910 270
11 (MoO 30~0 70)C 8Fe-12Ni 1070 245
12 ~MoO 25W0 75)C 15Co-5Ni-8Fe 900 230
13 (MoO 20W0 80)C 15Ni-40Co 600 240
14 (MoO 20W0 80)C 30Fe 950 245
(MoO 20W0 80)C 10Ni-5Co 1260 240
16 (MoO 20W0 80)C 5Ni-5Co-2Fe1280 210
Comparison
17 (MoO 80W0 20)C 5Ni-5Co 1410 110
18 (MoO 70Wo 30)C 2Ni-4Co 1700 go
19 (MoO 50Wo 50)C 60Ni-15Fe 420
(MoO o5Wo 95)C 3Ni 1820 75
Example 4
The solid solution (MoO 5Wo 5) prepared in Example 2
was mixed with carbon powder, Cr3C2 powder and Co powder as
a diffusion aid in such a proportion that the final compo-
( 0~45~0~45cro~1o)c1 o~ subjected to primarycarburization at 1800 C for 1 hour in a hydrogen stre~m and
then to secondary carburization at 1500 C for 1 hour in a
hydrogen stream. It was found by X-ray diffraction that the
~7~309Z
l resulting powder had a crystal structure of simple hevagonal
WC type and by measuremen-t of the particle size thereof that
it had a mean particle size of 4.0 rnicrons.
This carbide powder was mixed with Ni powder and Co
powder in such a proportion that the final composition be
(MoO 45~lO 45CrO 10)C - 30 ~ Ni - 15 % Co and then sintered
at 1220 C in a vacuurn of 10-2 morr. The thus obtained
alloy had a struc-ture consisting of two phases oL MC and
binder metals. When this alloy was polished with a diamond
paste to gi~e a mirror surface and subjected to a test by
immersing in an artificial sweat for 24 hours, there was
hardly found corrosion thereof.
The alloy obtained in this example is suitable I or use
as a watch frame because of its light weight as well as
excellent scratch proofing property.
~ xample 5
Heading dies were made of the Alloy Sample I~os. 1, ~,
12 and 14 or ~xample 3 and WC - 25 ~ Co alloy for comparison
and the life tests thereof were carried out by subjecting
to cold forging of bolts of S45C steel, thus obtaining results
shown in Table 4:
Table 4
Tool ~ife (Number of ~olts Processed/Die) x 10-~
Alloy Sample No. o 2 4 _ 6 8
-x
o :~
12
14 x
~IC - 25 ,~ Co ~
-24_
~809Z
l Example 6
910 g of the solid solution (Mo 7W0 3) prepared in
Example 1 was mixed with 90 g of carbon powder and 3 g of
cobalt powder as a diîustion aid and then subjected to (1)
carburization at 1450 ~ for 1 hour in a nitrogen stream or
(2) carburization at 1350 C for 1 hour in a nitrogen stream.
It was found by ~-ray diffraction that both of the resulting
carbides were uniformly of (MoO 7Wo 3)C and measurement of
the particle size of these carbides by means of ~isher Sub
Sieve Sizer showed that the carbide prepared by the process
(1) had a mean particle size of 4.2 microns, while the car-
bide prepared by the process (2) had a mean particle size
of 1.9 microns.
Each of these carbides was mixed with 30 % of cobalt
powder, ball milled with alcohol medium, pressed in a
desired shape and then sintered.
Heading dies were made of the thus obtained alloys and
the life tests thereof were carried out by subjecting to
cold forging of bolts of S45C steel, thus obtaining results
shown in Table 5:
Table 5
Tool Life (Mumber of ~olts Processed/Die) x 105
~lloy Sample Mo. 0 2 4 6 8
Alloy from x
Process (1)
x
~lloy from x
Process (2)
x
Example 7
Piercing punches for punching a steel plate OI 5 mm
in thickness were made of the Alloy Sample ~los. 7 and 15 of
Example 3 and a WC - 12 ~' Co alloy for co;nparison and used
-therefor 100,000 times. The amounvs of wear of the pier-
cing punches at that time are shown in Table 6:
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117809Z
l Table 6
Allo~ Sample No. Amount of wear (mm)
7 0.07
0.08
~IC - 12 ~ Co 0.21
Example 8
68.5 % of an (MoO 7l~10 3)C powder with a particle size
of 3 microns, 30 % of Ni powder, 1 % of Mn powder and 0.5 %
of Re powder were mixed while adjusting the guantity of
carbon to 97 ~ of the theoretical quantity 6.10 %, and the
mixed powder was sintered at 1250 C for 1 hour in a vacuum
of 10-2 Torr. The resulting alloy was non-magnetic and had
the following properties:
Density: 9.9 g/cm3 Hardness (HRA): 84.5
Transverse Rupture Strength: 290 Kg/mm2
Example 9
75 ~ of an (MoO 5~llO 5)C powder with a mean particle
size of 4 microns, 10 ~ of ~i powder, 13 $ of Co powder,
1 ,~ of Re powder, 0.8 c~ of Mg powder and 0.2 ~0 of B powder
were mixed while adjusting the quantity of carbon to 98 ~
of th~ theoretical quantity of carbon 5.93 ~, and the mixed
powder was sintered at 1350 C for 1 hour in a vacuum of
10-2 Torr. ~he resulting alloy had the following properties:
Density: 10.1 g/cm3 Hardness (HRA): 86.5
Transverse Rupture Strength: 265 Kg/mm2
~or examination of the corrosion resistance, the above
described alloy and a ~lC - 20 ~ Co alloy for comparison were
subjected to tests using various corrosion solutions to give
results as shown in Table 7:
-2
ll7sasz
Table 7
Amount of Corrosion (mg/cm2/hr)
I* II** III***
l~lloy of Our Invention 2.5 0.2 0
WC - 20 ,~ Co 2.8 2.1 0
Note: I Ho. 10 ~;, I12S04 iSolu-tion
II 35 ^ HCl Solution at Room Temperature
III 10 f,S NaOH Solution at Room Temperature
Example 10
30 ~' of an (MoO 7Wo 3)C powder with a mean particle
size of 5 microns~ 35 % of an (MoO 3W0 7)C powder with a
mean particle ~ize of 0.5 micron, 25 % of Ni powder and 10
~ oî Co powder were mixed while adjusting the quantity of
carbon to 97.5 " of the theoretical ~uantity of carbon, i.e.
5.15 ,~, and the mixed powder was sintered at 1320 C for
hour in a vacuum of 10 Torr. The resulting alloy had the
Eollowing properties:
Density: 11.2 g/cm~ Hardness (HRA): 82.5
Transverse P.upture Strength: 280 I~g/mm2
Heading dies for nut former were made of the above
described alloy and a l~lC - 25 " Co alloy for comparison and
the liEe tests thereof were carried out by cold forging nuts
of S15C steel, thus obtaining results shown in Table 8:
Table 8
Tool Life (Number of Nuts Processed/Die) }~ 105
hllo;y Sample No. 0 2 4 6 8 10 12
I~lloy of Our
Invention ~ (12.0)
IJC -- 25 ~`J Co ~ (4.2)
--27--
