Manufacture of tungsten monocarbide (WC) spherical powder

Abstract

Powder metallurgy, in particular production of tungsten monocarbide spherical powders, which is a major component of metalloceramic hard alloys used for manufacture of tools, drill bits, steel alloying, wear-resistant coating cladding at elements operating in intensive wear conditions. The method includes melting of the starting material, and melt atomization with forming of spherical powder. As starting material a tungsten monocarbide grit is used. Melting and atomization of the material is implemented by continuous filling of grit into a rotating crucible of a centrifugal atomization device under an inert atmosphere and melting it by a plasma arc. After that an annealing of the obtained powder is made at a temperature of 1200-1400° C. during a time necessary for W.sub.2C breakup with subsequent cooling of the powder in a furnace. And, the production of tungsten monocarbide spherical powder with WC content of more than 70%.

Claims

1. A method for the manufacture of tungsten monocarbide (WC) spherical powder said method comprising the steps of: initial melting of a starting material, and subsequent melt atomization with forming of spherical powder wherein a grit of tungsten monocarbide is used as the starting material, melting and atomization of the starting material is performed by continuous filling of grit into a rotating crucible of a centrifugal atomization device under an inert atmosphere and melting by plasma arc to form a powder comprising WC, W.sub.2C, and C, and after that an annealing of the powder is made at a temperature of 1200-1400° C. during a time necessary for W.sub.2C breakup and subsequent cooling of the powder in a furnace.

2. The method according to claim 1, wherein the starting material is a grit of tungsten monocarbide WC with a particle size in the interval 20-80 μm.

3. The method according to claim 1, wherein the annealing of the obtained powder is performed during 1.5-2 hours.

4. The method according to claim 1, wherein a powder with particle size from 10 μm to 2.5 mm is produced.

5. The method according to claim 1, wherein the inert atmosphere comprises at least one gas selected from the group consisting of nitrogen, argon, and helium.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 presents an electron micrograph of tungsten monocarbide spherical powder (a) and a cross section of particles after etching (b).

(2) FIG. 2 presents a typical radiograph of tungsten monocarbide spherical powder directly after atomization.

(3) FIG. 3 presents a typical radiograph of tungsten monocarbide spherical powder after annealing.

DETAILED DESCRIPTION

(4) In accordance with the state diagram, the tungsten monocarbide is formed by a peritectic reaction at a temperature ˜2500° C. At a temperature of complete melting (2870° C.) the tungsten monocarbide is disintegrated to W.sub.2C and carbon. During melt crystallization under the atomization process the phases of WC, W.sub.2C and free carbon are formed. Hereby, spherical powder obtained during melt atomization is a mixture of the above described phases. Annealing of the obtained atomized powder mixture allows transforming it into an equilibrium state. Thereby the phase W.sub.2C in the particles transforms to phase WC. Herewith the choice of modes and conditions of cooling allows to obtain desired properties. It is experimentally determined that the annealing treatment of atomized spherical powder at temperature 1200-1400° C. with a holding time necessary for W.sub.2C decomposition is suitable. The subsequent slow cooling of the powder in a furnace allows to obtain a spherical powder with a tungsten monocarbide content of more than 70 vol %. The best results for the annealing of the powder is observed for annealing times of 1.5-2 hours at 1200-1400° C.

(5) In a first aspect there is provided a production method for the manufacture of tungsten monocarbide (WC) spherical powder, said method comprising the steps of: initial melting of a starting material, and subsequent melt atomization with forming of spherical powder wherein a grit of tungsten monocarbide is used as the starting material, melting and atomization of the starting material is performed by continuous filling of grit into a rotating crucible of a centrifugal atomization device under an inert atmosphere and melting by plasma arc to form a powder, after that an annealing of the powder is made at a temperature of 1200-1400° C. during a time necessary for W.sub.2C breakup and subsequent cooling of the powder in a furnace.

(6) In one embodiment the starting material is a grit of tungsten monocarbide WC with a particle size in the interval 20-80 μm. Particle size is defined as the largest possible distance between any two points on the surface of the particle. For a spherical particle the particle size corresponds to the diameter.

(7) In one embodiment the annealing of the obtained powder is performed during 1.5-2 hours. A skilled person can determine a suitable time for decomposition of W.sub.2C by routine experimentation in the light of this description. A suitable value is often in the interval 1.5-2 hours.

(8) In one embodiment a powder with particle size from 10 μm to 2.5 mm is produced. The particles size is measured on individual particles of the powder.

(9) In one embodiment the inert atmosphere comprises at least one gas selected from the group consisting of nitrogen, argon, and helium. In one embodiment the inert atmosphere is nitrogen. In one embodiment the inert atmosphere is argon. In one embodiment the inert atmosphere is helium. In one embodiment the inert atmosphere is at least one noble gas. In one embodiment the inert atmosphere is at least one inert gas.

(10) In a second aspect there is provided a tungsten monocarbide spherical powder with a particle size from 10 μm to 2.5 mm and which is produced by the method as described above.

(11) In one embodiment the WC content is more than 70 vol %, i.e. the content of tungsten monocarbide.

EXAMPLES

(12) Initial powder (starting material) of tungsten monocarbide with irregular shape particles (the grit) was atomized in a centrifugal atomization device from a rotating scull crucible.

(13) Grit melting is implemented directly in the rotating crucible using plasma arc heat burned between the plasmatron and the surface of the rotating crucible. Initial grit was continuously filled in the crucible.

(14) In order to study the influence of the gas atmosphere in the work chamber on the size of particles on its properties, the atomization under argon, helium and nitrogen were conducted under atmospheric pressure. Depending on atomization modes a spherical powder with a particle size from 10 μm to 2.5 mm was obtained.

(15) In all cases, the particles of obtained powder have spherical shape, see also FIG. 1.

(16) Independent of the particle size and atomization conditions the phase composition of atomized material is identical and represent a mixture of particles with following phases: WC (˜31-35 vol %.), W.sub.2C (˜42-58 vol %.), C (˜10-23 vol %.), FIG. 2, table 1.

(17) Annealing of obtained spherical powder at temperatures more than 1200° C. with different time of hold up were conducted.

(18) Table 1 shows the phase ratio in spherical powder material depending on atomization conditions and thermal treatment modes. FIG. 3 shows a typical diagram of tungsten monocarbide spherical powder after annealing.

(19) In all cases regarding the structure of powder material, the change of material phase composition after thermal treatment is observed in comparison to material phase composition directly after atomization.

(20) Depending on thermal treatment modes, W.sub.2C phase content is decreased sufficiently and it disappears practically. W.sub.2C phase content at the most effective treatment modes is not more than 0.5 vol % or even lower. This content is so low that it has no practical importance for most applications and thus it can be said that the W.sub.2C has disappeared.

(21) Thereby, the suggested technical solution provides practically pure tungsten monocarbide in form of atomized powder with spherical shape of particles. For most practical applications the tungsten monocarbide can be considered as sufficiently pure.

(22) TABLE-US-00001 TABLE 1 Phase composition of tungsten monocarbide spherical powder directly after atomization and after thermal treatments. Atom- Size of ization Thermal Content of Material particles, condi- treatment phases, vol %. No condition μm tions at T ° C. WC W.sub.2C C 1 Initial 20-80 100 grit 2 Spherical 15-42 nitrogen 31 57 12 3 powder 400-630 34 56 10 4 directly 1600-2000 32 58 10 5 after 125-200 argon 35 42 23 6 atomization  800-1000 31 48 21 7 15-42 helium 35 55 10 8 125-200 32 54 14 9 400-630 35 45 20 10 Spherical 15-42 nitrogen 1200° C.- 70.2 2.8 27 powder 5 h 11 after 15-42 1350° C.- 70.5 1.4 28.1 atomization 4 h 12 and thermal 400-630 1400° C.- 97.6 0.7 1.7 treatment 3 h 13 1600-2000 1350° C.- 70.1 2.9 27 4 h 14 125-200 argon 1350° C.- 81 0.6 18.4 7 h 15  800-1000 1400° C.- 94 0.5 5.5 4 h 16 15-42 helium 1350° C.- 70.4 1.2 28.4 4 h 17 125-200 1400° C.- 93 0.4 6.6 4 h 18 400-630 1250° C.- 86 2.2 11.8 6 h