EP3577242B1

EP3577242B1 - Method of making a double-structured bimodal tungsten cemented carbide composite material

Info

Publication number: EP3577242B1
Application number: EP17708582.6A
Authority: EP
Inventors: Marek TARRASTE; Der-Liang YUNG; Maksim ANTONOV; Irina HUSSAINOVA; Renno VEINTHAL; Jüri PIRSO; Philippe KAPSA; Vincent Fridrici; Ousseini MAROU-ALZOUMA; Alexandre TABOULET; Thomas Camus
Original assignee: Tallinn University of Technology
Current assignee: Tallinn University of Technology
Priority date: 2017-01-31
Filing date: 2017-01-31
Publication date: 2022-10-12
Anticipated expiration: 2037-01-31
Also published as: EP3577242A1; WO2018142181A1

Description

Technical Field

This invention relates to the field of cemented carbides and methods of making cemented carbides. Bimodal, i.e., having two different distinct sizes of grains in final sintered material, cemented carbides can be used in the same applications where conventional cemented carbides are used. Bimodal cemented carbides usually have better mechanical properties and higher resistance against wear. The combination of dispersed areas with predominantly coarse or extra coarse WC grains surrounded by continuous area with predominantly ultrafine WC grains (double-structured bimodal materials) allows to obtain even better properties as required for materials working in demanding impact-abrasive conditions.

Background Art

Cemented carbides are composite materials where one constituent is a hard carbide phase of one or more transition metals and second constituent is a ductile metal phase. The carbides of titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum and tungsten can be used. Ductile metal phase is the cement that binds carbide grains together. Usually, iron group metals - iron, cobalt, nickel or their alloys are used as a metal phase in cemented carbides. Different alloying elements may be added to improve different properties. Cemented tungsten carbides with a cobalt binder are the most commercially important among the various carbide-metal combinations due to an excellent combination of mechanical and tribological properties.
Tungsten carbide cemented carbides with cobalt binder (WC-Co) are still predominant hardmetals since the composition of the hard and brittle tungsten carbide phase that is cemented by the Co-rich binder phase provides an efficient complex of mechanical and tribological properties [1, 2]. Mechanical properties are dependent to a great extent on the Co content and WC grain size [3]. Lower Co content increases hardness and decreases transverse rupture strength (TRS); decreasing WC grain size improves both of these characteristics [4, 5].
When it comes to wear resistance, the effects of Co content and WC grain size are not so straightforward. Often only hardness is used in order to evaluate wear resistance of hardmetals, but many researches have pointed out that this approach is invalid, since wear resistance is functionally related to hardmetal plane strain bulk fracture toughness. Abrasive resistance increases when fracture toughness increases. Fracture toughness, K_IC of WC-Co alloys is known to increase as binder phase volume fraction, mean carbide grain size, and binder phase mean free path are increased. The hardmetals with coarser carbide grains have higher toughness than finer grained grades yet lower hardness [6]. Generally, concurrent improvement of hardness and toughness has been the main research topic since cemented carbides were invented.
It is found that introducing coarse grains in otherwise fine-ultrafine structure can increase fracture toughness without sacrificing the hardness [7]. It results in a microstructure where different grain sizes appear simultaneously, a so-called bimodal structure. Bimodal grain size distribution is achieved by mixing together WC-Co sources with different mean carbide particle sizes [8].
Known is US5593474 disclosing a cemented metal carbide comprising a plurality of regions of a first type of cemented metal carbide; and a plurality of regions of a second type of cemented metal carbide, the first type of cemented metal carbide having a larger average particle size than the second type of cemented metal carbide and the second plurality of regions being interspersed with the first plurality of regions, the regions collectively forming the body of cemented metal carbide with the two types of regions being approximately uniformly distributed throughout the body. Such a double-structured bimodal material exhibits improved wear resistance without sacrificing toughness. The material is produced by preparing two grades of WC+Co powders (with different grain size) by milling and granulating it individually and then mixing the two amounts of these granules carefully without breaking down the granules that are followed by consolidation and sintering.
Also known is US7384443 disclosing a hybrid composite material comprising a cemented carbide dispersed phase and a cemented carbide continuous phase. The contiguity ratio of the dispersed phase of embodiments may be less than or equal to 0.48. The hybrid double-structured bimodal composite material may have a hardness of the dispersed phase that is greater than the hardness of the continuous phase. The method includes making a hybrid cemented carbide composite by blending partially and/or fully sintered granules of the dispersed cemented carbide grade with "green" and/or unsintered granules of the continuous cemented carbide grade to provide a blend that is later consolidated and sintered.
Mechanical and thermal activation is a novel method for producing fine and ultrafine grained WC-Co hardmetals by reactive sintering [9-11].
Known in US6293986 disclosing a bimodal hard metal or cermet sintering of body consisting of WC-containing hard metal phase and a binder phase, and WC platelets embedded therein as reinforcement produced by microwave-assisted reactive sintering while the given material was having only one type of structure, i.e., it is not double-structured.
WC-Co hardmetals with a bimodal structure have been produced using reactive sintering without microwaves. In [12], milled and activated tungsten and graphite powders were mixed with commercial coarse-grained WC-Co powder and then sintered. The microstructure of produced materials was without pores while double-structured material could not be produced because of the breakage of granules and mixing of WC grains from dispersed and continuous areas. What is needed is an alternative method for producing cemented tungsten carbides that combine double-structure with bimodal grain distribution to obtain improved mechanical properties and increased wear resistance in impact-abrasive conditions.

Summary of Invention

These and other goals of the invention are achieved by a method for producing a double-structured bimodal tungsten cemented carbide composite material, said method comprising:

milling of tungsten, carbon (e.g., graphite or carbon black) and cobalt elemental powders, resulting in a first mixture W+C+Co of ultrafine particles, wherein the average W particle size after milling in said first mixture is from 0.1 to 100 nm;
milling of tungsten carbide powder and cobalt elemental powder, resulting in a second mixture WC+Co of coarse or extra coarse particles, and average WC grain size in said second mixture is from 1 000 to 20 000 nm;
granulating said second mixture into a granulated second mixture;
mixing of said first mixture W+C+Co and said granulated second mixture WC+Co, resulting in a third mixture;
consolidating said third mixture; and
sintering said consolidated powder mixture, resulting in a final material.

Preferably, the average size of 95 % of WC grains of the dispersed areas of the finished material resulting from the second mixture is from 5 to 50 times larger than average size of 95 % of WC grains of the continuous areas resulting from the first mixture.
In principle, the invention combines two cemented carbide production methods, i.e., reactive sintering and conventional sintering, in order to achieve a double-structured bimodal microstructure, thereby increase hardness, strength, and wear resistance without compromising the fracture toughness.
Two powder mixtures are prepared: 1. elemental powders of W and Co are milled with a carbon source, such as graphite or carbon black, and 2. coarse or extra coarse WC and Co are milled and granulated. Granules are substantially spherical. Then the powder mixture of W+C+Co is mixed together with the granules of WC-Co and consolidated. Following sintering is done using conventional methods. During sintering, the in-situ reaction takes place to form ultrafine WC embedded in Co matrix. Coarse or extra coarse WC grains experience some growth during sintering. The final microstructure has the double-structured bimodal appearance, comprising closed areas of coarse or extra coarse WC-Co (originating from granules of WC-Co) while these areas are surrounded by an ultra-fine grained WC-Co matrix (originating from W+C+Co mixture). The main advantage over conventional methods is that after conventional sintering the WC grain size distribution is unimodal, i.e. close to normal or Gaussian distribution while with the bimodal approach described here a clear distinction is achievable. WC grains formed by reactive sintering from the first mixture have sufficiently smaller size than those obtained from the second mixture. The fact that during reactive sintering WC grains are first formed (this takes time) and then start to grow enables to obtain finer size than during conventional sintering when existing WC particles activated by preliminary mechanical milling tend to grow intensively during the heating-dwelling-cooling steps of the sintering process. The addition of well-known grain-growth inhibitors such as chromium, vanadium, zirconium, tantalum, titanium or their carbides, nitrides, carbonitrides such as VC, Cr₃C₂, TaC and TiC helps to further refine the microstructure of continuous areas resulting from the first mixture. The fact that grain-growth inhibitors can be added to the first mixture only reduces their possible negative effect due to their segregation along grain boundaries and resulting reduction of fracture toughness. The contiguity ratio of material (measurement of the degree of contacts between ceramic grains) should be as small as possible while magnetic saturation (indicating the presence of Wand C in the Co binder phase) should be as high as possible (indicating the absence of additives in the Co binder) so as to provide the highest possible resistance against impacts. The Co content in the first mixture and in the second mixture can be from 3wt % to 50 wt%, preferably from 10 wt% to 30 wt%, most preferably from 12 wt to 15% to provide final material with best wear resistance in impact-abrasive conditions. The Co content can be same or different in the first and second mixtures. The same Co content in both mixtures provides a reduction of thermal stresses generated during sintering while different contents can result in preferential pre-stressed conditions of either the first or second mixture. The invented method is simpler and could be implemented by most of the companies exploiting a conventional sintering process than the one described in US6293986 since a microwave generator is not required. The invented method allows to produce double-structured bimodal cemented carbide composite materials without breakage of granules of the second mixture as it took place in reference [12] due to the presence of Co in the first mixture that facilitates the pressing (consolidation) process. The given method is simpler that those given in US5593474 and US7384443 since it involves the granulation of only one mixture instead of two. The proposed production method is cheaper than methods described in US5593474 and US7384443 since it uses W for the first mixture instead of the more expensive WC. Additionally, intensive milling required to produce ultrafine WC grains for conventional sintering leads to partial oxidation of these grains and a subsequent higher risk of brittle phases formation, which is avoided in materials obtained by reactive sintering.
Another object not part of the invention is a double-structured bimodal tungsten cemented carbide composite material as prepared by the described method.
Yet another object not part of the invention is a tool insert for mining, tunnelling, construction and drilling, including earth-boring applications comprising a bimodal tungsten cemented carbide composite material as described above.

Brief Description of Drawings

The invention is described in the following figures:

Fig.1 shows the structures of conventional, bimodal, and novel double-structured bimodal materials, the latter being produced by the invented method.
Fig.2 is a flowchart of a method of producing a double-structured bimodal structure of the cemented carbide according to one embodiment of the invention.
Fig.3 is a SEM image of the composite material produced according to one embodiment of the invention.
Fig.4 is an enlarged image of fig 3.

Description of Embodiments

According to the one embodiment of the invention, a double-structured bimodal (Fig 1) cemented carbide was prepared. First, a mixture of elemental powders of W and Co and graphite as C source were milled for 72 hours in a ball-mill with hardmetal lining and hardmetal balls (Fig 2, step 1). Ball-to-powder weight ratio was 10:1. The average initial particle size of W and Co powders was 2-8 µm and the average initial particle size of the graphite powder was 17-19 µm. Alcohol was employed as milling medium. The Co weight ratio of the mixture (W+C+Co) was 15 wt%. C weight ratio of W+C was 7.1 wt% which is approximately 1% (depends on sintering methodology and equipment used) over the stoichiometric C content of WC (6.13 wt%). Excess of C is needed to compensate decarburization that occurs during sintering and to achieve stoichiometric ratio in the final material.
WC and Co powders were milled for 24 hours in ball-mill with hardmetal lining and hardmetal balls (Fig 2, step 2) with ball-to-powder weight ratio 5:1. The average initial particle size of WC was 3-4 µm and the average particle size of Co was 2-8 µm. Alcohol was employed as milling medium. The Co weight ratio of the mixture (WC+Co) was 15 wt% alike the first mixture. After milling, the second mixture was granulated using organic resin, namely rubber, and spray drying method (Fig 2, step 3).
Said first mixture (W+C+Co) and granules of said second mixture (WC+Co) were mechanically mixed inside a soft (plastic) rotating container for 24 h (Fig 2, step 4). This was done to reduce the fracturing of granules as well as to reduce the refinement of carbides. Steel springs were included in the container to facilitate the mixing procedure. Said first and said second powder mixtures were mixed with the ratios of 1:3 (Table 1, E2), 1:1 (Table 1, E3) and 3:1 (Table 1, E4). After mixing, the organic resin was added to the new powder mixture in order to facilitate the consolidation process.
Powder mixtures were consolidated into green specimens using a uniaxial press with a pressure of 90 MPa (Fig 2, step 5). Conventional cemented carbide and reactive sintered cemented carbide specimens with 15 wt% Co ratio were prepared as the reference (Table 1, grades E1 and E5 respectively).
Sintering of specimens was carried out in a vacuum furnace at 1410 °C with a 5 min dwell (Fig 2, step 6). The final temperature was reached with a ramp speed of 10 °C/min. Vacuum level during sintering was 0.3-0.9 mbar.

The microstructures were investigated with SEM (Zeiss EVO MA-15). Vickers hardness was measured in accordance to the ASTM Standard E384. The fracture toughness (K_IC) was determined by measuring the crack length from the tip of the indentation made by Vickers's indentation (Palmqvist method). Indentation diagonals and crack lengths (emanating from the indentation tip) were measured using the Buehler Omnimet software. The toughness is calculated by the following equation [13]

K_{IC} = 0.0726 \frac{P}{C}

where P is the load of Vickers indentor (N) and C is half of the diagonal plus crack lengths (in mm). Mechanical properties of experimental grades E2 to E4 as well as reference materials E1 and E5 are exhibited in Table 1.

[Table 1]

Grade	First mixture, W+C+Co, wt%	Second mixture, WC+Co, wt%	Hardness, HV50	Average crack length, µm	*Fracture toughness K_IC, MPam^1/2**
E1	0	100	1134 (+/-6)	49 (+/- 12)	18.5 (+/- 0.5)
E2	25	75	1308 (+/- 6)	42 (+/- 11)	20.8 (+/- 0.6)
E3	50	50	1251 (+/- 8)	52 (+/- 10)	19.7 (+/ -0.5)
E4	75	25	1211 (+/- 4)	46 (+/- 9)	19.5 (+/- 0.4)
E5	100	0	1343 (+/-10)	74 (+/- 10)	19.6 (+/- 0.4)

The wear rate of double-structured bimodal cemented carbide according to sample E2, Table 1 in combined impact-abrasive conditions (where hardness and fracture toughness are both important; see reference [PTL4] for an explanation of the testing method) and when tested by high-velocity (40-140 m/s) impacts of coarse (3.0-5.6 mm) abrasive particles was at least 20 % less than that of conventional (Table 1, E1) or reactive-sintered (Table 1, E5) cemented carbides. Testing of the proposed double-structured bimodal materials by the conventional ASTM G132 method (static sliding against SiC sand paper) has not revealed better wear resistance as compared to conventional or reactive sintered reference grades.

Citation List

Citation List follows:

Patent Literature

PTL1: Patent US5593474
PTL2: Patent US7384443
PTL3: Patent US6293986
PTL3: Patent EE05780

Non Patent Literature

NPL1: Schatt, W., Wieters, K. P. Powder Metallurgy: Processing and Materials. European Powder Metallurgy Association (EPMA), Shrewsbury, 1997
NPL2: Brookes, K. J. A. World Directory and Handbook of Hardmetals and Hard Materials: Sixth Ed. International Carbide Data, East Barnet Hertfordshire, 1996
NPL3: : Saito, H., Iwabuchi, A., Shimizu, T. Effects of Co Content and WC Grain Size on Wear of WC Cemented carbide Wear 261 2006: pp. 126 - 132, http://dx.doi.org/10.1016/j.wear.2005.09.034
NPL4: Upadhyaya, G. S. Cemented Tungsten Carbides: Production, Properties, and Testing. Noves Publications, 1998 .
NPL5: Gille, G., Szesny, B., Dreyer, K., van den Berg, H., Schmidt, J., Gestrich, T., Leitner, G. Submicron and Ultrafine Grained Hardmetals for Microdrills and Metal Cutting Inserts International Journal of Refractory Metals &
NPL6: Konyashin, I., Ries, B., Lachmann, F. Near-nano WC-Co hardmetals: Will They Substitute Conventional Coarse-Grained Mining Grades? International Journal of Refractory Metals & Hard Materials 28 2010: pp. 489 - 497, http://dx.doi.org/10.1016/j.ijrmhm.2010.02.001
NPL7: Liu, C., Lin, N., He, Y., Wu, C., Jiang, Y. The Effects of Micron WC Contents on the Microstructure and Mechanical Properties of Ultrafine WC-(micron WC-Co) Cemented Carbides Journal of Alloys and Compounds 594 2014: pp. 76 - 81; http://dx.doi.org/10.1016/j.jallcom.2014.01.090
NPL8: Petersson, A., Ågren, J. Sintering Shrinkage of WC-Co Materials with Bimodal Grain Size Distribution Acta Materialia 53 2005: pp. 1665 - 1671.
NPL9: Pirso, J., Viljus, M., Juhani, K., Letunovits, S. Microstructure Evolution in WC-Co Composites During Reactive Sintering From Nanocrystalline Powders Proceedings of the 2008 World Congress on Powder Metallurgy and Particulate Materials CD-ROM.
NPL10: Juhani, K., Pirso, J., Viljus, M., Letunovits, S., Tarraste, M. The Influence of Cr3C2 and VC as Alloying Additives on the Microstructure and Properties of Reactive Sintered WC-Co Cermets Materials Science (Medziagotyra) 18 (1) 2012: pp. 79 - 83.
NPL11: Tarraste, M., Juhani, K., Pirso, J., Viljus, M. Erosion Wear of Reactive Sintered WC-TiC-Co Cermets Key Engineering Materials 604 2014: pp. 63 - 66; http://dx.doi.org/10.4028/www.scientific.net/KEM.604.63
NPL12: Tarraste, M., Juhani, K., Pirso, J., Viljus, M. Reactive Sintering of Bimodal WC-Co Hardmetals Materials Science (Medziagotyra) 21 (3) 2015: pp. 382-385.
NPL13: Lawn, H. R. and Fuller, E. R. Equilibrium penny-like cracks in indentation fracture Journal of Materials Science 10 1975: pp. 2016-2024.

Claims

A method for producing a double-structured bimodal tungsten cemented carbide composite material, said method comprises:
milling of tungsten, carbon such as graphite or carbon-black, and cobalt elemental powders, resulting in a first mixture W+C+Co for obtaining ultrafine tungsten carbide particles in a final material;

milling of tungsten carbide powder and cobalt elemental powder, resulting in the second mixture WC+Co for obtaining coarse or extra coarse tungsten particles in the final material;

granulating of said second mixture resulting in a granulated second mixture, wherein said granules comprise a plurality of grains;
- mixing of said first mixture W+C+Co and said granulated second mixture WC+Co, resulting in a third mixture;

- consolidating said third mixture; and

- sintering said consolidated powder mixture resulting in the final material.
A method as in claim 1, wherein the carbon weight ratio in said first mixture is selected to achieve close to the stoichiometric ratio in the final material.
A method as in claims 1 to 2, wherein the final material has a microstructure of the tungsten cemented carbide composite material comprising two distinct areas: separate dispersed areas with coarser WC grains in the Co matrix and a continuous area with ultrafine WC grains in the Co matrix.
A method as in claims 1 to 3, wherein said first mixture is from 1 wt % to 99 wt % of the third mixture.
A method as in claim 3, wherein said first mixture is from 10 wt % to 50 wt % of the third mixture.
A method as in claim 3, wherein said first mixture is from 15 wt % to 35 wt % of the third mixture.
A method as in claims 1 to 6, wherein the Co fraction in the final material is from 3 wt % to 50 wt %.
A method as in claim 6, wherein the Co fraction in the final material is from 10 wt % to 30 wt %.
A method as in claim 6, wherein the Co fraction in the final material is from about 12 to about 15 wt %.
A method as in claims 1 to 9, wherein carbide grain-growth inhibitors are added in said milling step only to the first mixture.
A method as in claim 9, wherein said grain-growth inhibitors are selected from a group consisting of chromium, vanadium, zirconium, tantalum, titanium, or their carbides, nitrides, carbonitrides.
A method as in claim 11, wherein the weight fraction of said grain-growth inhibitors in the first mixture is from 0.1 to 5 wt %.