CA1164893A - Process for the production of a hard solid solution containing molybdenum - Google Patents

Abstract

TITLE OF THE INVENTION

A process for the production of a hard solid solution containing molybdenum ABSTRACT OF THE DISCLOSURE

The present invention relates to a process for the production of a solid solution constructed of at least one hard phase having a crystal structure of simple hexagonal type and selected from mixed carbides or carbonitrides of molybdenum and tungsten, which process comprises preparing an alloy powder consisting of a solid solution of molybdenum and tungsten, adding to the alloy powder carbon in an amount necessary for forming (Mo, W)2C and/or (Mo, W)2(CN), heating the mixture at a temperature at which (Mo, W)2C and/or (Mo, W)2(CN) is stable, adding to the (Mo, W)2C and/or (Mo, W)2(CN) carbon in an amount necessary for forming (Mo, W)C and/or (Mo, W)(CN) optionally with an iron group metal and then heating the mixture at a temperature at which (Mo, W)C and/or (Mo, W)(CN) is stable.

Description

I .t 6 4 ~! 9 3 1~ FIELD OF THE INVENTION
The present invention relates to a process for the production of a solid solution carbide of (Mo, W)C or a carbo-nitride of (Mo, W)(CN), which are used as a raw material for cemented carbide alloys, and more particularly, it is concerned with a process for the production of such a carbide or carbo-nitride with a uniform particle size.

2. DESCRIPTION OF THE PRIOR AR~
Up to the present time, as a starting material for - cemented carbides, there has been used tungsten carbide (WC) as a major component, but tung~ten is found in only a few parts of the world and thus is very expensive. ~ately, the tendency is to replace WC molybdenum carbide (MoC) having the ~ame cry-stal structure as WC as well as mechanical properties similar to WC and since MoC i8 un~table, MoC is stabilized by dissol-ving WC therein to form a solid solution of (Mo, W)C which is used as a starting material for cemented carbide alloys.
When using such a carbide or carbonitride as a raw material for cemented carbides alloys or hard alloy~, it is most important how to control the particle size of the hard phase in the allo~s and the thickness of the binder phase corre-3ponding thereto and to this end~ the particle size and evenness of the raw material powder such a~ (Mo~ W)C or (Mo, W)(CN) powder are most important for making even the particle size and distribution there ofthe hard phase.
In the production of mixed carbides of Group IVa, Va and VIa metal~ of Periodic ~able, metal oxides~ carbides and carbon are mixed correspondingly to the compo~ition of an object compound and reacted at a high temperature, or the reaction is promoted by adding an additive to increase the diffusion rate.
When a solid solution i~ produced by the solid phase reaction ~.

~ ~64~93 of powders, however, the degree of reaction is scattered depen-ding on the mode of mixing the powders, the particle size and size di tribution of the powders used. In order to form a uniform solid solution, a heating operation for a long period oî time is necessary as in the process described in Japanese Patent Application (OPI) No. 146306/1976 in which a part of Mo in MoC
is replaced by W to stabilize the (Mo, W)C phase of simple hexagonal type. ~hat is, in the production of a uniform solid solution by the diffusion among powders of metals such as Mo lO and W and carbides, heating at a high temperature such as 1600 C
or higher for a long time is required, in particular~ for diffu-sing and dissolving metallic powders of Mo and W with a particle size of several microns.
As a result of examining the particle size and the par-ticle ~ize distribution of (Mo, W)C and (Mo, W)(CN) prepared by such a known method, a reaction mechanism is found as shown in Fig. 1. In the method as shown in Japanese Patent Applica-tion (OPI) No. 146306/1976 and Japanese Patent Application (OPI) No. 104617/1978 wherein predetermined amounts of MoC and WC to 20 give a final carbide (Mo, W)C are previously mixed, large amounts of carbon and an iron group metal such as Co or Ni for stabili-zing (Mo, W)C are added before the reaction (a-1). During the course of the reaction, there appears once a stable form of (Mo, W)2C + C(a-~), but when this is converted into (Mo, W)C
by a ~ubsequent heat treatment, the particle size or diameter fluctuates (a-3).
If the powder particles are very fine, on the other hand, diffusion proceeds well and a large amount of an iron group metal as a diffusion aid is not required, resulting in 30 a good quality carbide. However, it is difficult on a commer-cial scale to obtain powders of metals and carbides with a particle size of 0.5 micron or less.

~ 1 648g3 1 We, the inventors, have hitherto found that when Mo and W are mixed in the form of ammonium salts of Mo and W, in the state of their solution~ or in the form of their oxides or hailides, mixing can better be accomplished and a uniform solid solution can more readily be obtained at a relatively low tem-perature as compared with combinations of metal powders and/or carbide powders. In this case, for example, W and Mo are uni-formly mixed at the stage of forming their oxides and reduced with hydrogen to form a solid solution of (Mo, W) which is then reacted with carbon to give a solid solution carbide.
This has already been proposed as a commercially feasible process (US Patent No. 4,216,009).

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process for the production of a hard solid solution containing molybdenum.
It is another object of the present invention to provide a process for the production of a solid solution carbide of (Mo, W)C or a solid solution carbonitride of (Mo, W)(CN) having an even particle size.
It is a further object of the present invention to provide a process of producing a solid solution of (Mo, W)C or (Mo, W)(CN) by two carburization stages.
~ hese object~ can be attained by a process for the pro-duction of a hard solid solution con~tructed of at least one hard phase having a crystalline structure of simple hexagonal type, which process comprises preparing an alloy powder consis-ting of a solid solution of molybdenum and tungsten, adding to the alloy powder carbon in an amount sufficient to form (Mo, W)2C and/or (Mo, W)2(CN), heating the mixture at a tempe-rature at which (Mo, W)2C and/or (Mo, W)2(CN) is stable, adding 1 3 ~4~'33 1 to the (Mo, W)2C and/or (Mo, W)2(CN) carbon in an amount suffi-c:;ent to form (Mo, W)C and/or (Mo, W)(CN) optionally with an iron group metal and then heating the mixture at a temperature at which (Mo, W)a and/or (Mo, W)(CN) is stable~

BRIEF DESCRIP~ION OF THE DRAWING

~ he accompanying drawings illustrate the principle and merits of the present invention in more detail.
Fig. 1 and Fig. 2 show reaction models to illustrate a process of formation of a solid solution (Mo, W)C, the model of Fig. 1 being according to the prior art method and that of Fig. 2 being according to the present invention.
Fig. 3 is a graph showing the relationship between the carbon content and the strength as to alloys of the present invention and comparative alloyæ of the prior art.
Fig. 4 and Fig. 5 are micrographs magnified 150 times showing the dispersed state of (MoO 7Wo 3)2C as to alloys of the present invention and the prior art respectively.

DETAILED DE~CRIP~ION OF ~HE INVENTION

~ he present invention aims at making uniform the par-ticle size of a solid solution powder of (Mo, W)C and (Mo~ W)(CN)to be obtained finally. That i8 to say, the present invention provides a process for the production of a solid solution con-structed of at least one hard phase having a crystal structure of simple hexagonal type and selected from mixed carbides or carbonitrides of molybdenum and tungsten and solid sQlutions of tungsten and molybdenum, which process comprises preparing an alloy powder consisting of a solid solution of molybdenum and tungsten, adding to the alloy powder carbon in an amount necessary for forming (Mo, W)2C and/or (Mo, W)2(CN), heating 3o ~1~4~g3 l the mixture at a temperature at which ~Mo, W)2C and/or (Mo, W)2(CN) is stable, adding to the (Mo, W)2C and/or (Mo, W)2(CN) carbon in an amount necessary for forming (Mo, W3C
and/or (Mo, ~)(CN) optionally with an iron group metal and then heating the mixture at a temperature at which (Mo, W)C
and/or (Mo, W)(CN) is stable.
In the process of the present invention, an alloy powder containing molybdenum and tungsten is prepared b~ a mixed powder obtained (1) by mixing an ammonium salt of tungsten (e.g., ammonium tungstate) and an ammonium salt of molybdenum (e.g., ammonium molybdate) in the form of a solu-tion to coprecipitate parasalts of tungsten and molybdenum, (2) by coprecipitating W03 and MoO3 with nitric acid or hydro-chloric acid, or (~) by mixing previously prepared oxides or hydroxides completely in a mechanical manner. Thus, the allo~
powder of (Mo, W) is previously synthesized, mixed with onl~
carbon in a minimum quantity necessary for forming (Mo, W)2C
and subjected to a primary reaction (Fig. 2, b-1). If the reaction temperature is suitably chosen during the same time as mentioned hereinafter, a uniform particle growth can be carried out because of absence of excess carbon becoming a bar to the particle growth of (Mo, W)2C. ~he (Mo, W)2C powder (b-2) grown by this method is mixed with carbon in an amount necessary for the final carbide composition and optionally with an iron group metal such as Co or Ni (b-3), and subjected to a secondary carburization at a temperature at which (Mo, W)C
is stable5 thus obtaining (Mo, W)C powder with a uniform parti~
cle size distribution (b-4).
In the case of producing a carbonitride of (Mo, W)(CN),

3 a mixture of carbon mixed in an analogous manner to the case of (Mo, W)C is subjected to carburization steps in which the carburization atmosphere is changed ~~ that containing N2 r I 16~93 l partly or throughout the steps.
For the practice of the present invention, it is desired t~at the quantity of carbon to be added before the primary reac-tion is in the range of z = 0.4 - 0.6 in carbides or carbonit-rides represented respectively by (Mo, W)Cz or (Mo, W)(CN)z.
If z is less than 4, the carbide is not stabilized as (Mo, W)2C, while if z is more than 0.6, the carbide after the primary reac-tion is under such a state that (Mo, W)2C, (Mo, W)C and (Mo, W)~C2 coexist and thus a uniform particle growth is not carried out.
Moreover, it is desirable to control the quantity of carbon to be added before the secondary reaction so that the final carbide composition be in the range of z = 0.9 - 1.0 in (Mo, W)Cz or (Mo, W)(CN)z. If z is less than 0.9, the strength of the final alloy is insufficient, while if z exceeds 1.0, it is difficult to sinter the final alloy.
Where the carbide or carbonitride is represented by (Ma Wb)Cz Wb)(CN)z, the primary heating condition when a ~ 0.8 and b c 0.2 is preferably 1400 C or higher. If lower than 1400 C, (Mo, ~)2C is not so stabilized and, accordingly, a higher tem-perature i~ rather desirable. On the contrary, the secondaryreaction is preferably carried out at a temperature of 1400 C
or lower.
When a C 0.8 and b ~ 0.2, the primary heating condition is preferably 1400 C or higher, more preferably 1800 C or higher.
1~ seco~ala.^~
~ ) When the~carburiz~tion reaction is carried out at a temperature . ~..
of 1800 C or lower, the carbide i~ stabilized as (Mo, W)C and at a temperature of 1400 a or lower~ it is more stabilized.
In order to accomplish the reaction surely and in a short time, it is further desired that the primary carbide is once cooled to room temperature and then subjected to a treatment to impart a mechanical ~train, such as grinding.
The following examples are given in order to illustrate the present invention in detail without limiting the same.

g 3 1 Example 1 54 g of Mo powder and 46 g of W powder were dissolved in 28 % aqueous ammonia and gradually neutralized with hydro-chloric acid to precipitate needle crystals. The thus copre-cipitated W03 and MoO3 were well mixed. These oxides were ~intered at 800 C in the air. The mixed powder was charged in a Ni boat, covered and then reduced at 1000 C in an H2 stream to obtain an alloy powder of 2 microns.
The resulting alloy powder (MoO 7W0 3) was mixed with

4 5 ~ by weight of carbon powder and ball milled for 36 hours.
This mixed powder was reacted within a temperature range wherein the subcarbide (MoO 7Wo 3)2C was stable, i.e. at 1900 C in an H2 stream for 1 hour. The carbide was once cooled and ball milled for 1 hour. Measurement of the particle size of the (Moo 7Wo.3)2C powder showed that it was a uniform powder with a particle size of 8 microns and a narrow particle size distri-bution.
The primary carbide powder wa~ mixed with 4.5 % by wei~ht of carbon powder and 1 % by weight of Co203 powder and subjected again to carburization at a temperature at which the monocarbide was stable, i.e. at 1400 C in an H2 stream. When the properties of the resulting carbide were examined, it was fou~d that the carbide Was a monocarbide of WC type containing combined carbon in a substantially theoretical quantity as shown in Table 1:
Table Combined Carbon Total Carbon Free Carbon Combined Carbon x 100 Theoretical Carbon ,, 8.9~ % 0.02 ~ 8.91 ~ 99.8 %
Example 2 A solid solution carbide of (MoO.85W0.15) trial by the procedure of Example 1. An alloy powder of i ~1 64~33 1 (~loO 85W0 15) was previously prepared in an analogous manner to E`xample 1 and well mixed with 5.0~ by weight of carbon powcier. The mixed powdex was charged in a graphite boat, heated up to 1600C for a period of time of about 3 hours, held at the maximum temperature for 1 hour and cooled to room tem-perature for 10 hours. The quantity of carbon in the powder i5 shown in Table 2. The reactivity was 50.2~. The analy-tical result of X ray diffraction showed a peak of (Mo, W)2C

only.
Table 2 Total Carbon Free Carbon Combined Carbon Reactivity*
4.91 % 0.07 % 4.84 % 50.2 Note: * Reactivity = COmbined Carbon x 100 Theoretical Carbon The subcarbide powder of ~o0.85W0.15)2C
mixed with 4% by weight of carbon powder and 0.3% by weight of Co powder, charged in a Tammann-furnace and heated at 1250C for about 40 minutes in an H2 stream. The properties of the resulting carbide were examined thus obtaining results as shown in Table 3:
Table 3 Total Carbon Free Carbon Combined Carbon Reactivity 9.57 % 0.21 % 9.45 % 95 %
X ray diffraction showed that the peak of (Mo, W~2C sub-stantially disappeared and the carbide had substantially a crystal structure of WC type.
Example 3 The alloy powder of (MoO 7W0 3) obtained in an analogous manner to Example 1 was mixed with 4.5~ by weight of carbon powder and ball milled for 36 hours. The mixed powder was reacted at 1800C in an N2 stream for 1 hour, cooled to ~ 1 6~93 1 room temperature and ball milled further for 1 hour. The nitrogen content, as analysed in the powder, was 0.10%. The resulting carbonitride of (MoO 7Wo 3)2(CN) was mixed with 4.:3% by weight of carbon powder and 0.3~ by weight of iron powder and subjected to carburization at 1500C in an N2 stream, thus obtaining a carbonitride with the following analytical data:
Table 4 Total Carb n Free Carbon Combined Carbon Nitrogen Reactivit~
8.60 ~ 0.00 % 8.60 % 0.15 % 97.5 %
The thus resultant carbonitride had a particle size of 7 microns and, according to X ray diffraction analysis thereof, there was substantiall~ a peak of ~7C type with a neglifible amount of ~Mo, W12C.
As ap~arent from these results, the carbides and carbonitrides obtained according to the present invention had a mean particle size of 4 to 8 microns, suitable for use as a raw material of cemented carbides alloys for hot use.
Example 4 The alloy powder of ~MoO 5W0 5) obtained in an analogous manner to Example 1 was mixed with 4.0% by weight of carbon powder and ball milled for 36 hours. The mixed powder was reacted at 1700C in an H2 stream for 1 hour, and cooled to room temperature. The resulting ~oO 5Wo 5~2C powder was then mixed with 4.0% by weight of carbon powder and 0.3~ by weight of cobalt powder, and subjected to carburization at 1450C in an H2 stream, thus obtaining a carbide with the following analytical data;

_ 9 _ I 1 6~93 1 Tàble 5 -_tal Carbon Free Carbon Combined Carbon Nitrogen Reactivity 7.87 % 0.01 % 7.86 % 0.02 % 99.S %
Example S
The alloy powder prepared in Example 1 was mixed with 8.9% by weight of carbon powder, ball milled for 36 hours and then reacted at 1700C in an H~ stream for 1 hour to form a ;Mo0 7W0 3)C powder (B).
The (Mo0 7W0 3~C powder (A) and (Mo O 7W0 3)C powder (B) were respectively mixed with 30% by weight of Co in a motar, compacted in a mold and sintered at 1300C in a high vacuum of 10 4 mmHg or less for 1 hour. The ally ~C) from the powder (A) and the alloy (D) from the powder ~B) were res-pectively subjected to examination of the particle size dis-tribution of the carbide using an image analyser, thus obtaining results shown in Table 6:
Table 6 (~ by volume~ ~
d<0.5~0.5~-d<l~ 1~-d<3~3~-d~5~ 5~-d<10~ 10~-d Our Alloy ~C) -0 0 0.9 11.1 77.9 10.1 20 Comparison Alloy (D) 4.56.1 16.7 20.3 40.2 12.2 Note: d = particle diameter Example 6 The ~Mo0 7Wo 3)C powder ~A) prepared in Example 1 and the (Mo0 7Wo 3)C powder ~B~ prepared in Example 5 were res-pectively mixed with 30% by weight of Co, ball milled by wet process, compacted in mold and sintered at 1300C in a high vacuum of 10 4 mmHg or less for 1 hour to obtain alloys with the following properties:

~.

~ 1648~3 1 Table 7 (Mo W 3)C powder Density Hardness Transverse 0 7 0 tg/cc) (HRA) Rupture (Kg/mm2 ) -Ou:r Alloy (E) A 10.2 83.5 290 Comparison Alloy (E) ~ 10.2 83.0 250 Example 7 Header tools for nuts were made of Our Alloy ~E) and Comparison Alloy ~E), prepared in Example 6, and used, for test, as a header die for producing a wire rod of SCr 4. The results are shown in Table 8 with those of a marketed WC-25 wt Co-Alloy:
Table 8 i Tool Life (x 10 4~
~ 10 20 30 40 50 I (pieces~
! Our Alloy ~ ~ o o - O
I Comparison x35 ¦ - Alloy (F) x28 x46 WC-25%Co Alloy x17 xlO
- xll o : usable x : broken Example 8 The (MoO 7Wo 3~C powder ~ prepared in Example 1 and the (MoO 7Wo 3)C powder (B) prepared in Example S were res-pectively mixed with 35~ by weight of Co powder and in each case, six sample alloys were prepared with varying the carbon content in the range of 5.30 to 5.90% by weight. The properties of these alloys are shown in Table 9:

I t64~3 1 Table 9 Density Hardness Transverse Analytical Values ~g/cc) (IIRA) Rupture Total Carbon Free Strength (%) Carbon (Kg/mm2 ) ( % ) Our Alloy No.
1 10.0 82.6 185 5.32 0.00 2 10.0 82.9 260 5.46 0.00 3 10.0 82.3 320 5~60 0.00 4 10.0 82.4 295 5.72 0.00 9.9 82.3 230 5.80 0.06 6 9.9 82.4 180 5.90 0.15 7 10.0 82.3 180 5.30 0.00 8 10.0 82.2 167 5.48 0.00 ~ 10.0 82.5 225 5.62 0.00 10.0 82.1 248 5.70 0.00 11 9.9 82.2 210 5.81 0.06 12 9.9 82.2 175 5.90 0.14 The alloys prepared in Example 8 were subjected to Charpy test, thus obtaining results shown in Fig. 3 ~Curve A:
Alloy Nos. 1-6 of the present invention; Curve B: alloy Nos. 7-12 of the prior art~.
The alloys of Example 8 (No. 2 and No. 81 were com-pared as to the dispersed state of (Mo0 7Wo 3)C by taking micrographs magnified 150 times, as shown in Fig. 4 and Fig. 5 respecti~ely.

, ~

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the production of a hard solid solution comprising at least one hard phase having a crystal structure of simple hexagonal type and being represented by (MoaWb)Cz or (MoaWb)(CN)z wherein z=0.9 to 1.0 and a+b=1, which process comprises:
(a) preparing an alloy powder consisting of a solid solution of molybdenum and a tungsten and adding to the alloy powder carbon in an amount sufficient to give a mixture wherein z=0.4 to 0.6, (b) heating the mixture at a temperature of at least 1400°C, and in an atmosphere containing nitrogen in the case of forming the carbonitride, (c) cooling the mixture, (d) grinding the mixture with carbon in an amount sufficient to give z=0.9 to 1.0, and (e) then heating the mixture at a temperature of at most 1400°C in the case of a > 0.8 and b < 0.2 or at a temperature of at most 1800°C in the case of a < 0.8 and b > 0.2, and in an atmosphere containing nitrogen in the case of forming the carbonitride.

2. The process according to claim 1 wherein an iron group metal is included in said mixture ground with carbon in step (d).

3. The process as claimed in claim 1 wherein the alloy powder is prepared by mixing molybdenum and tungsten in the form of compounds thereof selected from the group consisting of oxides, hydroxides, chlorides, sulfates, nitrates, metallic

Claim 3 continued...

acids and mixtures thereof and then reducing the mixture with at least one member selected from the group consisting of hydrogen and ammonia.

4. The process as claimed in claim 3 wherein ammoniacal solutions of molybdenum and tungsten are mixed.

5. The process as claimed in claim 1 or 2 wherein the heating is carried out in a hydrogen stream.