METHOD OF MAKING ULTRAFINE WC-CO ALLOYS

Abstract: The present invention relates to a method of making a cemented carbide using submicron WC grain size (manufactured by carbothermal process) and containing WC and 6-24 wt.% Co using a cobalt powder having deagglomerated spherical grains of submicron average grain size and with a narrow grain size distribution wherein at least 80 % of the grains have sizes in the interval x+0.2x provided that the interval of variation (that is 0.4x) is not smaller than 0,1 mu m. The invention is characterised in adding < 1 wt.% grain growth inhibitor such as VC and/or CR3C2 and selecting a carbon content close to etaphase formation.

Description: Method of making ul rafmp WC-Co alloys The present invention relates to a method of making ultrafine WC-Co alloys from a well dispersed mixture of fine and non-agglomerated WC and Co powders, optimised grain growth refiner additions and carbon content using low temperature smter/smter-HIP conditions. It is well known that decreasing the WC grain size confers performance advantages to cemented carbide in many applications e.g. PCB (Printed Circuit Board) machining, wood machining, metal cutting. Maintaining a submicron WC grain size requires the use of gram growth refiners such as VC, Cr3C2, TaC etc. and the finer the WC gram the greater the necessary addition of said re- fmers. In some applications e.g. metal cutting fineness of the WC gram size should not greatly reduce toughness, otherwise edge life will suffer. Gram refiners may reduce toughness if used in excessive amounts. In other applications e.g. PCB machining, fineness of WC gram size is of paramount importance with toughness demand being secondary. Commercially available ultra fine cemented carbide grades already use a gra size of about 0.4 μm. But to reduce the WC gram sizes to below 0.4 μm requires novel raw material and process- g technique. DE 40 00 223 (Mitsubishi) discloses a cemented carbide based on WC with 6-14 wt-% binder phase containing vanadium and chromium whereby the ratio Cr/ (Cr+V) is <0.95 and >0.50. US 4,539,041 discloses the making of metallic powders by a process for reducing oxides, hydroxides or metal salts with the aid of polyols. Particularly, when starting with cobalt hydroxide it is possible to obtain powders of metallic cobalt as essentially spherical, non-agglomerated particles. Such Co powder is herein referred as polyol cobalt. In US 5,441,693 it is disclosed a method of making cemented carbide with an extremely uniform structure by using Co-powder produced according to the above mentioned polyol method and with submicron grain size. It is an object of the present invention to provide a method of making cemented carbide with WC grain size less than 0.8 μm and with a low content of grain refiners . According to the method of the present invention cemented carbide composition with extremely fine micro- structure, average grain size <0.8 μm, essentially no grains larger than 1.5 μm, suitable for toughness demanding machining operations are made by milling deagglomerated submicron WC powder produced by carbothermal reaction with a cobalt powder having deagglomerated spherical grains of about 0.4 μm average grain size and with a narrow grain size distribution wherein at least 80 % of the particles have sizes in the interval xj_0.2x provided that the interval of variation (that is 0.4x) is not smaller than 0.1 μm. Preferably the cobalt powder is polyol cobalt. If the carbon content of the powder mixture to be sintered is held close to etaphase formation no or relatively low amount <1 wt-% of grain growth refiners such as VC and Cr3C2 need to be added. Sinter- ing with HIP takes place at relatively low temperature <1400 °C i.e. The sintered cemented carbide has a Co- content of 70-85% in terms of cobalt magnetic measurements assuming pure cobalt . In a preferred method the average WC grain size is further reduced to below 0.4 μm by using an optimum VC + Cr3C2 addition in which the ratio as VC/Cr3C2 in wt-% is 0.33-1.0, preferably 0.5-0.9, most preferably 0.7-0.8 for PCB-applications and 0-0.5 for metal cutting, preferably 0-0.2 for non ferrous machining and 0 for ferrous machining. Preferably sintering is performed using gas pressure sintering also referred to as sinter-HIP. In a first embodiment particularly useful for finish and general machining of non-ferrous materials the cemented carbide consists of 6-10% Co, 0.0-0.3 VC, 0.3-0.75 Cr 3 C and rest WC <0.8 μm. In a second embodiment particularly useful for rough machining in demanding work materials e.g. austenitic stainless steels the cemented carbide consists of 10-16% Co, 0.5-1.2 Cr3C 2 and rest WC <0.8 μm. In a third embodiment particularly useful for very tough machining operations, or those with very low cutting speed, e.g. broaching the cemented carbide consists of 16-20% Co, 0.8-1.8 Cr 3 C 2 and rest WC <0.8 μm. In a fourth embodiment particularly useful for PCB and non-metallic routing and slot drilling the cemented carbide consists of 5-8% Co, 0.1-0.6 VC, 0.25-0.6 Cr 3 C and rest WC <0.4 μm. In a fifth embodiment particularly useful for PCB micro drilling the cemented carbide consists of 8-12% Co, 0.2-0.9 VC, 0.4-0.9 Cr 3 C 2 and rest WC <0.4 μm. In a sixth embodiment particularly useful for wood- machining of solid wood or fibreboard the cemented car- bide consists of 2-5% Co, 0.05-0.2% VC, 0.1-0.25% Cr 3 C 2 and rest WC<0.4 μm. πxam l e 1 PCB drill blanks were produced from submicron WC made by carbothermal reaction and milled deagglomerated with cobalt powder having special deagglomerated grains of about 0.4 μm average grainsize and with a narrow grain size distribution and VC+Cr3C2. The following compositions were made containing in addition to WC : A. 8 wt-% Co, 0.3 wt-% VC and 0.4 wt-% Cr 3 C 2 with a carbon content according to the invention. For comparison blanks with the same composition but with carbon content according to prior art. B. 9 wt-% Co and 0.35 wt-% VC and 0.45 wt-% Cr 3 C 2 with a carbon content according to the invention. For comparison blanks with the same composition as prior art from A were made . C. 7 wt-% Co and 0.26 wt-% VC and 0.35 wt-% Cr 3 C 2 with a carbon content according to the invention. For comparison blanks with the composition (in wt-%) 6.5 Co, 0.6 VC and 0.32 C^C2 according to prior art were made. The blanks were pressed and sintered with HIP at 1340 °C. Magnetic cobalt content, CoM, coercive force, He, were measured and performance was tested in a micro- drilling and a routing test. The microdrilling test was performed under the following conditions: Drill diameter 0.3 mm Speed 80000 to 120000 rpm Feed 15 μm/rev increasing at every 500 hits by 5 μm/rev until failure Material tested three stacked PCB copper lined FR4 resin The routing test was performed under the following conditions : Diameter 2.4 mm Measurement of wear levels after 50 m routing at speeds ranging from 30000 to 42000 rpm (8 μm tooth at 2.4 mm diameter) Material three stacked PCB copper lined FR4 resin The following results were obtained Performance: PCB microdrilling examples: CoM He Tool life ratio of invention with respect to prior art A. invention 5.80 38.3 1.27 prior art 7.34 37.0 1 B. invention 7.33 40.5 1.59 prior art 7.34 37.0 1 Performance: PCB Routing invention 6.04 40.7 1.1 prior art 5.11 41.6 1 Fixampl e 2 Endmill blanks were produced from submicron WC made by carbothermal reaction and milled deagglomerated with cobalt powder having special deagglomerated grains of about 0.4 μm average grainsize and with a narrow grain size distribution and Cr3C2. The following compositions were made containing in addition to WC : D. 10 wt-% Co and 0.5 wt-% C^C2 with a carbon content according to the invention. CoM was 8.3 and Hc24 kA/m. E. 12 wt-% Co and 0.6 wt-% Cr 3 C2 with a carbon content according to the invention. CoM was 10.6 and He 21.5 kA/m. F. For comparison blanks with 10 % Co, 0.5 % Cr3C2 but with carbon content according to prior art and with 0.8 μm grain size. The blanks were pressed and sintered at 1360 °C. Magnetic cobalt content, CoM, coercive force, He, were measured. The blanks were ground to 8 mm diameter end- mills PVD coated with TiCN. Performance was tested in an end milling and slotting operation. An edge milling test was performed under the following conditions: Work piece material: Inconel 718 age hardened Speed 20 m/min Feed 0.021 mm/tooth Depth of cut : 8 mm Radial depth of cut: 4 mm Flood coolant Result D (Invention) reached 0.9 m cutting distance tool life and the reference F achieved 0.42 m cutting distance . A slot milling test was performed under the following conditions: Work piece material: 316 austenitic stainless steel Speed 50 m/min Feed 0.042 mm/tooth Depth of cut : 4 mm Width of cut : 8 mm Flood coolant Result E (Invention) reached 8.5 m cutting distance tool life and the reference F achieved 5 m cutting distance.

Claims: Cl aims 1. Method of making a cemented carbide using submicron WC grain size (manufactured by carbothermal process) and containing WC and 6-24 wt-% Co using a cobalt powder having deagglomerated spherical grains of submicron average grain size and with a narrow grain size distribution wherein at least 80 % of the grains have sizes in the interval x+0.2x provided that the interval of variation (that is 0.4x) is not smaller than 0.1 μm c h a r a c t e r i s e d in adding <1 wt-% grain growth inhibitor such as VC and/or C^C2 and selecting a carbon content close to etaphase formation. 2. Method according to claim l c h a r a c t e r i s e d in adding the VC and C^C2 in such proportions that the ratio VC/Cr 3 C in wt-% is between 0.33-1.0 for PCB-applications and 0-0.5 for metal cutting.

Inventors: AUCOTE JOHN, CARPENTER MICHAEL JOHN, GREARSON ALISTAIR, AUCOTE, JOHN, CARPENTER, MICHAEL, JOHN, GREARSON, ALISTAIR
Applicants: AUCOTE JOHN, CARPENTER MICHAEL JOHN, GREARSON ALISTAIR, SANDVIK AB, AUCOTE, JOHN, CARPENTER, MICHAEL, JOHN, GREARSON, ALISTAIR, SANDVIK AB (PUBL)
Application year: 1998
Main classification: C22C29
IPC codes: B23B27, B23B51, B23D43, C22C1, C22C29