Method of manfacturing an improved fine-grained WC-Co cemented carbide

Abstract: The present invention relates to a method of making a WC-Co-based cemented carbide with a fine WC-grain size. The cemented carbide is produced from well deagglomerated or easy to deagglomerate WC powder with round morphology, a Co powder also well deagglomerated or easy to deagglomerate and with a grain size equal to or smaller than the WC grain size and grain growth inhibitors. According to the invention the metal part of the grain growth inhibitors is added as part of the binder phase i.e. is included in the Co powder and alloyed therewith.

Description: The present invention relates to an improved method of making fine-grained WC-Co cemented carbide. The traditional way to produce time grained cemented carbide is to wet mill the desired proportions of WC, Co and grain growth inhibitors, and pressing agent like PEG or A-wax, in a ball mill with milling bodies of WC-Co (in order to avoid unwanted impurities in the material) extensively in alcohol/water or any other milling liquid. The final grain size of the tungsten carbide is determined during this process. The tungsten carbide is often strongly agglomerated and this is also valid for the cobalt powder. The milling process is often very long in order to: 1. Determine the final grain size of the tungsten carbide. 2. Get an even dispersion of the grain growth inhibitors to avoid grain growth in any part. 3. Have the cobalt evenly dispersed to avoid porosity and cobalt lakes in the sintered material. After milling, the slurry must be dried, often in a spraydrier, to get a free-flowing powder. This powder is then pressed and sintered to blanks followed by grinding to the final dimensions and often coated. For submicron material grain growth inhibitors are used as a rule: Cr 3 C 2 and/or combinations of VC+Cr 3 C 2 are used for finer grain sizes. However all cubic carbides in Groups IV and V of the periodic table act as grain growth inhibitors for WC-Co-alloys: TiC, ZrC, HfC, VC, NbC, TaC but also the hexagonal Mo 2 C and the orthorombic Cr 3 C 2 of Group VI. For WC-Co-alloys with a sintered mean grain size of 1.0-1.6 µm for the tungsten carbide, TaC is a very common grain size stabilizer/grain growth inhibitor, but also NbC is used often in combination with TaC. Mo 2 C can be used as well, both in the submicron and micron grain size area (0,8-1.6 µm). For instance, WO-A-9913120 discloses that up to 3 wt.% of Vanadium and/or chromium, Titanium and/or Niobium is added only in the form of carbides on mixing. The object of the present invention is to avoid the production disadvantages described above and also to increase the performance level for the sintered material, mainly the toughness. The present invention relates to a method of making a WC-Co-based cemented carbide with a sintered we mean grain size of 0.6-1.4 microns, containing up to 3 wt-% of vanadium and/or Cr, Ti and Ta and/or Nb as a deliberately added grain growth inhibitor. The invention is given by claim 1 and consists of the following basic concepts: A well defined, narrow grain size distributed WC raw material with rounded morphology is used in which its final (sintered) grain size is already determined when it is produced via reduction and carburization. The WC must be deagglomerated into single grains or be easy to deagglomerate. If a cemented carbide with a sintered WC mean grain size of 1.3 µm is wanted the original WC must have a mean grain size of about (1.0-) 1.2 µm because a certain small, but controlled, grain growth can never be avoided. A well defined, narrow grain sized Co raw material, also with rounded morphology and with a mean grain size equivalent to or smaller than the mean WC grain size with which it will be mixed is selected. The cobalt powder must also be easy to deagglomerate. This Co raw material already includes at least the metal part of the grain growth inhibitors, i.e. the addition of the grain growth inhibitor is part of the Co powder production process. This means that also the cobalt is 'tailor made' for the final sintered alloy, because the amount and type of grain growth inhibitor additions are dependent on both final (sintered) WC grain size and the amount of binder phase desired. A short milling time which is rather a blending and mixing than a traditional milling. The use of the concepts listed above gives a cemented carbide with better production economy combined with better compacting properties (less cracks and better tolerances i.e. better shape stability) and increased toughness. The toughness increase is due to a better morphology with more rounded and less triangular and prismatic WC grains. With the grain growth inhibitors present where they are wanted/needed, i.e. the contact surfaces between Co and WC, the amount of grain growth inhibitors can often be decreased. Because these inhibitors, especially VC, are well known to decrease the toughness, a decrease of these elements but still the same effect because they are placed where they are needed, a better toughness can be obtained. EXAMPLE 1 Two powder batches were produced, one according to established technology and one according to the invention. Known technique : 89.5 w/o WC, 0.8 µm (FSSS) 10.0 w/o Co standard (1.5 µm) 0.5 w/o Cr 3 C 2 Milling time: 30 h Invention : 89.5 w/o WC, 0.70 µm (FSSS) 10.43 w/o Co-Cr (0.65 µm) 0.07 w/o C (carbon compensation) Milling time: 3 h The Co-Cr alloy according to the invention contains Co and Cr in the proportions 10/0.43 and is easy to deagglomerate as well as the WC according to the invention. The mills were identical as well as the total amount of powder in the mills. The slurries were spray dried with the same process parameters. The two powders were pressed to insert blanks, SNUN 120308, in tools for 18% shrinkage when sintering. The compacting pressure was 145 MPa for the powder produced according to existing technique and 110 MPa for powder according to the invention. Desired compacting pressure is 100±20 MPa. The pressed compacts were then sintered in the same batch and had the same hardness in as-sintered condition, 1600±25 HV3. EXAMPLE 2 Of the same powders as in example 1, test pieces 5.5x6.5x21 mm were produces. They were sintered together and then tested in a 3-point bending test with the following results, mean values: Known technique Invention 2725±300 MPa 3250±200 MPa EXAMPLE 3 Two alloys with the same composition were made, one according to the present invention and one according to known technique. Known technique 93.5 w/o WC 1.2 µm FSSS 6.0 w/c Co standard (1.5 µm) 0.5 w/o TaC Milling time: 40 h Invention 93.5 w/o WC 1.0 µm (FSSS) 6.4 w/o Co-Ta 0.8 µm 0.1 w/o C (carbon compensation) Milling time: 4 h The two variants were produced according to example 1. When pressing the same test inserts, SNUN 120308, the compacting pressure for 18% shrinkage was 160 MPa for the powder according to existing technique and 115 MPa for the powder according to the invention. After sintering both variants had the same hardness, 1750±25 HV3.

Claims: Method of making a WC-Co-based cemented carbide with a sintered mean WC grain size of 0.6-1.4 microns, containing up to 3 wt-% of vanadium and/or Cr, Ti and Ta and/or Nb as a deliberately added grain growth inhibitor, in which a well deagglomerated or easy to deagglomerate WC powder with rounded morphology is mixed with the well deagglomerated or easy to deagglomerate cobalt based binder powder, characterized in that the binder powder is a alloy of cobalt and at least one of the said metal grain growth inhibitors. Method according to claim 1 characterised in that the Co-powder has a mean grain size equal to or smaller than that of the WC-powder.

Inventors: OSKARSSON ROLF, OSKARSSON, ROLF
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 2000
Main classification: C22C1
IPC codes: B22F7, C22C1, C22C29