FIELD OF THE INVENTION

The invention relates to method for producing hard metal powder suitable for manufacturing hard metal products comprising metal carbides and a metallic binder. The expression "hard metal powder" refers here to a powder comprising fine hard particles of carbide and a binder metal. The hard metal powder can especially be used for providing products with a wear resistant coating.

BACKGROUND OF THE INVENTION

Hard metal powders can be manufactured in multitude manners. Various chemical agents, machines and devices are typically used in the manufacturing processes of the powders.

Desirable is to have a manufacturing process which is easy to carry out and which provides a hard metal powder which has high quality and can be readi- ly used e.g. for spraying a hard metal coating on a product. Known manufacturing processes do not fulfil these requirements in all respects, they require e.g. chemicals, such as reducing agents to prevent undesired oxidation; alternatively oxidation can be prevented by carrying out method steps in a non-oxidizing environment. Preventing undesired oxidation typically makes the manufacturing process complicated and expensive. A conventional process for making agglomerated hard metal powders includes a synthesis which is carried out separately from a spray drying of the completed mixture comprising carbides and a metal matrix (binder). US 20030075012 Al discloses a method for producing a refractory hard metal powder comprising milling a slurry formed of carbide powder, powdered metallic binder and water; subsequently mixing pressing aid components, in the form of a wax emulsion with the slurry; and drying the slurry to form the hard metal powder.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a method for produc- ing hard metal powder, which method is easy to carry out and which provides hard metal powder having high quality and nano sized or fine sized carbides.

This object of the invention is achieved by a method comprising the steps of

a) dissolving in water water soluble raw materials comprising a hard particle source and a metallic binder source to form an aqueous solution, b) drying in a drying process the aqueous solution to form a precursor powder having the raw materials homogenously distributed throughout the precursor powder,

c) decomposing the precursor powder in a first heat treatment step by heating it in a temperature of 370°C to 430°C in an inert atmosphere to remove gas evolved in the decomposition of the raw materials,

d) grinding the precursor powder and mixing it with a liquid media to produce a suspension,

e) spray drying the suspension to agglomerate the precursor powder, and

f) heat treating the agglomerated precursor powder in a second heat treatment step at 900 °C to 1150 °C in an inert atmosphere to form a hard metal powder containing agglomerates of carbides evenly distributed and bonded to a metallic matrix. Preferred embodiments of the invention are disclosed in the dependent claims.

The carbides in the agglomerates in step f) are fine sized, preferably nano sized.

An essential feature of the invention is that two drying steps, prefera- bly spray drying steps, are carried out between which drying steps an intermediate first heat treatment step is carried out. In the method of the invention, a nano carbide synthesis and an in-situ matrix takes place in the form of an agglomerate. Thus, in the invention, the carbide synthesis step and in-situ matrix take place in the heat treating of the agglomerated powder. Thus conventional separate syn- thesis and a spray drying of a finished carbide-metal matrix solution is not made for producing the agglomerated hard metal powder. In the invention there is no need to deal with separate nanoparticles when the carbides are bonded to the metallic matrix in the form of agglomerates, providing thus health and safety advantages.

During drying of the suspension, the powder is not oxidized, because it is already in an oxidized state prior to the drying of the suspension, and this enables to use preferably hot air for drying. Thus, reducing agent is not needed in hot gas drying with air, neither is an inert gas needed for the hot gas drying. In the hard metal powder, the binder is homogenously mixed with the metal carbides providing a dense powder. The powder is readily usable and has good flowability. The hard metal powder has a nanostructure and it is easy to control the composi- tion of the hard metal powder. The expression nanostructure refers in this invention to carbides. The hard metal powder produced by the method is readily usable and has good flowability; it can be used for manufacturing wear resistant powder metallurgy products and coatings. A multitude of manufacturing techniques are available for manufacturing the products and coatings, e.g. thermal spraying including additive manufacturing. More conventional manufacturing techniques include e.g. sintering in a mold.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of a preferred embodiment with reference to the attached drawing, where the main steps of the invention have been presented in a general form.

DETAILED DESCRIPTION OF THE INVENTION

The attached figure shows the main steps of the method of the invention for producing hard metal powder. The expression "hard metal powder" re- fers here to a powder comprising fine hard particles of carbide and a binder metal. Typically, the hard particles of carbide are WC, and the binder metal is typically Co. As the figure tells, the steps for producing hard metal powder are: a) dissolving in water water soluble raw materials comprising a hard particle source and a metallic binder source to form an aqueous solution, b) drying in a drying process the aqueous solution to form a precursor powder having the raw materials homogenously distributed throughout the precursor powder, c) decomposing the precursor powder in a first step by heating it in a temperature of 370°C to 430°C in an inert atmosphere to remove gas evolved in the decomposition of the raw materials, d) grinding the precursor powder and mixing it with a liquid media to produce a suspension, e) spray drying the suspension to agglomerate the precursor powder, and f) heat treating the agglomerated precursor powder at 900 °C to 1150 °C in an inert atmosphere to form a hard metal powder containing agglomerates of carbides evenly distributed and bonded to a metallic matrix.

The hard particle source in step a) above comprises in addition to car- bon (C) one or more of the chemical elements W, Cr, V, Ti, Zr, Mo, Ta, and Nb. Glycine, C2H5NO2, is preferably chosen as the carbon source. If the hard particle source comprises wolfram (W), ammonium meta tungstate, (ΝΗ4)6Η ₂ νΐ2θ4ο·Η2θ, is preferably chosen as the W source.

The binder source in step a) above preferably comprises one or more of the chemical elements Co, Cr, Ni and Fe. If the binder source comprises Co, co- bait acetate, Co(C ₂ H30 ₂ ) ₂ -H ₂ 0, (or C4H6C0O4), is preferably chosen as the binder source.

The drying process of the aqueous solution in step b) is preferably carried out by spray drying in hot gas, preferably in hot air having a temperature preferably above 100 °C. Spray drying is an efficient process for producing a homogenous solution.

The gas removed in the decomposing of the precursor powder in step c) is preferably NH3 and H ₂ 0, and removing these gases will reduce 30 % of the mass of the precursor powder.

A ball mill is preferably used for grinding the precursor powder in step d) . Preferably, the precursor powder is grinded when being mixed with the liquid media.

The liquid media used in step d) to produce a suspension, is preferably water or ethanol.

The spray drying of the suspension in step e) is preferably carried out with a hot gas without the use of a reducing agent and preferably using a hot air dryer. A reducing agent is not needed in the present invention, because the substances of the suspension are in a state which does not react with oxygen. Thus, air can be used as the hot gas in the dryer.

Nano carbide synthesis takes place in step f). The inert atmosphere for heat treating the agglomerated powder in step f) is preferably argon. Preferably, the inert gas can alternatively be i) a mixture of argon and hydrogen, preferably 96-98 % Ar, the rest (2-4 %) H ₂ ; ii) nitrogen; or iii) a mixture of nitrogen and hydrogen, preferably 96-98 % N ₂ , the rest (2-4 %) H ₂ . Gas evolved in decomposition reactions in the step f) is small enough to prevent agglomerates to break up. The powder formed can, as such safely be handled and used for various applications.

The total weight loss is roughly 70 %, typically 60 % to 80 %, when the nano carbide synthesis is made with water soluble raw materials. The size of the agglomerates in step f) is preferably 5 μιη to 50 μιτι, more preferably 10 μιη to 40 μπι and even more preferably 20 μιη to 30 μιη. A size of 20 μιη to 30 μιη is applicable for most applications. The distribution of the binder in the nanoparticles is homogenous in the hard metal powder produced by the method of the invention. The size of the carbides (typically WC) defines the hard metal powder produced by the method of the invention to be a nanomaterial.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The de- tailed description does not limit the invention. The invention may in its details vary within the scope of the claims.