Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C29/00—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides
  • C22C29/02—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides based on carbides or carbonitrides
  • C22C29/06—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides based on carbides or carbonitrides based on carbides, but not containing other metal compounds
  • C22C29/08—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides based on carbides or carbonitrides based on carbides, but not containing other metal compounds based on tungsten carbide
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/04—Making non-ferrous alloys by powder metallurgy
  • C22C1/05—Mixtures of metal powder with non-metallic powder
  • C22C1/058—Mixtures of metal powder with non-metallic powder by reaction sintering (i.e. gasless reaction starting from a mixture of solid metal compounds)
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/10—Alloys containing non-metals
  • C22C1/1084—Alloys containing non-metals by mechanical alloying (blending, milling)
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
  • B22F2998/10—Processes characterised by the sequence of their steps

Definitions

• 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 allows to obtain even better properties as required for materials working in demanding impact-abrasive conditions.
• 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.
• 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.
• 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.
• 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
• WC-Co hardmetals with a bimodal structure have been produced using reactive sintering without microwaves.
• 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.
• 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.
• 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.
• 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 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 aspect of the invention is a double-structured bimodal tungsten
• cemented carbide composite material as prepared by the described method.
• Yet another aspect of the invention is a tool insert for mining, tunnelling,
• 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
• a double-structured bimodal (Fig 1 ) cemented carbide was prepared.
• 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 pm and the average initial particle size of the graphite powder was 17-19 pm. 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 pm and the average particle size of Co was 2-8 pm.
• Alcohol was employed as milling medium.
• the Co weight ratio of the mixture (WC+Co) was 15 wt% alike the first mixture.
• the second mixture was granulated using organic resin, namely rubber, and spray drying method (Fig 2, step 3).
• NPL1 Schatt, W., Wieters, K. P. Powder Metallurgy: Processing
• EPMA European Powder Metallurgy Association
• 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
• 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 & Hard Materials 20 2002: pp. 3 - 22, http://dx.doi.org/10. 1016/S0263- 4368(01 )00066-X
• NPL6 Konyashin, I., Ries, B., Lachmann, F. Near-nano WC-Co hardmetals:
• NPL7 Liu, C, Lin, N., He, Y., Wu, C, Jiang, Y.
• NPL8 Petersson, A., Agren, 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
• NPL10 Juhani, K., Pirso, J., Viljus, M., Letunovits, S., Tarraste, M.
• NPL1 1 Tarraste, M., Juhani, K., Pirso, J., Viljus, M. Erosion Wear of
• 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

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• Mechanical Engineering (AREA)
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• Organic Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Powder Metallurgy (AREA)

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Citations (7)

• 2017
  • 2017-01-31 EP EP17708582.6A patent/EP3577242B1/de active Active
  • 2017-01-31 WO PCT/IB2017/050505 patent/WO2018142181A1/en unknown

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