EP2647731B1

EP2647731B1 - Method of making a cemented carbide body

Info

Publication number: EP2647731B1
Application number: EP12163181.6A
Authority: EP
Inventors: Michael Carpenter; Jane Smith
Original assignee: Sandvik Intellectual Property AB
Current assignee: Sandvik Intellectual Property AB
Priority date: 2012-04-04
Filing date: 2012-04-04
Publication date: 2017-07-19
Anticipated expiration: 2032-04-04
Also published as: EP2647731A1; ES2643688T3

Description

The present invention relates to a method of making a cemented carbide body where the WC raw material is single crystal and has an angular or spherical morphology. The raw materials are subjected to a non-milling mixing operation by using an acoustic mixer.

Background

Cemented carbide components are well known in applications such as metal machining, as wear parts, in mining applications etc. Cemented carbide raw material powders, usually WC and cobalt, used for making sintered bodies for e.g. cutting tools, wear parts etc. are usually made by first forming a slurry by milling the powder constituents together, organic binder (e.g. polyethylene glycol) and a milling liquid in either a ball mill or an attritor mill for several hours. The slurry is then usually subjected to a spray drying operation to form granulated WC-Co powders which can be used to press green parts that are subsequently sintered.
The main purpose of the milling operation is to obtain a good binder phase distribution and good wettability between the hard constituent grains and the binder phase powder, and in some cases to de-agglomerate WC crystals. A good binder phase distribution and good wettability is essential for achieving cemented carbide of high quality. If the phase distribution or wettability is poor, pores and cracks will be formed in the final sintered body which is detrimental for the material. However, obtaining a good binder phase distribution and wettability is very difficult for these types of materials and requires a high input of energy, i.e. quite long milling times, usually 10-40 hours depending on the type of mill used and/or the grade produced. To achieve coarser grain size grades the milling time is relatively low such to minimize WC crystal breakdown whilst trying to ensure good binder distribution.
Ball mills and attritor mills both provide good, homogenous mixing of the powder constituents, binder metal powders and the organic binder. These processes provides a large energy input that can overcome the static friction and binding forces that is required to obtain a good binder phase distribution and good wettability. However, such mills will subject the powders to a milling operation. Hence, the powders, both hard constituent powders and binder metal powders, will partly be grinded so that a fine fraction will be formed. This fine fraction can cause uncontrolled grain growth during the subsequent sintering. Hence, the grain size distribution of a narrow sized raw material can be destroyed by milling.
Conventional manufactured WC powder used for cemented carbide is characterized as fairly agglomerated and with different grain shapes and ranges. The non-uniformity of WC powder results from the heterogeneity of the W powder produced by reduction and this can become even more mixed during the subsequent carburization stage. Furthermore, during sintering any WC agglomerates may form larger sintered carbide grains and contain an increased frequency of sigma2 boundaries, i.e. carbide grains together without cobalt layer. Single crystal WC raw material having an angular or spherical morphology are usually manufactured by being carburized at high temperature and after the W metal has been deagglomerated.
Single crystal WC raw material having an angular or spherical morphology and narrow distribution, are commonly used in applications that requires a superior toughness: hardness relationship e.g. mining applications. In such applications, it is important that the narrow grain size distribution and the morphology are preserved as much as possible. In order to minimize the milling time, the milling step has been combined with other methods to obtain a good mixing between WC and cobalt.
One method is the sol-gel method as described in patent EP752921 . Such methods are quite complicated and costly and indeed still require a milling step.
Resonance acoustic mixers are known in the art, see e.g. WO2008/088321 and US 7,188,993 . Such mixers use low-frequency, high intensity sound energy for mixing. EP0682577B1 shows examples of cemented carbides however it does not disclose a method of using acoustic waves to mix the powder blend.
It is an object of the present invention to provide a method making it possible to maintain the grain size, distribution and the morphology of the in the sintered material while still achieving a good mixing.
To achieve this object the invention provides a method as defined in the appended claims.

Brief description of drawings

Fig. 1 shows the grain size distribution comparing Invention 1 and Comparative 2 from Examples 1 and 2.
Fig. 2 shows a histogram showing the grain size distribution comparing Invention 2 and Comparative 1 from Examples 1 and 2.
Fig 3 shows a LOM micrograph of Invention 1 from Example 1.
Fig 4 shows a LOM micrograph of Comparative 2 from Example 3.

Detailed description

A method of making a cemented carbide body comprising the steps of:

forming a powder blend comprising WC raw material powder and a metal binder phase powder, where the WC raw material is single crystal and where the WC grains after sintering have a spherical or angular morphology,
subjecting said powder blend to a mixing operation using a non-contact mixer wherein acoustic waves achieving resonance conditions is used to form a mixed powder blend,
subjecting said mixed powder blend to a pressing and sintering operation.

The WC raw material is suitably a single crystal WC having a spherical or angular morphology. These types of WC are typically manufactured by carburizing at a high temperature and subsequently being de-agglomerated.
The actual determination of the shape of the WC crystal, i.e. spherical or angular, is usually done by first choosing the correct raw material, i.e. a WC powder made by deagglomerating spherical or angular tungsten-metal powder followed by high temperature carburization to maintain the rounded particle shape and keep a mono crystalline nature in the tungsten carbide powder. The WC raw material powder is usually examined in a Scanning Electron Microscope to determine if the powder is single crystalline or agglomerated and what morphology or shape the grains have. The shape is then confirmed by measurements after sintering.
By spherical is herein meant grains that have a "round" shape, not the exact mathematical definition of spherical.
'Spherical' WC herein refers to the grain morphology as measured after sintering. This can be analyzed using a micrograph of a large number of grains and measuring the ratio between the diameter of the largest circle that may be inscribed within the grain dimension, d1, and the diameter for the smallest circle that the grain dimension fits into, d2. The Riley ratio (ψ) is then determined by the equation: $ψ = \sqrt{\frac{d 1}{d 2}}$
A sphere has the Riley ratio of 1 whereas "rounded" grains are considered in the art to have a ratio below 1.3.
In one embodiment of the present invention, the WC grains are spherical after sintering and suitably have a Riley ratio of below 1.5, preferably between from 1.2 to 1.5.
By angular WC is herein meant that the WC has the shape of truncated tri-gonal prisms. Angular WC grains suitably have a Riley ratio of above 1.5.
The WC raw material suitably has an average grain size (FSSS) of from between 0.2 to 30 µm, preferably 1 to 8 µm, more preferably from 2 to 4 µm and most preferably from 2.5 to 3.0 µm.
The amount of WC added is suitably between 70 to 97 wt%, preferably between 83 to 97 wt%, more preferably between 85 to 95 wt%.
The binder metal powders can either be in a powder of one single binder metal, or a powder blend of two or more metals, or a powder of an alloy of two or more metals. The binder metals are selected from Cr, Mo, Fe, Co or Ni, preferably from Co, Cr or Ni. The grain size of the added binder metal powders is suitably between 0.5 to 3 µm, preferably between 0.5 to 1.5 µm. The amount of binder phase is suitably between 3 to 30 wt%, preferably between 3 to 17 wt%, more preferably between 5 to 15 wt%.
In one embodiment of the present invention the cemented carbide is suitably for mining applications, the amount of binder phase is suitably between 5 to 7 wt% and the WC suitably have an average grain size of between 2 to 3 µm.
In another embodiment of the present invention the cemented carbide is suitable for another mining application, the amount of binder phase is suitably between 7 to 12 wt% and the WC suitably have an average grain size of between 1 to 3 µm.
The cemented carbide can further comprise hard constitutes selected from borides, carbides, nitrides or carbonitrides of metals from groups 4, 5 or 6 of the periodic table, preferably tungsten, titanium, tantalum, niobium, chromium and vanadium. The grain size of the hard constitutes can have a mean size of below 1 µm and up to 8 µm, depending on the grade application.
An organic binder is also optionally added to the powder blend in order to facilitate the granulation during the following spray drying operation and/or also to function as a pressing agent for any following pressing and sintering operations. The organic binder can be any binder commonly used in the art. The organic binder can e.g. be paraffin, polyethylene glycol (PEG), long chain fatty acids etc. The amount of organic binder is suitably between 15 and 25 vol% based on the total dry powder volume, the amount of organic binder is not included in the total dry powder volume. The organic binder can either be added before or after the mixing step.
The mixing of the raw material powders are suitably performed using a non-contact mixer wherein acoustic waves achieving resonance conditions, preferably in a resonant acoustic mixer apparatus. Acoustic mixers are known in the art, see e.g. WO2008/088321 and US 7,188,993 . Such mixers use low-frequency, high intensity sound energy for mixing. A mechanical resonance, also called natural vibration or self-oscillation, is a general phenomenon of a vibrating system where the amplitude of the vibration becomes significantly amplified at a resonance frequency. At resonance frequency even a weak driving force applied to the system can provide a large amplitude, and hence a high mixing efficiency of the system.
One advantage with the method according to the present invention is the short treatment (mixing time) to achieve homogeneity of the mixture and that little or no mechanical damage, fracture or stresses are induced in the WC crystals. Furthermore in the utilizing of this process in the system gives the advantage that the energy consumption is low. Thus no change is made to the grain size or distribution of the hard constituent powders by the acoustic mixing process.
In one embodiment of the present invention the vibrations are acoustic vibrations. Acoustic waves are utilized to put the system in resonant condition. The acoustic frequencies are considered to be within the interval 20-20 000 Hz. In another embodiment of the present invention the vibrations has a frequency of 20-80 Hz, preferably 50-70 Hz.
In one embodiment of the present invention, the mixing is done without any mixing liquid, i.e. dry mixing. In one embodiment the organic binder can then be added in a solvent, preferably ethanol or an ethanol mixture, to form a slurry after mixing but prior to drying.
In another embodiment of the present invention, a mixing liquid is added to the powder blend to form a slurry prior to the mixing operation. Any liquid commonly used as a milling liquid in conventional cemented carbide manufacturing can be used. The milling liquid is preferably water, alcohol or an organic solvent, more preferably water or a water and alcohol mixture and most preferably a water and ethanol mixture. Also, other compounds commonly known in the art can be added to the slurry e.g. dispersion agents, flocculation agents, pH-adjusters etc.
Since the drying of the slurry requires energy, the amount of liquid should be minimized in order to keep costs down. However, enough liquid need to be added in order to achieve a pumpable slurry and avoid clogging of the system. The properties of the slurry system are dependent on the solids and liquid content.
Drying of the slurry is preferably done according to known techniques, in particular spray-drying. The slurry containing the powdered materials mixed with the organic liquid and possibly the organic binder is atomized through an appropriate nozzle in the drying tower where the small drops are instantaneously dried by a stream of hot gas, for instance in a stream of nitrogen, to form agglomerated granules. The formation of granules is necessary in particular for the automatic feeding of compacting tools used in the subsequent stage. For small scale experiments, other drying methods can also be used, like pan drying.
Green bodies are subsequently formed from the mixed powders/granules. Any kind of forming operation known in the art can be used, e.g. injection molding, extrusion, uniaxial pressing, multiaxial pressing etc.. If injection molding or extrusion is used, additional organic binders are also added to the powder mixture.
The green bodies formed from the powders/granules made according to the present invention, is subsequently sintered according to any conventional sintering methods e.g. vacuum sintering, Sinter HIP, plasma sintering etc.. The sintering technique used for each specific powder composition is preferably the technique that would have been used for that powder composition when the powder was made according to conventional methods, i.e. ball milling or attritor milling.
In one embodiment of the present invention, the sintering is done by gas pressure sintering (GPS). Suitably the sintering temperature is between 1350 to 1500°C, preferably between 1400 to 1450 °C. The gas is preferably an inert nature e.g. argon. The sintering suitably takes place at a pressure of between 20 bar to 1000 bar, preferably between 20 bar to 100.
In another embodiment of the present invention the sintering is done by vacuum sintering. Suitably the sintering temperature is between 1350 to 1500 °C, preferably between 1400 to 1450°C.
Suitable applications for cemented carbides made according to the method above include wear parts that require a combination of good hardness (wear resistance) and toughness properties.
The cemented carbide manufactured according to the above can be used in any application where cemented carbide is commonly used. In one embodiment, the cemented carbide is used in oil and gas applications such as mining bit inserts.

Example 1 (invention)

Samples of cemented carbide comprising the hard phase WC and the binder phase Co were manufactured. The WC raw material was a single crystal WC having a typically spherical morphology, as determined by visual investigation in a Scanning Electron Microscope with an average FSSS grain size of 2 µm.
The powders of WC and Co were mixed with an ethanol-water - PEG mixture in a LabRAM acoustic mixer. The mixing was done for 5 minutes at an effect of 100% intensity.
After mixing the slurry was spray dried forming agglomerates which was then pressed to bodies of the shape of drill bits. The pressed bodies were GPS sintered at vacuum at a temperature of 1410°C to dense samples of cemented carbide. The characterization of sintered grain size was done according to ISO4499. The WC grains after sintering were generally spherical with a particle size of 1.5 um and a distribution that is characterized by a Gaussian distribution, see Figures 2 and 3. The amounts and properties of the different raw materials are given in Table 1. Table 1

Co content (wt%) WC morphology WC grain size (µm, FSSS) prior to mixing

Invention 1 6 spherical 1.5

Invention 2 11 spherical 1.5

Example 2 (prior art)

Samples of cemented carbide comprising the hard phase WC and the binder phase Co were manufactured. Powders of WC and Co according to Table 2 were wet milled in a ball mill for 10h at a ratio of milling bodies to powder of 3.6:1, spray dried and pressed to bodies of the shape of drill bits. The pressed bodies were GPS sintered at vacuum at a temperature of 1410°C to dense samples of cemented carbide. The sample is denoted Comparative 1. Table 2

Co (wt%) WC morphology WC grain size (µm, FSSS) prior to milling

Comparative 1 11 angular 4

Example 3 (prior art)

A cemented carbide has been manufactured by the sol-gel method according to EP752921 using a cobalt acetate to coat the WC raw material with spherical morphology. After coating the slurry is dried and the Co acetate reduced with hydrogen at 450°C. The coated dry powder containing 2wt% Co is added to a milling vessel together with the additional 4 wt% Co adjusted to achieve the grade composition as comparative 2, including an ethanol-water mixture and a lubricant and followed by a "gentle milling", wet milling in a ball mill for 4h at a ratio of milling bodies to powder of 2.7:1 to achieve homogeneity. The raw material powders are defined in Table 3. Table 3

Co (wt%) WC morphology WC grain size (µm, FSSS) prior to milling

Comparative 2 6 rounded 4

Example 4

The cemented carbide samples from examples 1 and 2 3 were analyzed with regards to grain size, hardness and porosity. The coercivity was measured by the standard method ISO3326.
The grain size and the Riley ratio was measured from a micrograph from a polished section with mean intercept method in accordance with ISO 4499 and the values presented in Table 1 are mean values. The hardness is measured with a Vickers indenter at a polished surface in accordance with ISO 3878 using a load of 30 kg.
The porosity is measured in accordance with ISO 4505, which is a method based on studies in light microscope of polished through cuts of the samples. Good levels of porosity are equal to or below A02maxB00C00 using the ISO4505 scale. The grain size of the WC raw material is also included for comparison.

The results can be seen in Table 4.

Table 4

	WC raw material (µm)	WC sintered (µm)	Hardness (HV30)	Magnetic sat. %	Hc (kA/m)	Riley ratio	Porosity
Invention 1	1.5	2	1270	93	5.6	1.16	A02,B00,C00
Invention 2	1.5	1.5	1250	90	8.2	1.29	A02,B00,C00
Comparative 1	4	4.5	1250	90	8.4	1.75	A02,B00,C00
Comparative 2	6	4.5	1300	90	6.8	1.17	A02,B00,C00

As it can be seen in Table 4, the physical properties of the samples according to the present invention, Invention 1 and 2, shows equal or improved properties as compared to the prior art samples, Comparative 1 and 2.

Claims

A method of making a cemented carbide body comprising the steps of:
- forming a powder blend comprising WC raw material powder and a metal binder phase powder,

- subjecting said powder blend to a mixing operation using a non-contact mixer wherein acoustic waves achieving resonance conditions is used to form a mixed powder blend,

- subjecting said mixed powder blend to a pressing and sintering operation wherein the WC raw material is single crystal and where the WC grains after sintering have a spherical or angular morphology.
The method according to claim 1 characterized in that the grains after sintering have a spherical morphology and a Riley ratio of below 1.5.
The method according to claim 1 characterized in that the grains after sintering have an angular morphology with a Riley ratio above 1.5.
The method according to any of the preceding claims characterized in that an organic binder is added to the powder blend.
The method according to any of the preceding claims characterized in that a mixing liquid is added to the powder blend to form a slurry prior to the mixing operation.
The method according to claim 5 characterized in that the slurry is spray dried.
The method according to any of the preceding claims characterized in that the binder metal powder is any of one single binder metal, or a powder blend of two or more metals, or a powder of an alloy of two or more metals where the binder metals are selected from Cr, Mo, Fe, Co or Ni.
The method according to any of the preceding claims characterized in that the sintering is done by gas pressure sintering at a sintering temperature of between 1350 to 1500°C.
The method according to any of claims 1-7 characterized in that the sintering is done by vacuum sintering at a sintering temperature between 1350 to 1500 °C.
The method according to any of the preceding claims characterized in that grain size of the WC raw material is between 0.2 to 30 µm.
The method according to any of the preceding claims characterized in that the amount of binder phase powder is between 3 to 30 wt%.
The method according to any of the preceding claims characterized in that the amount of binder phase is between 5 to 7 wt% and the WC grain size is between 2 to 3 µm.
The method according to any of claims 1-11 characterized in that the amount of binder phase is between 7 to 12 wt% and the WC grain size is between 1 to 3 µm.